JJ asked about the brace.
Several ranges are now preventing shooters from using an AR pistol with the SIG like brace, unless the owner has a copy of his/her tax stamp. I know, ARAK pistol is not an SBR, but this is where some of the ranges/ clubs want to avoid run in with ATF, or what ever agency shows up. So much for these ranges/ clubs being "pro2A", much to the overall detriment of the Second Amendment.
Rainier Arms has several pistol type buffer assemblies, one or two with rubber or padding along the tube. They have a different profile than a rifle buffer tube, so an AR type rifle stock can not fit. If there's enough padding to have a friction fit against the forearm, it can help stabilize the pistol.
http://www.rainierarms.com/phase-5-t...ol-buffer-tube
I'm sure there are other choices, but this is an idea. Some versions also fit the SIG braces.