A P90 fires a proprietary juiced up pistol ammo if my memory serves. I was looking to buy a P90 (not SBR) and did a bit of research I remember it using very expensive ammo and not re loadable. Technically since it fires pistol ammo it wouldn't be a rifle
The ATF doesn't care though. If I am going through the hastle of a NFA purchase I think there are better options. A P90 is cool looking though, I have a air-soft one.
nah, a P90 fires proprietary
rifle ammo that just so happens to be small enough to fire out of their large-framed pistol. You can reload 5.7 ammo fired out of a p90 or a FiveseveN, just not out of the AR-57 upper, it blows the neck out of it.
As for cost, as I recall, 5.7x28 price is actually within reaching distance, and not even as expensive as some of the boutique rounds we have out there today. Some sites online (if you don't mind ordering ammo of the internet) have it priced similarly to .40/.45, with 9mm being only slightly cheaper.
As it stands though, and I agree a p90 would be cool, I wouldn't advise building one, since BATFE strips away the two coolest parts about it: Full auto with a tremendous yet controllable cyclic rate and ss190 rounds. Without the full auto, it loses a lot of the cool factor, and without the military-restricted rounds, you are shooting something slightly stronger than .22magnum. And that is an awful lot of money to spend on something that won't give the bang-for-your-buck I'd imagine you're looking for.
MP5 could be kind of cool, but for the price of building one, you could do a Bushmaster Carbon in 9mm, SBR, and probably still afford a suppressor and tax stamp for that. And that would be all kinds of awesome. Not that the MP5 isn't, it's a solid choice.
I have to agree the krinkov/74u would be a pretty cool project, you don't see that sort of thing floating around a whole lot. Personally, I'm a big fan of SBR SCAR-L, but that isn't so much a "project" as it is "I'm gonna order a barrel and a tax stamp". Maybe that's
less work than you had in mind.
Whatever it is, most important thing is for you to have fun with it.