New to ARs entirely. I have a few other firearms, my favorite being my M1A Scout Squad in a JTAC Rogue bullpup chassis. A few weeks back I decided I wanted an AR pistol, but not in 5.56/.223. Looked at several pre-made pistols (RRA and Sig being two) w/o buffer tube extension. Then I discovered what a 'pistol brace' was and then changed my mind to making my own. She's almost complete, now:

This isn't even my final form.
http://oi59.tinypic.com/6jhvu0.jpg

http://oi59.tinypic.com/6jhvu0.jpg

Parts List

SigTac Pistol Brace
Generic buffer tube from Amazon. Had to file the end down, opposite buffer retention pin, to get the upper to settle in correctly. There was a 3mm, or so, lip above the threads. Helped to retain the buffer retainer pin, but protruded too much at the top and prevented the upper from closing.A few minutes with a file and it was near perfect.
Custom '300' laser etched endplate. Thought this was nice touch :)
DPMS slick upper (no forward assist, no dust cover, no brass deflector)
DPMS lower
CMMG custom lower kit (no trigger, no grip)
Del-Ton 2-stage trigger (my M1A has a 2-stage, and I like it, so wanted to replicate the 'feel')
Hogue molded grip; again, same as trigger above, in that I've been using one on my bullpup and like it
Troy Alpha rail. A bit pricier than other options, but I really liked its appearance
8.5" Rainier Arms .300 BLK Select Barrel, SS Matte. Was planning on Noveske, but none were available, that I could find. Though after research, Rainier had good reviews, so I might use them again.
Wilson Combat lo-pro gas block, DPMS Pistol gas tube

Though not arrived yet, last part is AIM NiB BCG.

Still need to tweak and tighten things more appropriately, but I think she looks pretty good, so far.

Oh yeah, along with the buffer tube, I also had to modify the Troy rail. Unfortunately, the Troy Alpha Rail does not play well with the DPMS upper receiver. The receiver design has a small extra bit at base of rail where it meets the 'tube' of the receiver (visible as a light line at base of receiver rail in pic above.) I presume this is a design by DPMS that may make the upper slightly stiffer. Not really sure, but that's as close a reason as I could figure out. The rail has a small 'pin' of sorts that is supposed to mate up against the base of the upper rail. The design of the receiver prevents this from occurring. My only option was to file the pin on the rail. I tried to remove only enough to get it to seat, but unfortunately the whole pin had to go. Presumably the rail might rotate if the barrel nut loosens, or the rail clamps loosen, but I don't this is that big a deal. I can tighten them back up if needed.