Troubleshooting feeding and BHO on a new build

[dennisuello]

13.9" Rosco barrel, mid length gas system, standard buffer. When doing the last round BHO test, it fails to lock open. Tried different magazines and replaced the bolt catch. Tried adjustable gas block, it just fails to extract altogether when reducing gas pressure. The only thing I haven't tried is heavier buffer, but I would think I need less mass not more. I am thinking the gas port is too small. Any other ideas?

[Disciple]

Aren't barrels under 14.5" with mid-length gas typically biased toward suppressor use? Maybe you need a lighter than standard buffer or carrier if you aren't using one. Try pulling the weights out of the buffer and see what happens.

[SteveL]

What ammo? What buffer spring? Also check for gas leaks around the gas block and check to make sure the gas key isn't loose.

[dennisuello]

Aren't barrels under 14.5" with mid-length gas typically biased toward suppressor use? Maybe you need a lighter than standard buffer or carrier if you aren't using one. Try pulling the weights out of the buffer and see what happens.

I'll give that a try. Suppressor is planned for this one, but I'd like it to run without one.


What ammo? What buffer spring? Also check for gas leaks around the gas block and check to make sure the gas key isn't loose.

No leaks around gas block, that bolt was used in that upper with a different barrel prior and no issues.

I might just try a different barrel, with a carbine gas length.

[SteveL]

Are you sure the gas block is properly aligned? What about gas tube to gas key alignment?

[dennisuello]

Are you sure the gas block is properly aligned? What about gas tube to gas key alignment?

Checked that first. I can see the gas residue around the gas port when I swapped to adjustable gas block. Gas tube protrudes the right amount into the upper receiver.

[Jim D]

13.9" Rosco barrel, mid length gas system, standard buffer. When doing the last round BHO test, it fails to lock open. Tried different magazines and replaced the bolt catch. Tried adjustable gas block, it just fails to extract altogether when reducing gas pressure. The only thing I haven't tried is heavier buffer, but I would think I need less mass not more. I am thinking the gas port is too small. Any other ideas?
Ditch the adjustable block, they'll never seal up and be as efficient as a fixed block. Commonly when you have short stroke issues on systems where the port size is adequate, it points to the efficiency of the rest of the system (notably the gas tube fit and the BCG). I'd try another BCG in that upper before anything else.

[markm]

Bolt speed it too fast or too weak. If the recoil is "poppy" that "standard buffer" is too light. If the recoil is smooth or sluggish, you're not getting enough gas to fully cycle the bolt group.

[HKGuns]

Since you are swapping gas blocks around, buy yourself a set of inexpensive pin gauges and measure the gas ports on your rifles when servicing them.

Not enough information to otherwise isolate your issue. But I do agree, ditch the AGB and let the good folks here know what RE - Buffer and Spring and bolt.

Complete rifle, built rifle (what components) or what?

More information.

[Todd.K]

Swapping in a known good BCG if available is always a good idea, right after confirming the bolt catch moves freely.

Check the gas tube for alignment, and for length.

[26 Inf]

13.9" Rosco barrel, mid length gas system, standard buffer. When doing the last round BHO test, it fails to lock open. Tried different magazines and replaced the bolt catch. Tried adjustable gas block, it just fails to extract altogether when reducing gas pressure. The only thing I haven't tried is heavier buffer, but I would think I need less mass not more. I am thinking the gas port is too small. Any other ideas?

So I imagine yours is the K9 Greenline Tactical in 5.56. FWIW I have 13.7 Purebred .223 Wylde - it runs like butter on an A5H3.

I recently spent several days at the range dialing in a 16 Dissipator with a rifle length gas system and a rifle length receiver extension.. Getting it to function and pass lock back was easy, getting it to cycle the way I wanted was harder, expecially since I had my choice of rifle, a5 with spacer and carbine with spacer.

What proved invaluable was that ability to change weights around in buffers. I buy tungsten weights from purtungsten.com and all the other components (steel weight, aluminu, weights and rubber spacers as well as buffer bodies from https://kakindustry.com/ar-15-parts/buffer-tubes?page=2. I use a cheapo HF scale - https://www.harborfreight.com/digital-scale-95364.html?_br_psugg_q=scale
to check weights.

I'd suggest anyone puts together AR's do the same.

Note: if you are building rifle buffers I would not recommend the KAK rifle buffer body as the base has the aluminum spacer as an integral part of the body itself so you won't really be able to build hevy rifle buffers.

You also need to consider that some times it takes some shooting to seal the gas system - although I've never had that problem using Giessele gas blocks.

You mentioned changing bolt catches. No need to even think about that unless the bolt doesn't lock to the rear when you cycle it with empty magazines inserted and seated.

You also don't mention checking your bolt - look for leakage around the gas key - between the bolt carrier body and the gas key. Check your gas rings:

Per the TM 9-1005-319-23&P.

4. Check bolt assembly (5) for proper fit with bolt cam pin removed. Turn key and bolt carrier assembly (6) and suspend so the bolt assemblyis pointed down.

NOTE: The bolt assembly must not drop out. If weight of bolt assembly allows it to drop out of key and bolt carrier assembly, replace bolt rings (p 3-21).

It's also nice to be able to swap in a known good BCG.

Good Luck!