60 Years Of The AR-15
Understanding America's Rifle
Yes indeed, 1959 was the year. Before miniskirts, before the first televised presidential debate, the pill, or 8-tracks. Despite how some folks would have you believe it is only a recent modern war device too dangerous for mere civilians,...back then, even Californians could order one through the mail directly to their door at any age, no registration or paperwork. Jeez, how did everyone not get murdered?! (Sarcasm,...can you smell it?)
But in fact, the AR15 was actually merely a later evolution from its original big brother, the AR10, which had first appeared in 1956! Elvis is new, Eisenhower still in his 1st term, the civil rights movement is just getting rolling.
Having been around so long, there is a lot of history and related areas and stories that could fill books on this subject, and have. What I will try to impart here in only a mere single article is the essential yet comprehensive summary of how it came to be, where its been, where we are now and the variety it has evolved into. In doing so, my hope is that this will illuminate why it has become the standard base rifle for the American public as well as the armed forces.
Many people wonder what the AR15 name stands for. Well, the designer, Eugene Stoner, worked for a company in Hollywood called Armalite https://en.wikipedia.org/wiki/ArmaLite , a gun designing division of the aircraft company Fairchild https://en.wikipedia.org/wiki/Fairchild_Aircraft, which has had several planes successfully attain U.S. defense contracts over the decades. Thusly, when created, it stood for Armalite Rifle 15. Other Armalite products had the AR prefix, such as the AR7 and AR18, neither of which looked like or were based on the most famous one we are discussing here.
As part of an aerospace manufacturing conglomerate, Armalite was from the beginning focused on utilizing aerospace materials and technology. Lightweight alloys and plastics instead of wood and iron. The products they turned out looked truly from the future and were very unconventional not only in their materials and manufacture, but even in outward appearance. The AR10 was developed specifically to compete in the 1957 U.S. Army trials to select a new service rifle to replace the WW2 and Korean era M1Garand. While it impressed the users who tested it alongside the FN/FAL (which was adopted by most of NATO), the fix was in and the M14 (essentially an enhanced and modified M1Garand) was adopted instead.
Some countries bought small quantities of the AR10, but it just seemed too much too soon for most traditional military institutions to take the plunge.
But almost as soon as the M14 was adopted, it became clear it had limitations in facing opposing rifles of the new "assault" type, and an insurgency within the Pentagon was on to save the U.S. Military from itself and it's new "old" rifle. There were Army officers who had witnessed the Armalite's performance during the trials and were very impressed. They wondered if perhaps it could be scaled down slightly to chamber a true intermediate assault rifle type caliber, and if they might be able to somehow sneak such a weapon development program through the cracks and have a stealth assault rifle program under the radar. As it turned out, the Air Force had the greater number of forward-thinking generals to go along with such a scheme, and enough independent budget with minimal oversight. By 1959, what we today know as the AR15 was already prototyped and in testing with the Air Force. It was in service by 1960. 3 years later, in 1963, the M14 was declared obsolete after the shortest service life of any U.S. Rifle (just 6 years), and no new production runs were made. (However, the Marines refused to give theirs up until 1967 by an act of Congress.)
The M14 had provided a larger reservoir of ammo than the M1Garand it had replaced, but it was still a full-power "battle rifle" caliber. Great for engagements between foes at long distances, but not for the new and evolving land warfare doctrines that emphasized mechanized units and shock troops maneuvering to surprise and quickly assault and over-run the opposing force. Such actually required something of a more intermediate caliber, allowing for more effective close-in fighting, and allowing troops to carry more ammo for the same weight.
The story of what made the AR15/M16 so revolutionary is really of 2 different parts;...the physical rifle itself ?" it's design and materials;...and the particular cartridge it debuted to the world, along with all it's unique properties. The debate still rages today as to which has had the greater independent impact on weapon development since.
