Yes, although some pistol uppers use odd bolt carriers. Not all though.

Bushmaster has a shortened bolt carrier. I've never had one, I've only seen pics. Its so the receiver extension can be made shorter.

According to what I read on the AR Pistol forum it's completely legal to build up a 11.5" AR pistol using a carbine receiver extension as long as you do not put the the stock on.

What I've seen people do is wrap the receiver extension with 550 cord so that it would add addtional security that a stock could not be slipped on by accident.

If you setup your AR pistol this way all the parts except the stock should be identical to a SBR.

Also, it looks like if you use the carbine receiver estension that it's not too difficult to shoot it off your shoulder with just the bear receiver extension.  

I've considered doing this but I'd really like to have a stock so I may have to pony up and go the SBR route for what I want...

Quoted: According to what I read on the AR Pistol forum it's completely legal to build up a 11.5" AR pistol using a carbine receiver extension as long as you do not put the the stock on. What I've seen people do is wrap the receiver extension with 550 cord so that it would add addtional security that a stock could not be slipped on by accident. If you setup your AR pistol this way all the parts except the stock should be identical to a SBR. Also, it looks like if you use the carbine receiver estension that it's not too difficult to shoot it off your shoulder with just the bear receiver extension.   I've considered doing this but I'd really like to have a stock so I may have to pony up and go the SBR route for what I want...

Your lower receiver must never have been assembled as a rifle, either.  From what I read the sfaest (legal) way to do it is either buy a complete AR pistol, or buy a lower "certified" as being "virgin" by the mfgr.  I've also heard folks say that on the 4473 make sure it gets listed as a pistol, not a rilfe.

Quoted: Your lower receiver must never have been assembled as a rifle, either.  From what I read the sfaest (legal) way to do it is either buy a complete AR pistol, or buy a lower "certified" as being "virgin" by the mfgr.  I've also heard folks say that on the 4473 make sure it gets listed as a pistol, not a rilfe.

Yeah, good point I forgot to mention that.