My Buddy with the 24" Stoner 223 barrel bought it because he liked the performance on my Stag Model 6 Super Varminters, with similarly configured 24" 1:8" Bull barrels. All of my 223 chambers are either Wylde or 5.56 NATO.
My AR's shoot IMI 77gr Razor Core, M855 (and soon an M855 clone using the 62gr Gold Dot), M193, Federal Fusion MSR 62gr, or a Handload using Starline brass, CCI-400/BR4, HDY 75gr HPBT Match, and 23.5-23.7gr of Varget.
He bought the PSA PA-10 18" Upper and had lock back issues with it, which we attributed to the mid-length Gas system. He added an adjustable gas block and resolved his issue.
Based on his experience, I opted instead for the
PSA 20" Upper, which uses a rifle length gas system. Once I had my PSA-10 assembled, upgraded, and scoped, I fired 10rd through it (about two weeks ago) to confirm feed and function. I had no issues whatever.
So I'd suggest the 20" Upper.
For the 308, I've been using IMI 7.62x51 NATO 168gr Razor Core Semi-Auto Match in my Savage 11VT 24" 1:10" F Class rifle. My Buddy tried some in his 1" PA-10 and it cycles well, shoots quite accurate. It's a clone of the 168gr FGMM, pretty much.
I have the Bushnell AR Drop Zone BDC308 scope on it. It has trajectory accurate aim points for this load at 100yd increments from 100-500yd and has an trick to provide a 600yd aim point. I use this scope on both of my 308's and both of my 6.5 Grendels, which share a nearly identical trajectory curve out to 500yd.
As of this past week, all of my 223, 6.5G, .260, and .308 rifles use various versions of the Bushnell AR Drop Zone BDC scopes. The 260 uses the 6.5CM version. The BDC Reticle simplifies aiming solutions; requiring only a 600yd LRF to determine range, then aligning target and reticle aim point for that distance. The first round hit probability is improved, and happens faster. It's limited to 600yd as a slam dunk solution, but that's really not such a bad limitation. The only real issue is density altitude; I live at 4350ft ASL.
I am working on a 308 handload for my two rifles, mostly a slight alteration to my 168gr FGMM clone. It's the Speer 168gr Gold Dot over 43.5gr of IMR-4064, in Starline Brass, with WLR primers, which have a more positive ignition capability. Assuming this load works as expected, I'll be doing a run of it to set some aside.
I'm also developing a 6.5 Grendel Handload employing Starline brass, the Speer 120gr Gold Dot, WSR Primers, and W748, charge to be determined, but we're close (testing cautiously in the 30gr+ range right now). The Grendels are a pair of AR Stoner sourced Uppers, both factory 20", and homebuilt 24". Lowers are PSA PA-15's.
My AR's use 6-position collapsible stocks with
this recoil pad, which makes the Grendel more pleasant to shoot and convinced me that I didn't need a brake on the 308. In addition to the PSA 2 Stage trigger I linked above, I also use a
Bolt Release Extension on my AR's. AR Stoner
Version.
I know there are more answers here than you asked for, but you could probably use the info in days to come, and others here, maybe sooner.
Greg