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Posted: 5/6/2026 1:19:24 PM EDT
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I just bought a 7.5" 300 BLK upper (pistol length gas) that isn't reliably cycling subsonic ammo, even with a suppressor, CAR buffer, and Springco white (standard power) spring. I tried 3 different types of FMJ ammo, and they all perform about the same. None of them will lock back on an empty mag, and some of them do not reliably strip a fresh round from the mag. Is this just par for the course with this setup, is it worth buying a reduced power spring, or is there another issue perhaps? The carrier key is tight and secure, and the gas block visually appears ok. I know BCM only recommends supersonic ammo to ensure reliability with their 7" 300 BLK upper, and 7.5" isn't much different. Any ideas? |
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Make sure the gas block is properly indexed and installed before drilling the barrel. Also make sure the BCG isn't a source of gas leakage. When I have opened gas ports at home I insert a brass rod into the barrel and go with the absolute smallest drill bit that opens the port and as soon as it hits brass stop. |
USAF (Ret) 2003-2024
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I'd be interested in how long the barrel was that the ammo was confirmed subsonic in. If the proofer used a 16" barrel, then a 7.5 is going to have them going a LOT slower (and dwell time will be lower as well). depending on the powder they used, it may not be getting enough pressure to cycle properly. I have a 6" blackout that wouldn't cycle suppressed any of the subsonic loads my 11.3" works perfectly fine with (even unsuppressed). I loaded up some new ones with a good bit more CFE BLK, and while they were occasionally (about 35-40% of them) supersonic out of the 11.3, i want to say they were hovering just shy of 1000fps out of the 6". and they cycled perfectly. If you aren't going to handload at all, as previously mentioned you may need to open the gas port a bit. |
| Hmm I'm shooting my home build 9" .300 this weekend for the first time, waiting on a yhm resonator k for it was well. I heard the resonator k isn't as high backpressure as one would think. I also have a polo 30 i could use on it, which will be the suppressor I test on it this weekend. |
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Originally Posted By GimmeLibertee: I just bought a 7.5" 300 BLK upper (pistol length gas) that isn't reliably cycling subsonic ammo, even with a suppressor, CAR buffer, and Springco white (standard power) spring. I tried 3 different types of FMJ ammo, and they all perform about the same. None of them will lock back on an empty mag, and some of them do not reliably strip a fresh round from the mag. Is this just par for the course with this setup, is it worth buying a reduced power spring, or is there another issue perhaps? The carrier key is tight and secure, and the gas block visually appears ok. I know BCM only recommends supersonic ammo to ensure reliability with their 7" 300 BLK upper, and 7.5" isn't much different. Any ideas? If you would like us to take a look at it, please PM me. I would verify that it runs with supers, then pick a subsonic round that you like, and dial it in with the right buffer, spring, and maybe even an adjustable gas block to tune it to work just right. Thanks, Josiah |
Palmetto State Armory
3760 Fernandina Rd
Columbia, SC 29210
Web Sales: 1-803-724-6950
3760 Fernandina Rd
Columbia, SC 29210
Web Sales: 1-803-724-6950
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Originally Posted By panthermark: As other have mentioned, tested with supers first. Is it functioning fine without a can? Is it overgassed with a can? The gas port might be on the small size. I would rather have that (easy enough to open up) than vice-versa. I've only tested it with a can, shooting subsonics. I would imagine that if it were over-gassed, then I wouldn't have the issues. I just haven't had time yet to test it more. |
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I have a 7.5" PSA upper too and mine will run subs with or without the can (over gassed), I had to put an AGB on it. Then again, I reload and have a chrono so I tune my loads to 1000-1050 FPS out of the 7.5" barrel with Hornady 212 grain ELD-X and 225 grain ELD-Match. The 225's are a compressed load using CFE Black. My 212 grain load is 11.2 grains of the CFE Black and the 225's run 12.1 grains to keep velocities in the same range. My SBR lower is running an A5 buffer tube, Warthog spring and Kynshot hydraulic buffer that Amphibian recommended since I also run a 9mm upper I built from CMMG's FE kit with a 5" barrel Amphibian made for me. The 300 upper will cycle subs, without a can, using both the standard carbine buffer and spring or the Warthog/Kynshot set up, so I know it was way over gassed. I shot some 110 grain supers out of it when I first got it, but I built it to be a sub shooter. If you don't reload, then you'll need to tune it to whatever commercial ammo you intend to run. As others have said, check gas block alignment first, and if it's fine then you have some options. Lighter spring, drill the port if it's not getting enough gas, check for leaks somewhere, try different ammo, etc. If it's just shy of cycling enough, then do as the others said and run some supers to get it broken in. It might be enough to overcome the issues. |
