Governor Greg Gianforte refers to House Bill 258, which was passed and signed into law in 2021. From HB 258, now MCA 45-8-368:

Prohibition Of Enforcement

45-8-368. Prohibition of enforcement. (1) A peace officer, state employee, or employee of a political subdivision is prohibited from enforcing, assisting in the enforcement of, or otherwise cooperating in the enforcement of a federal ban on firearms, magazines, or ammunition and is also prohibited from participating in any federal enforcement action implementing a federal ban on firearms, magazines, or ammunition.

(2) An employee of the state or a political subdivision may not expend public funds or allocate public resources for the enforcement of a federal ban on firearms, magazines, or ammunition.

(3) Nothing in this section may be construed to prohibit or otherwise limit a peace officer, state employee, or employee of a political subdivision from cooperating, communicating, or collaborating with a federal agency if the primary purpose is not:

(a) law enforcement activity related to a federal ban; or

(b) the investigation of a violation of a federal ban.

Governor Gianforte informs Attorney General Merrick Garland of the definition of a ban under the law