The Hyper-Speed Bullet
It had been well understood by WW2 that, in a given bore diameter, the lighter and more unstable a bullet pushed at an even greater velocity than the heavier and more stables ones, would produce a dramatic wounding effect in flesh of significantly greater grievousness, incapacitation and potential lethality. By the 50's, American hobbyist rifle tinkerers had found that specialty cartridges originally designed for prairie dog and coyote eradication, using little .22 caliber pills traveling over 3000 feet per second (about 50% faster than most military calibers of the era) produced destruction in flesh far out of proportion to what their small size would seem to indicate could be expected. They did this with far less recoil energy expended at the shooter. Prairie dog hunters of the day exclaimed how, with these little "mouse guns", they could blast away at prairie dog towns all day long without fatigue.
The U.S. Army Wound Ballistics Laboratory took note of this, and began experimenting as well. When the semi-secret alternative rifle program that became the AR15 came calling, the USAWBL provided them with a cartridge they had partially developed in partnership with Remington, called the .223 Remington. (The U.S. Army's version, slightly different, was called the 5.56mm. These are similar, but INDEED different,...which we shall discuss later.)
Why was this so radical and important?
Among the data points to emerge after WW2 was that, despite the great distances of engagement that full power battle rifles of the day were capable of,...most engagements between soldiers shooting at each other occurred at distances of LESS THAN 300 yards. If so, then why put up with the abuse and fatigue of shooting a rifle meant to hit soldiers as far as 600 or 800 yards? As the Germans had shown with the Stg44,...it's little 8mmKurtz round, effective out to 300 yards, weighed only 65% of its bigger brother, the 8mm Mauser, which was the main German battle cartridge. Ergo,...a soldier could carry 35% more ammo into battle if he had the StG44, which allowed for tactics using a higher volume of fire to overwhelm the enemy.
But what if we could go even lighter?
The development of these hyper-velocity varmint hunting rounds seemed to make that possible. And with even less recoil and shooter fatigue than even the intermediate rounds like the 8mm Kurtz or Russia's AK47 cartridge, the 7.62x39.
Indeed, the typical .223 or 5.56 cartridge weighs about only 45% of the .308(7.62NATO) round it replaced. A U.S. soldier could literally carry more than double the number of rounds into combat for the same weight penalty in his overall load. If the soldier merely carried the same number of rounds, he then was less encumbered by load, fatigued less, and was therefor more combat effective.
All of this was very controversial at the time, and there was great difficulty overcoming institutional bias in favor of "battle rifles" over these puny and anemic "mouse guns".
Combat, however, tends to settle all arguments. And this was true of the new 5.56 ammunition when it saw use in Vietnam. The new round was so effective in its wounding, creating horrendous cavitation and crippling tissue damage, that complaints were lodged in Geneva by the Vietnamese that this new ammo was in violation of the conventions of war for being needlessly cruel and torturous, designed primarily to wound and not to kill. That was in fact partly true, but the complaints never got traction in international court, primarily because the ballistic science at play was new and not widely understood by those in positions of judgment.
The South Vietnamese soldiers LOVED the new rifle and ammo. Being small people, they more than anyone else could benefit from the lessened kick the package produced. And in the jungle, where you almost never saw the enemy until he was close enough to hit with a rock,...big blasters designed for trench warfare across the plains of Europe were just silly.
In fact, as any veteran will tell you, a soldier spends more of his life carrying his rifle and ammo than shooting it, even in a war zone. Soldiers labor like mules under heavy loads they must transport on their backs wherever they go, and every ounce is cursed about to the lord above. While soldiers issued the new AR15/M16 might have thought it looked like a plastic toy and had ammo that wasn't very inspiring to look at,...everyone loved it when it came to long road marches, as opposed to the heavy beast that was the M14.
In The Age Of The Jet,
Aerospace Materials & Engineering
America's military has had a particular lesson drilled into it during it's history. Ever since the Civil War, America's access to the latest technology, when applied to the battlefield, has been central to victory. It did not always mean America had the best weapons. But our industrial might has on several occasions made the difference.
