HELP: AR15 in 300AAC under cycles with subsonic ammo

[armedpolak]

I have an integrally suppressed upper (https://www.radicalfirearms.com/product-p/rfisu300.htm) and shooting it several times recently it started jamming with 220gr S&B subsonic ammo. It would either not pickup the next round from the mag, or the round would be halfway in and pinned by the bolt.

I initially thought mags were the issue, but after trying mags specifically designed for 300AAC (Pmags) the issue persisted. Yesterday I started to suspect the issue is with not enough gas in the system (supersonics cycle all day long without a single glitch), and after the bolt didn't get locked back after the last round several times in a row I'm 100% sure now.

The gas block is custom to the upper so I can only tweak spring and buffer weight but dont know how they're rated.

Please recommend a replacement spring and buffer for use with sub sonic ammo exclusively. (EDIT TO ADD: I just checked my buffer, it weights exactly 2.95oz; the spring weight 1.9oz give or take; the buffer tube is mil-spec with six positions for collapsible stock)

AP

P.S. the dimensions are 17" overall length with 8.5" barrel and 8.5" mono core baffles, barrel and baffles are all one part.

[ss1]

First inspect the system for excess fouling or displacement such as the gas block or gas key. If all are in good order and If nobody from Radical can give you a specific solution or no one has this problem with this upper, I would start with an adjustable buffer like Odin works and start lowering the weight until you get a reliable cycle but no lower than 2.1 oz. If that doesn’t work, get a reduced power spring and start with normal 3 oz and do the same procedure. There are low mass BCG systems out there for race guns but you shouldn’t have to go down that rabbit hole.

[armedpolak]

I realized something last night... the gun ran fine for a long time until few months ago I put in some fancy 2 stage trigger in... I'm pretty sure that's when it all started... perhaps it adds extra drag on the bolt carrier and that combined with weaker subs is causing cycling issues?

[osprey21]

Radical is known for poor quality, but aside from that, it sounds like a gassing issue to me, as in under gassed.

[armedpolak]

Radical is known for poor quality, but aside from that, it sounds like a gassing issue to me, as in under gassed.

I wouldn't know; I only have their integrally suppressed upper in 300AAC and am very happy with it. the issues started with the installation of a new trigger; I think the way I installed it may have caused too much tension on the hammer spring: so I moved the tails of the spring to rest at the bottom of the lower vs on top of the trigger pin (which is not backing out because of the change), I think I had the build kit trigger installed the same way and gun ran flawlessly. I'll take it out later in the week to test it...

[ss1]

I hope it works, but I would be surprised. The amount of tension difference would be nominal IMHO and as you said, the trigger pin is kept from walking with the hammer spring. That said, maybe there is the chance that your trigger install upset a marginally functional gassing balance. But if you are manually cycling the bolt and you don't notice an abnormal hangup with the bolt and 2 stage hammer/trigger then my guess is that it's a gassing issue that occurred coincidentally with your trigger change. Process of elimination and bring your old trigger to the range and if your problem continues with you tucking the hammer spring below the trigger pin, then swap back to your old trigger. If it runs, then its your new trigger, if it persists, then you have a gassing issue.

[556pro]

open up the port and get an adjustable gas block. tune it. done

[armedpolak]

open up the port and get an adjustable gas block. tune it. done

I dont want to mess with the gas port; it's integrally suppressed and has a custom gas block.

[zeebaron]

I find it hard to believe that for something that has to have a tax stamp for a permanent factory suppressor, knowing all the problems people complain about with over/under gassing adjustment issues with any supressor, that they were so incompetant/cheap that they did not put an adjustable gas system on it. Ridiculous.

[armedpolak]

I find it hard to believe that for something that has to have a tax stamp for a permanent factory suppressor, knowing all the problems people complain about with over/under gassing adjustment issues with any supressor, that they were so incompetant/cheap that they did not put an adjustable gas system on it. Ridiculous.

it is not an adjustable gas block
but the gas port and tube do look pretty wide; it would be hard to expand it further without some of it leaking out around the gas block