This is NOT legal advice. If you mess around with National Firearms Act weapons and do something that you should not do, you will very likely go to prison. Seek competent legal advice.
Destructive Devices is an interesting category. Did you know you can legally own a hand grenade or a live artillery shell? You just need to go through the red tape and pay the $200 transfer tax. Of course, you only get to use something like that once. Then it is literally $200 up in smoke. Maybe if I win that MegaMillions jackpot...
Destructive Devices also covers things like rifles over .50 caliber that are not deemed for "sporting use" by the BATF. For instance, the .577 Tyrannosaurus (look it up on YouTube - it's a hoot watching folks shoot it) is deemed sporting purpose. The Swiss Lahti 20mm rifle from WWII, which was legally imported and sold in this country over the counter until 1968, is not and is considered a DD. As I understand it, the 40mm grenade launcher made famous as the "bloop tube" in Vietnam or as the M203 when attached under a M16 has a rifled barrel and is a DD (not to mention each exploding grenade is one as well). However, the exact same thing, except 37mm with a smooth barrel is not a destructive device, and you can shoot solid "training" rounds or projectiles filled with smoke or marking agents that don't explode and there is no DD restrictions.
Back in the late 80s, there was a 12 gauge shotgun called the Street Sweeper or the Striker imported from South Africa. It had a revolving 12 round cylinder and a folding stock. It was hugely cumbersome and not very practical... especially when you can add a magazine extension tube to a Remington 870 and have nearly the same firepower capabilities. But, it got a bad reputation from the cocaine wars in Miami and so BATF declared it to be a DD. Right here in Virginia, it is actually the only individual weapon that is completely illegal to own.
There have been rumors lately that BATF is contemplating adding other types of "riot" shotguns to the DD list, including anything with a pistol grip. I think those rumors have pretty much faded away as of right now.
NFA for SHTF - Useful or Not?
So, should a prepper drop the coin for an NFA weapon?
For a machine gun, I really can't see it. They are SO ridiculously expensive thanks to Hughes, and they eat a ton of ammo. If the Hughes amendment is ever done away with and you can get a select fire M4 for just a little more than a semiauto one (plus $200), then there might be an argument in favor of it.
I think suppressors have an awful lot of practical use. They can be used for pest control, shooting in the backyard (where legal) without bothering the neighbors, and protecting your hearing. I understand that in some places, they are legal for hunting, even. A good suppressor can be had for about $350 for a .22 or $750 - $1,250 for a centerfire pistol or rifle (plus $200). I'd say that if you can do it where you live (although legal under federal law, not all states allow them) that might be a good thing to have if you have some spare money and no debt.
A SBR/SBS can also be very useful, especially if you get it as an AOW and it only has a $5 transfer tax. Moving up to a $200 tax SBR, there is a lot of argument to be made for a Ruger Charger with a shoulderstock and bipod (threaded for a suppressor even?) or a shoulderstock on a Glock... again if you have the extra money.
Another AOW that might be of use is a vertical foreend grip for something like an AR15 pistol or a "Krinkov" AK47 pistol. It can make them very controllable. Only $5 (plus all the red tape) or pay $200 to make it a SBR and stick a folding stock on it.
I don't see much practical use for going to the trouble and expense of a Destructive Device, but they sure could be a lot of fun if you have the money and it doesn't hit the fan.
In closing the National Firearms Act of 1934 was a backdoor attempt to tax certain guns out of existence. The owners of NFA weapons are few in relation to owners of standard weapons, and they are overall a wealthier group with more disposable income and more reason to be vocal about attempts to further restrict NFA weapons among "civilians." They can have their place in a prepper's arsenal, but I really think they should be considered only after other preps are complete and the prepper has no debt. Of course, if SHTF and the UN blue-helmets are kicking in doors and hauling patriots off to the reeducation camps, then I'd imagine the BATF would not exist and anything you might pick up as a "war trophy" would be fair game...
For more information on NFA weapons, visit the BATF website, consult an attorney experienced in the subject, or read one of these books: