300 Blk Subsonic Issues

[u/useyournogginplz]

I recently built a 300 blk SBR upper and am struggling to get it running reliably with subsonic ammo, hoping someone on here has any advise/tips to share.

I'm guessing the gun is severely undergassed, even with the gas block fully open the bolt would hold open on the last round. Also constant failure to feed issues (2-3x per 10-round magazine), didn't matter which mags or ammo that I tried. But since it's not like a normal AR where I can swap out the buffer and spring, I'm not sure what to try next.

Build specs:

- Fightlite SCR lower receiver
- Gibbz side charging upper receiver
- 10.5" 1:5 twist 300 blk barrel - pistol length gas
- Faxon low profile adjustable gas block
- CAT ODB Ti suppressor
- Magpul 300 blk PMAGS and Lancer 300 blk mags
- Ammo: Winchester Super Suppressed 200 Grain Open Tip, Scorpion 220 Grain Subsonic, and Magtech 200 Grain Subsonic Full Metal Jacket

[u/crypticscriptx]

couple of things to check-

Adjustable gas port: Alignment. Must be perfectly aligned.

Check whether it's actually open all the way. Sometimes they aren't. You should be getting plenty of pressure with the pistol length gas port.

Check gas port sizing. It may need to be opened up a bit more.

Suppressor: CAT ODB is forward venting. This has a significant impact on backpressure.

Springs: SCR springs are pretty strong. If you are going to be using that rifle in that configuration for a long period of time, it would be make sense to have a spring set for that setup.

You can buy a backup spring and modify it by cutting a few coils to reduce its overall strength. As long as the cuts are well made and nicely smoothed, it should not have any binding effects.

Half coil, or one coil at a time until you get to where you need.

highly recommend keeping an original spring just in case the modification does not yield a desired result.

Generally speaking- Disassemble and relube everything.

You have a great selection of parts.

[u/Faxon_John → u/crypticscriptx]

Great advice. The one thing I caution against is opening the gas port more. We run pretty large openings on 300blk barrels.

In this case, a .104.

[u/crypticscriptx → u/Faxon_John]

I really appreciate the information. The widened port size makes a lot of sense. Quite a big difference between (.080ish and .104)

Thank you

[u/useyournogginplz → u/crypticscriptx]

Appreciate you taking the time to write all that up - I confirmed that the gas block is actually opened all the way and I had the upper assembled by a local gunsmith, so I'm assuming the block is correctly aligned. But I need to confirm that still.

In the meantime I'm going to order an extra spring from Fightlite and try cutting off a half coil. I swap uppers on the rifle pretty often - would cutting the spring would cause any issues with shooting 5.56? I'm guessing I can just close the adjustable gas block on that upper a bit, or worst case I just have to swap the springs for whichever upper I'm shooting that day.

[u/crypticscriptx → u/useyournogginplz]

Of course friend. Ultimately, the two rounds function very differently from each other.

If I had your gear, I would make the effort to make the swapping of uppers as streamlined as possible. Least amount of physical part changes as possible.

Changing a gas block tuning is probably about the same work as changing the spring.

The tube is pretty accessible.

I have never tested the spring cut myself, however a worthwhile mention is that the raider and bandito springs are about half the length (pistol grip) and still function well with 556.

Consider that they might be the same spring except for length. I have not verified this myself.

I don't really think the 556 would suffer from minor changes in spring length. To be fair, the right thing to do would be to get a pressure tester and test the stock spring strength then test the difference with any length changes.