I’m not so keen on a straight up AR-15. I have a couple, but their use is limited. Unless you get an ARAK-21 or similar gas driven upper, you can’t make it into a short gun. And, at close range, I see a toss up between a big stopper like a 1911 or a totally hidable cc like a Ruger LCR.
And, if you are gonna pick someone off at a distance, then an AR-15 or equivalent barrelled for a 300 blackout is the tool for the job. Subsonic loaded, suppressed and scoped.
OH! Those ARAKs are so sweet!
I used a Faxon barrel for my first AR build.
Excellent company.