barrel length + gas tube length come into play. You'll see a lot of 7.5" pistol gas that won't do it, at least not w/o adj gas block. Carbine gas on an 8.5" - 10" barrel is how I did mine. They will run subs, supers, suppressed, unsuppressed. I've also had a very hard time finding a good barrel as far as accy goes. Getting one to run is pretty simple - just throw carbine gas on it. But I went thru 2 DD barrels and could not get supersonic loads grouping well. Went thru all the usual suspects 30 cal projectiles from Sierra & Hornady from 110VMax up to 168gr SMK, lots & lots of ladders, and just couldn't find a good load for supersonic + a DD barrel. Subs were easy. I'm on a Faxon barrel now & pretty much gave up dicking around with supersonic loads. I get 1.25" @ 100 yds but quit chasing it and didn't work much for the new barrel. I really only shoot it suppressed with subs anyway & if i want a 30cal projectile moving real fast, well there's better stuff like 308WIN for that.....
In the end, now that have years of experience with it & lots of rounds downrange, lots of trial & error with different loads, if i knew then what i know now, would just build it 100% for subsonic suppressed performance. That's where it shines. In fact, I wouldn't have SBR'ed the 300BLK - I'd have just pinned the suppressor on.
I can drink a quart of monkey and still stand still...
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