During WW2, American engineers and designers had continually been impressed with and frightened by many Nazi "wonder weapons". What was so disturbing about these was that it was clear that if Germany had simply not been saddled by it's mentally defective dictator who quite often got in the way of many of these weapon systems being more widely adopted or used,...competent generals could have seen to it that ?" with the help of these systems more advanced than what America had ?" the war could have been much more bloody and taxing for the Allies, and indeed, might have ended with a negotiated partitioning of Europe rather than clear victory.
And so U.S. military thinkers coming up with future war-fighting doctrine were converting from the idea of having "the most" weapons, to having "the best" ones. America was rich when compared to other nations, and we could afford to spend on our weapons.
Nowhere was this being put into practice more vigorously than in the Air Force, with the world's greatest and fastest aircraft. It was before what we know today as "The Space Race", but for the Air
Force, it already saw itself in a race against Russia to create the future today. New materials and manufacturing processes were being developed as a result of all the research and development funds being dumped into the aerospace industry. Fairchild Aircraft, unfortunately, was not having as much luck with military contracts at that time as it had prior to WW2, so they looked to diversify a little and see what else they might be able to apply their aerospace engineers on that could bring in government money.
The result was Armalite, and the designs of Eugene Stoner.
As previously mentioned, Stoner brought aerospace tech to gun design. If a moving component used to operate the system could be replaced or eliminated, resulting in less weight, that was done as well. Stoner also understood ergonomics and kinetic energy, and realized keeping the whole occurrence of rearward recoil as low and inline to the shoulder as possible equaled better control and quicker return of sights on target. As well, the safety could be engaged or disengaged without the firing hand needing to move from its ready grip, allowing for soldiers (if trained appropriately) to have the weapon at the shoulder and ready fire, yet safe,...and still be able to engage immediately at the flick of a switch. This ergonomic focus is also part of the success of the AR15.
And perhaps most importantly ?" and you will hear this over and over in this article and elsewhere ?" Stoner emphasized modularity;...all of the system consists of modular sub-groups which can be repaired by simple replacement. But also, can be upgraded with an improved redesigned sub-component, usually without need of depot level armory technicians. While riflesmithing in the past required some skill as a machinist and as a blacksmith,...the Stoner design merely required a manual and a complete set of tools and gauges. This is key to why the basic design is still with us today and why individual shooters love it so much ?" customization.
As such, the AR15/M16 rifle cost more to produce than almost all other rifles in competition with it. While the AR15 cost approximately $350 (in 1970 dollars), the AK47 was being cranked out for less than $50. Today, due to the aerospace manufacturing methods used being more common, that disparagement in cost is not as widely different now. But a good AK47 averages for about $600-700, while similar quality AR15s run about the same. (There are exceptions to those guidelines, which we will cover later.) For prime Aks, expect to spend $1200 and up. The same goes for AR rifles, but their price ceiling is far higher.
If we were to look at the AR15/M16 timeline, 1984 was an important year, since that was when the 25-year patent on the basic AR15 expired. Suddenly, anyone could make and market AR15 copies. A few companies started making the lower receivers, and customers could now custom assemble their own AR as they would like it to be. This was the real beginning of the growth in development of the modular accessorizing of the series. While the AR15 had not previously been one of the more affordable semi-auto "assault rifles" available for the civilian consumer, it now began to be. Rather than buy it all at once, you could buy pieces of one as you could afford to, and increasingly at lower prices due to competition. (The expiration of patents now allow sharing them online as digital files, which is likely to be the future of weapon manufacture for many American citizens,...at home. More about that later.)
The Eminently Upgradable Rifle
As mentioned earlier, the U.S. government M-16 rifle has gone through many evolutions during its over 50 years of being issued to American troops. (The longest serving base rifle design in U.S. history, by the way.) To better understand how the system's modularity makes this possible and more economical by simply replacing the system with something else entirely, let us catalog what these changes have been.
