9x19 PDW

[TurboLag]

Since you live in a free state, you can always register 2 sbr's. I'd do a mag block in my opinion but I also only have 2 lowers for all my uppers to keep things cost effective/shooting consistent overall. One pistol (in your case sbr) and one rifle lower.

Personally I'm going to go 6.5" because it's longer than my actual handguns but shorter than my 8" .300 and 8" 5.56

[Ron3]

Consider the Beretta Cx4 Storm rifle.

These rifles are handy, light and reliable. (From Tula 115gr to 115 gr Gold Dot +p+ to 147 gr JHP's they eat it) You can get one for about $650 right now.
Use Beretta M92 or PX-4 mags depending which factory adapter you use. Beretta makes great 30 round mags for the M92 style.

Most controls are reversible, the trigger is not bad (Mine feels like a 6.5 Glock trigger) and it's quite adaptable in it's own way. (Various rails and stock lengths)

Take down is easy and so is cleaning. It is 100 yds or less rifle and really shines 50 yds in.

It loses points because it's not the shiny-est new thing nor something you get to enjoy building from the ground up. But at least you get to start with a reliable, handy weapon for not much money. (and you may already have M92 mags)

[Turnkey11]

MP5 is the way to go for a 9mm carbine, be prepared to pay to play (even clones are expensive).