Your 12" AR-10 has the following specs, then: 2415 feet per second on the round exiting the barrel.
I used this [
http://shootersnotes.com/Velinfo.php ] calculator:
Reference Load: - Bullet Dia.: 0.308
- Case Volume (gr. water): 52.0
- Bullet Weight (gr): 150.0
- Barrel Length (in.): 20.8
- Case Length (in.): 2.008
- Muzzle Velocity (FPS): 2690
- Charge Weight (gr): 41.8
I then put the "new" information as:
- New Bullet Weight (gr.): 150
- New Barrel Length (in.): 12.0
- New Charge Weight (gr.): 41.8
So, yeah: 7.62mm-diameter projectile traveling at 2,415 FPS out of a carbine that is, what I assume to be, ~30.5" overall length (took an M4A1's overall length and cut down the appropriate 2.5" off the barrel... with the extended magazine well, however, perhaps this AR-10 carbine would be ~31.5" overall length or so, stock fully retracted).
Here's a look at the Romanian Pm. Md. 90: Firing from a ~11.5" barrel, the carbine gets about 2,075 FPS out of the 7.62x39mm cartridge (video I found was using Silver Bear HPs). This is a difference of roughly 340 FPS. The Pm. Md. 90 also utilizes standard AK magazines, so you can go as small as you want (five rounds) or as large as you want (75 rounds) without sacrificing much more than weapon height and weight. Overall length, stock extended, is ~30.0" or so. Stock folded would be ~22.0" overall.