As first adopted by the Air Force, the AR-15 (and later designated the M-16 once fully adopted by the Pentagon) looked like this:
After the troubles experienced with it in Vietnam, the first upgrade came about. These were designated M16A1. Changes included a forward bolt assist button to help ensure the bolt could close fully on a live round if things were getting sticky due to grime and goo, chrome lining of the bore, as well as a redesigned flash-hider that did not as easily snag brush and twigs. It looked much the same, except like this:
At the same time, a smaller carbine version was also issued to select troops, whether because they needed to get in and out of vehicles or just needed to operate in confined spaces. This used the same lower and upper receivers, but had different/shorter barrel, handguard and even a collapsible butt-stock. It was at different times (depending on which armed service) called the CAR-15, XM-177 or other experimental designations. The first ones also lacked the forward assist and chrome lining, but later ones included the A1 upgrades. They looked like this:
And there things stayed until about 1981, when the M16A2 upgrade began. This was a much more inclusive redesign, which included new 1/7 twist rate barrel with a larger diameter, new adjustable sights, new directional compensating flash-hider, a forward assist button less prone to breakage, a 3-shot burst feature rather than continuous full auto fire, finger-grooved pistol grip, redesigned heat shield handguards, a 1 inch longer butt-stock to better fit larger Americans, as well as a cartridge case deflector to allow left-handed shooters to not get hot brass down their shirt. And it looked like this:
Covert special forces and commando units began growing rapidly under the Reagan administration, and after their successful operations in the 80s and 90s, had the clout to begin getting their own derivative weapon systems in the late 90s. Liking the improvements made in the A2 upgrade, but still often using the old and worn Vietnam era carbines like the XM177 and CAR15 on many missions, Special Operations Command (with help from the regular Army) got the Pentagon to approve purchases of a new carbine version, designated the M4. Recognizing that too much velocity and therefor lethality was lost with the super short 10 inch barrels of the past, the M4 came with a compromise length barrel of 14.5 inches. This gave good ballistics and wounding out to about 100 yards, but was still short enough to be much more useful than the full rifle. The commandos also wanted to more easily and securely mount night vision optic devices to these carbines than had been practical in the past, so the new weapon had a modified carry handle that was removable and provided for mounting such gear in its place. The new M4 looked like this:
It was during the late 80s and early 90s that an explosion of accessories for the AR15 series of rifle began to burst forth on the civilian market. The commando warriors in the special services, always keen to be a little different and to experiment, enthusiastically tried out all of them. Today it is not uncommon to see much more modified versions of these basic rifles in service, with all manner of other devices mounted to them, and covered in colorized finishes.
The Main Sub-Groups
Lower Receiver Assembly
This is, legally speaking, the "heart" of the gun. It bears the serial number, and so under U.S. law, the lower receiver IS the gun, even if all the other components that would allow it to shoot are not attached. Because of this, it can be less expensive to first invest in acquiring the lower receiver while laws and regulations are in your favor, and get the rest of the rifle's components at a later date via unregulated and unsurveilled sources. The feds also charge a 15% surcharge tax on all weapons, so buying a $100 lower receiver results in only a $15 tax paid, versus perhaps $150 if you bought a complete rifle for $1000. You can also "build" or complete your own lower receiver from a raw forging of aircraft grade aluminum, which federal law allows, and in this way leave no purchase record at all that you have one. Currently, legal circumstances allow for lower receivers that are only 80% completed to be sold as though they ARE NOT firearms, greatly reducing the work needed to be done by the purchaser to further fabricate it into a 100% functional firearm. Because they are currently unregulated and how many have been sold or completed is unknown, opponents of freedom have labeled these as "ghost guns". But those in gun circles simply call them "80 percenters" or "80% kits". (We will cover 80% receivers more in depth later.)
Upper Receiver Assembly
This is the section which has the barrel mounted to it, and nearly all of the major action that occurs mechanically when fired occurs within this half of the weapon. Because it can in modular fashion be detached completely from the lower half of the weapon, the possibility exists to mount a completely different other upper receiver assembly, consisting of different barrel length and weight (or even caliber) if the circumstances might warrant doing so. Thus, with one lower receiver but several uppers, you could have several different rifles in one.
