Hello. I am new to this forum (and forums in general 
and i thought I’d get some input on a project I’m starting. I ordered a pa10 complete pistol lower, and a blemished pa10 complete upper. I plan on using a 12.5" cmv 4150 barrel with qpq finish and carbine length gas system from aero precision, a cryptic coatings mystic black 308 bolt carrier group with a sun devil adjustable drop in gas key, midwest industries k series low profile keymod handguard, geissele ssa-e trigger, magpul asap qd end plate, magpul ms4 sling system, and a sig sbx brace. This isn’t my first build, or my first 308, but it is my first 308 ar build. I got a ptr 91 sccr 308 that i’ve made into a DMR of sorts, also a 4.8 lb 5.56 build on a mag tactical lower. I’ll try to post some pics tonight, I love them both but they each have their flaws. The ptr is probably the most reliable 1 moa rifle i’ve ever laid hands on, but it’s pretty heavy. The 5.56 has a lot of parts that i feel are untested and lacks the power i desire for protecting myself and my family. This will be my SHTF carbine (whoops meant to say pistol ;), and I will probably be feeding all sorts of ammo through it. Any input from you guys who are probably more experienced than me would be appreciated.
Sounds like you have good components selected and have done your homework. I see nothing wrong with your plan, other than the fact that it is such a short barrel length and is a pistol. Both the muzzle blast and comfort of shooting a 308 with the Sig Brace would be undesirable from my own personal viewpoint and preference. But I know that there are fans of these guns. So you are hardly alone.
I believe in having longer range capability myself, so I have a carbine 308 with a 2-7x scope with illuminated reticle. It is really more capability than one needs in our society for personal protection under normal circumstances. But I figured that if I was going to put together a serious SHTF rifle, I would go all out and have something effective out to 400 yards. Although one would never conceivably need that capability, unless there is a complete break down of law and order and society. And if that is the case, one will probably be screwed anyway.
Back to components, if you want another option for the handguard to consider, look at the Diamond Head USA VRS T-308L handguard. That is what I have on my carbine, and I just absolutely love the ergonomics. I find it most comfortable, and really like the more flat bottom design.
http://www.diamondhead-usa.com/products/308-vrs-t-free-floating-handguards
Thank you for your input. That is a neat handguard, i already ordered the midwest one though, it’s lightweight and it has 5 qd swivels built in. I really like the slim keymod rails, it’s easier for me to control. I think you would be surprised how accurate these 12.5 inch 308 pistols are. bigshooterist has a video on youtube where he shoots a 4 inch group at 300 yards with 145gr prvi ball (it think…). I think it would be effective and accurate at 400 yards, especially with some m118lr!!! When i try it for myself i’ll post results. With an a2 flash hider i think it should be decent recoil wise. You should try an ar pistol with the sig brace man, (or a padded buffer tube, i would hate to think you would shoulder an arm brace 
) I have one in 50 beowulf already and it’s probably the funnest “pistol” i’ve ever fired. I might SBR them both if i ever decide to sell my soul to the government… I doubt i will though.