There's basically two main classes of destructive devices: explosives and firearms with a bore over half an inch. The permits for the former are mostly issued t

There's basically two main classes of destructive devices: explosives and firearms with a bore over half an inch. The permits for the former are mostly issued to demolition and mining and it would be insane to allow anyone to "collect" explosives that degrade and become unstable. You don't actually need either the FFL or the permit if you just decommission the main gun, which is how most tank and artillery collectors get theirs. DD permits are rarely issued for explosives without a good reason (like construction or mining) so even if you get the permit for the gun, you'll struggle to get one for the shells.