Gas Operating System
When it was first designed, and for much of its life, the AR15 (and its big brother AR10) utilized only one type of system for operation; direct gas impingement. Not many other auto-loading rifles had used this system, but it was not new, although the AR15/M16 is the longest serving and most successful of the type. As the name suggests, the idea is to bleed propellant gas off from the barrel and have it flow down a tube to the bolt group, where it impinges directly upon the assembly, pushing it to the rear, setting in motion the process of cycling the gun. Eugene Stoner chose this type of gas operation system because it reduced not only overall weight, but mass which would be traveling under velocity within the gun, which resulted in reduced felt recoil.
However, such a system, to work cleanly and reliably, requires that the ammunition consumed within it it be engineered from the beginning to meet certain benchmarks. If the ammo used is of insufficient standards, mechanical mayhem can ensue. Not a very big deal when the only ammo available for the gun is that made by and provided for the Army, ensuring proper product will be fed to the weapon. But as years went by, and more and different nations and private manufacturers began making ammo in 5.56/.223, there was then a wide variety of quality control, and some was flat out poorly made.
And while testing of the direct impingement system (often simply called DI for shorthand) worked well in the laboratory, on the range and in field trials held in temperate North American climates and weather,...the crucible of combat in Vietnam illuminated shortcomings almost immediately.
As said previously, the ammunition (more accurately, the propellant) needed to be rather specific. Gunpowder is not a universal thing, and there is a wide variety. Within the large diversity of gunpowder propellants, each has a unique and specific "pressure curve", which is time it takes in micro-seconds for peak pressure to occur and abate, as well as how radically or sedately that peak occurs. As well, different methods of manufacture produce gunpowder granules of different physical appearance and properties. Some look like little short mechanical pencil leads, while others resemble grains of sand, while others still look like little tiny flat flakes. And due to these properties, the adverse affects they produce when burned can be starkly different as well. Some burn dirtier, some burn hotter, while others produce more flash. The powder selected for the new 5.56/.223 cartridge was of the "stick type", which in testing here in America showed to be cleaner burning and at lesser flame temperature. (Lessened burning temp was important to prolonged service life of the weapon, since barrels which get hotter quicker burn out sooner, needing to be replaced and increasing overall cost of the weapon system.)
But once in the humid jungle extremes of Vietnam, only then was it discovered that the propellant residue that had been no problem back in Alabama and Virginia, reacted completely differently. The light soot within the gas tube no longer simply got blown out the next time the gun was fired. If allowed to sit after being used, humidity would cause the sooty residue to congeal and fasten to the interior of the gas tube like an epoxy. Successive firing and sitting would see this build in layers, until soon, the gas tube became obstructed, and the gun would not cycle reliably.
Also, the stick type propellant, which had produced stable results in temperatures like those found in the U.S. or Europe, where it was intended to be used, now suddenly in the sweltering equatorial jungles was over-pressurizing when fired. This had the result in rifles that had not yet been fouled to the point of ceasing to cycle, to cycle so violently, that often the bolt would rip the rim right off the base of the cartridge, leaving the case stuck in the chamber. The only way to dislodge it was to unpack the cleaning rod and run it down the bore to pop the case out;...a process that made shooting such an afflicted weapon as slow as the muzzle-loaders of the Civil War. Reports began to come back of U.S. soldiers' bodies having been recovered from battlefields with their rifles beside them, cleaning rod in the bore,...showing the soldier had died while trying to get their malfunctioning weapon back in action.
The Pentagon soon recognized these issues and set about fixing them. The propellant was changed to the more temperature stable "spherical ball" type powder. This propellant burned hotter, reducing service life of the barrel, and produced thicker and sootier carbon fouling. But this fouling was of a chemical make up that did not turn into a baked epoxy. Yet it was still capable of eventually gumming up internal workings if not cleaned regularly. Soldier training was revamped to emphasize giving appropriate attention to rifle care and maintenance,...something which really should never have been neglected in the first place.
