SBR upgrades and gas blowback

General Firearms NFA
1

I wasn’t sure whether to post this under technical or NFA but we’ll start here. So I recently upgraded some parts on my SBR and I’m now receiving much more gas blowback than previously. I have a Sionics 11.5 standard barrel, Sionics gas block and gas tube with a SilencerCo SpecWar K attached via ASR flash hider. Here is my list of upgraded components:

BCM4 standard upper to Vltor MUR 1A
BCM bolt carrier to LMT enhanced carrier
BCM Mod4 to Geissele ACH

I don’t believe it matters but I swapped from an A5H4 to A5H3 buffer and Colt FCG to Geissele SSA. I’m having difficulty understanding why this is happening, especially with the e-carrier. The blowback is the worst during the first few rounds and seems to dissipate through the remainder of the magazine. I will RTV the ACH after I can unpack after moving here in the next few weeks to see if that helps.

I’m interested to hear any thoughts or suggestions.

2

Well well well, you should be confused. That makes no sense.

Maybe different ammo? Different conditions in environment? In indoor range vs outdoor? CH change that back and see if that’s the easiest culprit to check.

PB

3

Same ammo, all handload 69 grain Nosler and Sierra over 8208 XBR and I always shoot at the same outdoor range. It was definitely the warmest day I’ve fired the rifle all year.

I will try the BCM charging handle again to see if it makes any difference.

4

The bolt carrier and buffer were both switched to items that increase bolt velocity or need less gas drive to operate.

The barrel gas port is sized generously enough to run weak ammo and heavy buffers unsuppressed.

When a suppressor is added to the mix, bolt velocity increases further.

Throttling back the gas drive a bit would probably go a long ways to make it more enjoyable.

5

I agree with Clint. I would swap the A5H3 for the A5H2 and install a Sprinco green spring, then install either an SLR adjustable gas block, or get Clint to send you one of his Custom Micro Port kits. Tune your upper to run 100% suppressed. If needed, build an RTV dam on the charging handle. This should reduce the amount of gas to your face and result in a softer recoiling AR.

6

It is already running a Sprinco green action spring, I will continue to experiment with buffers. Clint, PM inbound…

7

To parrot what Clint said: The LMT enhanced carrier speeds up the carrier speed, you need MORE buffer, not less. So by adding that carrier and going with a lighter buffer, you are speeding up your carrier speed. A lot.

In the short term, run the A5H4.

8

I thought the cam pin slot in the enhanced carrier delayed unlock time to bleed off excess gas and pressure, not vice versa, ergo potentially slower carrier speed? I could very well be mistaken here, just curious about the physics…

9

When I first tested the LMT Enhanced Carrier, it would eject and lock back with one click less gas than a standard carrier.

10

There are two important changes to the carrier;

  1. a longer unlocking cam
  2. a bleed hole in the piston cylinder

These two items tend counteract each other in regards to carrier velocity.
The cam track reduces extraction force, increasing velocity.
The bleed hole reduces gas impulse, lowering velocity.

But the overall effect can vary depending on the application.

11

I received and installed a new adjustable gas block from Clint with the .060 insert a few weeks ago and did manage a quick 20 rounds suppressed yesterday at the range. I reverted no other parts and the gas issue was nearly non-existent and the rifle cycled fine. I will continue to experiment with buffers and springs and different gas inserts to see if I can dial it in a bit tighter. I’m guessing it will run without a can based on my brass pattern but I will test this definitively when I have more time.

12

Thanks for the update.