1911 guy said:
In the navy we didn't get issued SAW's (M249), so we get M-16's that are FA, not 3 round burst. Having spent some time behind those, unless you're using a pistol caliber in a heavy weapon, it's not practical except for suppressive fire in the general direction of the enemy. And if you have a heavy weapon, why is it chambered for a pistol caliber? I prefer handy sidearms and accurate rifles, not mediocre rifles chambered in a round I can have on tap in a much smaller package. I won't say it's not fun for a minute, but the novelty wears off and your wallet gets thin.
I don't think I'd call my SWD M11/9 SMG a "heavy" weapon. (On a Form 4, $200 tax stamp and all, nice and legal -- any other way is begging for major trouble!!!) There are pistols bigger than many subguns.
Subguns are tactically similar to shotguns. They are good when you want firepower without the penetration of rifle cartridges. VIP protection. House clearing. Close quarters. Supposedly useless and outdated (and detested by those who think an '03-A3 and .45 are advanced enough, thank you very much) they refuse to die. Cops, feds, special ops, security folks, they all swear by them. Note: I would not use a SMG for self defense in today's legal climate, unless professionally employed to do so!
Think of the pilot with the MP5 in "Blackhawk Down". Would he have rather had an old 7 shot .45? For some reason I don't think so. I'm sorry but I just can't believe that every cop, fed, secret service, and spec op with a subgun is stupid and missing the point.
Machine guns? (belt feds) Suppresive fire, yes, but also good for taking out vehicles. Plus, for some reason, I don't think the German MG42 gunners in Normandy where thinking "oh man, ja, I could do this with my 98K".
Automatic rifles? Good for making a lot of noise by the ill-trained. Seems like the good guys are using them in semi-auto under combat conditions.
And like everyone here has said -- if you want to shoot full-auto -- go the legal route. Spend the big bucks, or go to a range and
rent one. Plenty of places you can do that. There's one range that has a rental Uzi that has fired over a million rounds.
If you are interested in the academic "how do they work" go to the frigging library. Or go to a surplus store and for $5 buy a military M16 manual, they all show how the little internals do their things.
DO NOT touch, think about, hint, scheme, whatever illegal full-auto. It ain't worth it.