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.