It is a shotgun caliber that is exempt from DD in the law-- Title 18 GCA, Section 921 -- It doesn't matter what crazy name you give it, like magnum, super express. 12Ga From Hell, super duper magnum, shotgun from purgatory, etc, as long as it shoots straight sided 12ga shotgun cases.
It doesn't matter what cases are made of or their length, as long as it fires shorter ones if you have a longer chamber. Like 3" in 3.5" chambers. like 2.75" in 3" chambers, like 3" in 3.85" chambers, etc. 12ga chamber lengths range from 1.5" to about 4 inches.
It doesn't matter the barrel length, smooth, rifled, polygonal barrel, paradox bore.
Now you can't take a 40mm cannon case and neck it to 12ga and have an exempt 12ga, as chamber wouldn't fire other straight sided 12ga cases.But such a case can get a sporting use exemption by asking ATF for one like many big bore rifle cases that have done that.And that is part of the same law.Ed
It doesn't matter what cases are made of or their length, as long as it fires shorter ones if you have a longer chamber. Like 3" in 3.5" chambers. like 2.75" in 3" chambers, like 3" in 3.85" chambers, etc. 12ga chamber lengths range from 1.5" to about 4 inches.
It doesn't matter the barrel length, smooth, rifled, polygonal barrel, paradox bore.
Now you can't take a 40mm cannon case and neck it to 12ga and have an exempt 12ga, as chamber wouldn't fire other straight sided 12ga cases.But such a case can get a sporting use exemption by asking ATF for one like many big bore rifle cases that have done that.And that is part of the same law.Ed