After these and other changes were implemented, the weapon became far more reliable. But stories of those dead soldiers with non-operational weapons by their sides continued to spread, and plagued confidence in the weapon for decades after. And despite the greater reliability, the DI system still had the nasty habit of depositing far too heavy an amount of exhaust gasses directly back into the bolt carrier group, coating the interior of the carrier's pathway within the upper receiver. Cleaning was a never-ending job.
Beginning in the 1980's, experimenters began trying to create retrofit adaptations of the more tolerant piston system principle to the AR15. Some produced results which were impressive. I admit here my own bias for piston-driven systems and AR15s. I have never had a DI AR15 that did not gum up and begin to fail to operate after only 200-300 rounds without a cleaning. As typified by the AK47, piston-driven gas actuation is far more tolerant of the ammo propellent it is fed. And gas which is bled off for the purpose gets vented far forward of the internal workings of the gun, keeping things relatively clean and in continued operation.
Piston-driven AR15s are slightly heavier, recoil slightly more and cost a little more too. They also are just slightly less accurate (for reasons to be explained later). But in my opinion, worth every bit of bother. They clean up easier too. The Marines Corp has adopted in limited numbers a variant called the M27 which uses piston operation, and these have been fielded in Afghanistan. A recently announced new purchase of more of these occurred in the last year.
In recent years, however, a certain subset of civilian competition shooters that prize the lighter recoiling impulse of the DI gas system over the piston type, but also want improvement in the occasional sensitivity to crappy ammo, have gravitated to "mid-length" DI gas tubes. These merely relocate the hole that bleeds off gas to cycle everything 2 inches further forward. The Special Operations Command (SOCOM) has taken note of this and has fielded some rifles in Afghanistan with these mid-length gas systems as part of an evaluation for possible modification/inclusion/upgrade in further weapon purchases.
(Twist rate of spin)
As mentioned earlier, BOTH the rifle AND its ammunition were co-revolutionary. And while there were times (as described above) where the ammo was redesigned around the gun, the reverse was also true, where the gun was redesigned around the ammo. The best example of this is "twist rate".
Twist rate is the measurement in distance that a bullet travels down a rifled bore to make one full spiral revolution. A 1-in-12 twist means the bullet makes one full turn within the bore by the time it travels 12 inches. 1/9 twist means a full turn in 9 inches, 1/7 means one turn in 7 inches, and so on.
In ballistic science, there is an optimum twist rate for every caliber and bullet combination to induce stable flight.. Lighter bullets tend to need a "slower" twist, such as 1/12, while heavier longer projectiles tend to need a "faster" twist, such as 1/7. Sometimes, a compromise twist rate is used in an attempt to be able to stabilize sufficiently any and all projectile types, such as 1/9 is considered to be.
From the beginning, the AR15 threw light and short bullets weighing 55 grains. The designation for this round was the M193. The twist rate settled on at the time was 1/12. This worked fine. The only people shooting anything heavier than 55 grains in the 5.56/.223 caliber were varmint hunters and competitive marksmen, all of them shooting customized rifles, to which they had affixed specialty barrels with the faster rates of twist. But in military circles, this was unneeded. So 1/12 remained the standard through the 60's and 70's.
But as NATO embraced the smaller rifle caliber concept and also adopted the 5.56/.223 round, concerns about it lacking the ability of previous rifle calibers to punch through light armor or vehicles drove development of armor-piercing projectiles. The Belgians presented to NATO the fruits of their research into this problem; the SS109 round, which was a longer 62 grain bullet with a steel core. The Pentagon liked this ammo too, and adopted is as the M855. Unfortunately, it was too long to be reliably stabilized by the slow 1/12 twist of rifles then in inventory. Even worse, the new tracer round that had to be developed to compliment the M855 round was even longer, and often just tumbled end over end through the air with no accuracy at all. Eventually, all new production AR15/M16 weapons were given the "A2" upgrade, which included (among other improvements) a 1/7 rate of twist designed for the new tracer and armor-piercing ammo.
