The bore measurement requirement excludes any guns of .50 caliber or under (or around 12.7 mm in metric) from the “destructive device” category for legal purposes, which covers most modern small arms. Aside from a handful of experimental or exotic weapons, the only real exceptions are a few Eastern Bloc heavy machine guns and anti-materiel rifles, which you’d have a hard time getting ahold of in the States anyway.
It’s common for black powder weapons to have larger bores -- .5 to .8 inches were typical calibers for colonial-era muskets—but they’re excluded from the “destructive device” category by a separate provision.
The bore measurement requirement excludes any guns of .50 caliber or under (or around 12.7 mm in metric) from the “destructive device” category for legal purposes, which covers most modern small arms. Aside from a handful of experimental or exotic weapons, the only real exceptions are a few Eastern Bloc heavy machine guns and anti-materiel rifles, which you’d have a hard time getting ahold of in the States anyway.
It’s common for black powder weapons to have larger bores -- .5 to .8 inches were typical calibers for colonial-era muskets—but they’re excluded from the “destructive device” category by a separate provision.