I'll second the Uzi. Caliber conversion is easy (.45, .22, .40, even .41AE if you like roll your own oddball ammo). Rock solid, plenty of parts, mags, accessories, suppressors, web gear, history. A bunch of German surplus 32 round IMI made mags just came in, $15 each -- IMI Uzi mags are second only to Brit L2A3 mags in the reliability department, as far as I'm concerned.
For all you tacticool types, you can get all sorts of nifty stick on stuff, bayonets, scope mounts, rails, different colored furniture.
Oh, and don't forget a registered bolt if you want to spend the big bucks, there are still a few out there.
I've got a Vector full size Uzi and a Norinco 320 (the Chinese copy). My wife has a Vector mini Uzi carbine. They work fine with everything from 147gr sub sonic to WWB to +P+ subgun ammo. Some Vector guns have quality issues -- but they will stand behind them and make them work.
It is easier to clear rooms with a smaller platform than a larger platform, so pistols are easier in this regard than carbines.
SBR. Problem solved. Takes longer to fill out the paperwork than change the barrel on an Uzi.
For me there's the added joy of owning firearms with "Israel" on the side of them (but not all Vectors use IMI receivers, some are Group Industries). The Hebrew marked safety is just a wonderful bonus.
Heavier than the newer civilian type options, but you just can't beat the wide range of accessories. You can do far more with an Uzi than just about any other pistol caliber carbine I can think of. And they go bang every time you pull the trigger. Only downside is that with the IMI, Group Industry, Norinco, Vector, full size, mini, and pistol variants out there, they get addictive!
(I'm sure the 9mm AR variants are great too, but I've never had the chance to handle one.)
