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*always check your local laws and statutes before using or reloading 37mm rounds
From James O. Bardwell's (bardwell@netcom.com) FAQ On National Firearms Act Weapons

"Smooth bore 37mm projectile launcher - not a DD. Not even a title 1 firearm. This item falls under the "not a weapon" (signaling device) exception, I believe. Generally a large bore device for which no anti-personnel ammo has ever been made will NOT be a DD. This used to be true of the 37mm guns. However, according to ATF, some folks have started making anti-personnel rounds for these guns, and ATF has ruled that possession of a 37mm launcher and a bean bag or rubber shot or similar round is possession of a DD, and at that point the launcher needs to be registered. Put another way, before you make or buy anti-personnel rounds for your 37mm launcher, register it as a DD. The rounds themselves, not being explosive, incendiary or poison gas, are not regulated in themselves either. It is just the two together. See ATF Ruling 95-3."

Section 5845(f), Title 26, U.S.C., classifies certain weapons as "destructive devices" which are subject to the registration and tax provisions of the NFA. Section 5845(f)(2) provides as follows:

(f) Destructive device.--The term "destructive device" means * * *

(2) any type of weapon by whatever name known which will, or which may be readily converted to, expel a projectile by the action of an explosive or other propellant, the barrel or barrels of which have a bore of more than one-half inch in diameter, except a shotgun or shotgun shell which the Secretary or his delegate finds is generally recognized as particularly suitable for sporting purposes; ..."

A "sporting purposes" test which is almost identical to that in section 5845(f)(2) appears in 18 U.S.C. § 925(d)(3). This provision of the Gun Control Act of 1968 (GCA) provides that the Secretary shall authorize a firearm to be imported into the United States if the firearm is "generally recognized as particularly suitable for or readily adaptable to sporting purposes." With the exception of the readily adaptable' language, this provision is identical to the sporting shotgun exception to the destructive devices definition. The definition of "destructive device" in the GCA (18 U.S.C. § 921(a)(4)) is identical to that in the NFA.