And it worked fine.
However, there was still an awful lot of the the older ammo in reserve stockpiles. It was often observed that when the faster and lighter bullets were fired through guns with the faster twist barrels, the projectiles often shredded themselves from the stress.
I personally have witnessed this, where I observed gray puffs of cloud-like vapor magically appear just feet beyond the muzzle, which was actually the lead radially spinning off in all directions as the bullet came apart from the centrifugal force.
The military solved this problem by selling off or giving away to allies all of the older style ammo to those who still had the older rate of twist. Eventually, the only ammo in U.S. military stockpiles was the M855.
But civilian shooters were in a quandary. They wanted the latest version of the AR15, but they also wanted to be able to shoot the least expensive ammo they could get their hands on, which often was surplus M193. As the rest of the world that used the 5.56/.223 caliber also converted over to the new heavier bullets and faster twist rates, cheap surplus M193 type ammo was the primary supply for civilian consumption.
Finally, some companies responded to this by coming out with barrels with a compromise twist rate of 1/9. Fast enough to stabilize all but the long tracer round (which few civilians ever had ability or reason to get their hands on anyway), yet not so radical a twist rate that it would shred the 55 grain M193 projectiles.
The point of all this (or the twist, if you like) is that you need to know before you purchase your rifle or barreled upper receiver assembly what ammunition type(s) you are likely to feed it. Thusly, you can intelligently select which rate of twist you will require.
While the older style 55 grain ammo is not as prevalent as in decades past, it remains popular with some and in active production for civilian sales. Some shooters like the even lighter 52 and 50 grain hollow-point ammo due to how it penetrates very little before explosively shredding within the target. It also, when fired indoors, penetrates fewer layers of drywall or pine studs, reducing liability issues faced by errant rounds that might find an unintended victim. Fans of this type of ammo will NEVER want a "fast" 1/7 twist barrel.
At the other end of the spectrum, even heavier and longer .224 diameter bullets have seen greater and greater use lately, such as the 69 grain and 77 grain sniper-grade projectiles. These absolutely require a 1/7 twist barrel, although some have reported good results with some custom and hard to find 1/8 twist tubes.
My recommendation is the middle ground. I have long been a fan of the 1/9 twist rate barrels, and I see no reason to change that point of view. They will easily handle 90% of the ammo you are likely to ever encounter.
(Stable vs. Unstable - The Controversy Over The Puny Poodle-Puncher Projectiles And Terminal Performance)
But this esoteric science over twist rates and the affects on the projectiles is not just applicable to accuracy and putting projectiles on target. As it turns out, the bullet you choose (both weight, design and even who made it) also has a great deal to do with whether or not it has effective wounding capability on your foe. The .223/5.56 caliber has been the stuff of legendary failures or legendary lethality, depending on whom you are talking to. In the days before the internet allowed us to research things so easily and before calibrated gelatin was found to accurately simulate flesh for testing purposes,...unverifiable legends passed from person to person were all the info anyone really had on which to judge whether the caliber was deadly or not. Short of having actually used it to shoot other people, you were reduced to hearing stories from Vietnam veterans in bars and at surplus stores about how it never worked worth a damn or how it smote down VC like the fist of God. No one ever seemed to have a middle of the road story. The round was either completely ineffective, or like a laser death ray.
As usual, the truth was rather nuanced.
Recall that earlier I said that in development, it was discovered how HIGH-VELOCITY yet small calibers were found to be surprisingly effective. In the regular-sized rifle, which has a 20 inch barrel,...the caliber generates enough velocity to produce these results out to 200 yards. But many variants of the basic AR15/M16 rifle were tested and issued in Vietnam, with some having barrels as short as HALF that length. Less barrel means less time to burn propellant before the bullet leaves the bore, which means less velocity. And in the .223/5.56 round, that means less lethality on target. THIS IS A VELOCITY-DEPENDENT CALIBER.
