Mont. Code Ann. § 30-20-104

A personal firearm, a firearm accessory, or ammunition that is manufactured commercially or privately in Montana and that remains within the borders of Montana is not subject to federal law or federal regulation, including registration, under the authority of congress to regulate interstate commerce. It is declared by the legislature that those items have not traveled in interstate commerce. This section applies to a firearm, a firearm accessory, or ammunition that is manufactured in Montana from basic materials and that can be manufactured without the inclusion of any significant parts imported from another state. Generic and insignificant parts that have other manufacturing or consumer product applications are not firearms, firearms accessories, or ammunition, and their importation into Montana and incorporation into a firearm, a firearm accessory, or ammunition manufactured in Montana does not subject the firearm, firearm accessory, or ammunition to federal regulation. It is declared by the legislature that basic materials, such as unmachined steel and unshaped wood, are not firearms, firearms accessories, or ammunition and are not subject to congressional authority to regulate firearms, firearms accessories, and ammunition under interstate commerce as if they were actually firearms, firearms accessories, or ammunition. The authority of congress to regulate interstate commerce in basic materials does not include authority to regulate firearms, firearms accessories, and ammunition made in Montana from those materials. Firearms accessories that are imported into Montana from another state and that are subject to federal regulation as being in interstate commerce do not subject a firearm to federal regulation under interstate commerce because they are attached to or used in conjunction with a firearm in Montana.