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I ran about 60 rounds of supers through it which cycled 100% even with an H3 buffer and no can. However, it still won’t cycle subs with a CAR buffer and the can. If it were under-gassed, should it be expected to cycle supers so well with an H3 buffer and no can? |
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Originally Posted By GimmeLibertee: I ran about 60 rounds of supers through it which cycled 100% even with an H3 buffer and no can. However, it still won’t cycle subs with a CAR buffer and the can. If it were under-gassed, should it be expected to cycle supers so well with an H3 buffer and no can? |
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Originally Posted By Tiribulus: Maybe you missed THIS The part about contacting him or about trying to tune it? Yes, I have reached out. I don't know how to tune it considering it's short-stroking with the lightest buffer weight and added backpressure from suppressor. I do have a yellow (reduced power) Springco buffer spring on order to try, but if that's what it requires to function properly I'm not sure I trust it. ETA: Josiah is taking care of me, I will report back when its resolved. |
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Are you planning to run both supers and subs, or just subs? If subs, then if the lighter spring works, then you should be good. I too have a PSA 7.5", but mine is over gassed (will run subs without a can), so I put an adjustable gas block on it to cut the gas back. I put mine together to be a sub only shooter. That's the thing with the AR gas system, especially the shorties, it can be a juggling act to get them to shoot both supers and subs reliably and do it without beating the heck out of the internals. In that case you need some kind of adjustable gas...unless your port is too small, then you have to open it up, or lighten up buffer and spring and go sub only. |
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Originally Posted By EDL: Are you planning to run both supers and subs, or just subs? If subs, then if the lighter spring works, then you should be good. I too have a PSA 7.5", but mine is over gassed (will run subs without a can), so I put an adjustable gas block on it to cut the gas back. I put mine together to be a sub only shooter. That's the thing with the AR gas system, especially the shorties, it can be a juggling act to get them to shoot both supers and subs reliably and do it without beating the heck out of the internals. In that case you need some kind of adjustable gas...unless your port is too small, then you have to open it up, or lighten up buffer and spring and go sub only. My plan was to shoot mostly subs in this gun. Even if the lighter spring works, the gun is too close to the "edge of reliability" that I'm not sure I'd be able to trust it. Also, I noticed a fair amount of residue on the handguard just behind the gas block, which I know can sometimes be normal, but I'm going to have PSA take a look just to make sure the gas block is aligned and installed properly. I'm sure they'll take care of it if that is indeed the issue, so no big deal. I'll report back once I have it figured out. |
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Originally Posted By EDL: Ehhhh...I dunno about that being normal (lots of residue behind the gas block). None of my AR's do that. I think you probably found the issue if that's the case. Block is loose or misaligned (or both). Yeah it sounds like gas block leakage may be the issue |
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*Update* The repair ticket from PSA reads: "Inspected firearm for not cycling subs while suppressed. Found chamber to be out of spec. Replaced barrel, honed and polished chamber, verified gas port was to spec and checked head space. Used Dead Air Nomad suppressor and alignment rod to verify concentricity . Test fired Winchester 220 grain, AAC 220 Grain and Frontier 208 grain without any issues or malfunctions, cycled correctly. Cleaned and oiled. DR" I should have it back in hand tomorrow to test out. All in all a great customer service experience thus far. I wish I could say the same about some other companies in this industry (cough!!!CMMGcough!!) |
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