As seen in this photograph, these M193 projectiles were each fired into tissue simulant at different velocities. At velocities above 2500 feet per second, the M193 projectile breaks apart as it yaws (turns sideways) on its path. This is what produces a tremendous "splash" effect in human tissue (which is mostly water) and thereby performs the bulk of its wounding. This hydraulic force that shoves tissue violently aside, also causes these same tissues to try to fly further away from the wound path, stretching and tearing as they do.
However, if the M193 bullet encounters human flesh at velocities LESS than this, it tends to simply bore little .22 caliber holes,...producing a wound no more devastating than being shot with the common and lowly little .22LR rimfire that most people are familiar with. Yes, depending on where one is shot, that can still be quite lethal. But not nearly as dramatically and immediately incapacitating.
Some soldiers reported that they shot enemy combatants multiple times at close range with very little effect. That would be entirely consistent with the very reduced velocity from one of the shorter-barrelled weapons.
Because the wounding capacity of the .223/5.56 caliber involved the bullet tumbling through the body after hitting it, there was commonly a misunderstanding for a long time that this tumbling occurred due to the bullet somehow being inherently unstable in flight, and that if made to fly too stably, insufficient wounding would result. Thus, when the "A2" product improvement of the U.S. M-16 rifle occurred in the early 80's, with the new 1/7 twist barrel replacing the old 1/12,...rumors began to fly immediately that this would render very unimpressive results in flesh. And again, as some limited use of the new heavier bullet ammo and faster twist barrels began to trickle in from the few spots around the world where it saw some use,...the results were once again mixed. Sometimes the enemy was put down promptly,...and other times they seemed to shrug it off and keep on trucking.
Eventually, through extensive testing, it was discovered that much of the variance in effects on downrange persons which could NOT be explained due to lack of velocity and bullet energy upon contact, were in fact due to the wide variance of manufacturing specifications in crafting the projectiles, depending on where and when they were produced. Some NATO nations produced ammunition where the thickness of the copper jacketing was too much and prevented the projectiles from breaking apart and producing the explosive wound channel typical to the American product. Rather crucially, the American projectile has a built-in crimping groove (known as a cannelure) for the cartridge case to better keep hold of the bullet before firing. During fabrication, the making of the cannelure actually slightly weakens the bullet jacket at this location, producing a uniform place for the sideways forces under the stress of plowing through tissue to cause the bullet to break apart and become more lethal. This was not always present in ammo produced by other NATO nations. And since it is far more common than many people realize for ammunition from other NATO member states to end up being expended by American troops, this had an adverse affect on the terminal results observed on the opponents U.S. soldiers were occasionally shooting through the 80's and 90's. These variances were eventually remedied, and today nearly all NATO 5.56 ammo is within spec of the American production standards.
And if you are a soldier, that's fine. However, as a civilian shooter, having to source your ammo from a multifaceted marketplace, there is a lot of ammo out there meant "for training purposes only" that just does not measure up. It is not useless. But it is good to understand how to spot cheap bargain ammo that may not actually deliver the best results on people if you are ever unfortunate enough to have to do so. We will cover that later under "ammo selection".
It may seem we have been straying off target (I love puns) by talking so much about the .223/5.56 cartridge, but hopefully you by now see that the variety of barrel designs can be directly bound with the science of the ammunition's performance. And as part of that, here is another change that occurred when the M4 was adopted,...enhanced feed ramps. Sometimes an old slightly worn magazine will not fully lift cartridges into line with the chamber and these rounds might angle somewhat lower, causing them to "nosedive" into the non-ramped area just below the barrel face. The new feed ramps are a 2-part adaptation of both the M4 spec upper receiver and the newer barrels. Occasionally, assemblers of rifles from DIY kits who purchase the barrels and upper receivers from differing sources mistakenly get components not optimally matched for the feed cuts. Below is a picture showing how this can appear.