Supersonic/Subsonic 300blk in same gun?

[Captains1911]

My 9” BCM 300 BLK upper runs supers and subs reliably with an H2 buffer, suppressed and unsuppressed. It doesn’t seem to care. I can’t remember exactly what spring is in it, but I believe it’s a Sprinco “regular” stiffness.

[Matt in TN]

Mine runs fine with both subs and supers. However - one thing I haven't seen mentioned here is twist rate and accuracy. If you have a fast enough twist rate to stablize 220gr subs, it won't be ideal for 125gr supers. Mine doesn't shoot the light bullets worth a crap, but 155gr and 168gr supers shoot great groups.

Follow-up: I ditched the PSA 7.5" barrel and installed a Ballistic Advantage 8" barrel and it shoots much better now with both supers and subs.

[wanderson]

I run PSA .300bo uppers in 16” & 8”, the 16” does group 125 gr. supers better but you really don’t see any difference til you get out to 100 yards. Both are 1:7 Twist barrels which is the most common twist rate and seems to work for 125 to 220 gr.
My PSA 8” didn’t run subs at all Unsuppressed at first. Went to a lightweight BCG and a Sprinco yellow buffer spring, use std. weight buffer for subs and an H3 for supers.

[CMV]

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.