Ok I've been wanting a SBR for a long time but that looks like a lot of hassle and extra money just to put a stock on it.  Let me get this straight on a AR pistol all you have to do is register the new lower as a pistol and don't put a stock on it and your good to go?  If that is the case I have some questions.

1) what is the shortest reliable buffer tube setup that you can get
2) I was looking at the 7.5 barrels from cmmg and mstn and was wondering how well they function?
3) what ff railed handguards work best with these setups?
4) is there a bullet weight that should not be exceeded?

Thanks

1.  Typically the carbine buffer tube length (approx 7") is the shortest reliable buffer tube/sping/etc. that works with the standard bolt carrier assembly.  

1a. If you want to spend a little more money you can go with a 4" tube from Bushmaster, but also need a special spring/buffer/bolt carrier, which will run around $100-$120 in parts.  see the tacked thread about "Bushmaster Buffer" fora  better explanantion.

1b.  Model 1 Sales used to (and  Now Gunsmoke Enterpisesdoes ) sell a 3 1/2" long buffer tube that used something akin to a 1911 spring and smoe odd parts whcih allowed the use of a stnadard bolt carrier, but reliablity was hit or miss at best- some guns ran fine, others wouldn't function at all.

2.  They are DPMS barrels, and the DPMS "Kitty Kat" 7.5" uppers are very highly regarded, but not cheap.  The Model 1 Sales uppers are about $200 cheaper, but very occasionally may have a 'hiccup' or two in getting the to run.

3.  Several companies make free float handguards for the 7.5" upper.  Yankee Hill Machine has several on their website and seem to have the best prices- your basic  knurled tube with no rails runs around $35, and railed tubes run in the $80-90 range.

3a.  One other option if is to go with a carbine lnegth rail, which covers the gas block (need to shave down standard front sight base or use a "low prifle" type gas blcok).  All you will see poling out of the end of the free float tube is the flahs hider, which can be pretty neat.  With a railed carbine length tube you can always use a Yankee Hill Machine front sight that is designed to mount on the free float tube, if you care for such things as useless as iron sights on an AR pistol.

4.  You should be fine with any bullet weight appropirate for the barrel twist you are using.  Pretty much all 7.5" barrels are 1/9 twist, so they may or may not work with the heavy 75 to 77 gr ammo.  DPMS/CMMG might have 1/7 twist barrels.  (Heck, my build has a 1/12 twist as the barrel is a cut-down M16A1 barrel, go figure!)

Hope that helps...

When I did my build I used a Model 1 7.5in barrel and the current production Model 1 pistol buffer assy and model 1 pistol free float tube. The upper and bolt/carrier I got from Bravo and the virgin lower I got local, the only problem I have had was a usgi Colt stamped mag and a Magpull follower that was sticking in the front at the top of the mag and would cause the last few rounds to nose dive. It worked great in my carbine with the extended M4 ramps, but the pistol has standard ramps and it would nose dive into the bottom of the feed ramp. But other than that, the thing has run 100%.