AR Problem - failure to feed from mag while firing, but not manually?

[Greywolf]

OK, here's the scoop. I have a 6.5 CSS with 20" barrel, 7.62x39 bolt, adjustable gas tube, built on a Eagle Arms lower. It has a Magpul PRS stock with rifle buffer and spring.

When I load up my mag and use the charging handle or bolt release, it loads a round into battery every time. I pull the trigger, it goes BOOM, ejects spent casing every time. However, using multiple and various mags, a new round never chambers - it just stays in the mag. Now, if I put an empty mag in and pull back the charging handle and release, it will hold open the bolt. But if I have one round in the mag, pull the handle, chamber the last round, then fire it will eject the empty case but won't lock back the bolt on the empty mag.

So, am I correct in assuming that either:

1 - I'm not getting enough gas to throw the bolt far back enough to get past the rim of the next round so as to chamber it? I don't know about this theory since ejecting is no problem.

2 - My rifle spring or rifle buffer is too much (too heavy weight buffer or too stiff of a spring) to allow for full cycle.

3 - something else?

I am using reloads which perform very well, seem to be well within proper pressures, etc. The adjustable gas tube is almost fully open, so I'm getting 100% gas.

Thanks for the help!

 

[tyeo098]

Clean the gas tube?

 

[Greywolf]

Gas tube is clean.

 

[xfyrfiter]

It does sound as if you are short stroking .It could be buffer and or spring but I would try different ammo and check gas tube alignment with the gas key ,[may noy be centered].

 

[Skinsanity]

Have you tried different magazines? Could be the magizine isnt keeping up...if it doesnt the same thing with different magazines then yea, my next guess would be the buffer/spring combo...

 

[rbernie]

The action is short-stroking. I have seen this happen on many 'bigger than 223' builds with 20" barrels and rifle length gas systems, due to a mismatch between the chambering's pressure/volume curve and the gas system.

You can try to make this action more reliable by lowering the reciprocating mass (using a commercial and not a USGI or heavier-mass bolt carrier), and you can replace the gas tube to try and ensure that you have no impediments to flow. You might also contact the barrel manufacturer and ask for their gas port spec and measure yours, to make sure that your gas port is as expected. Finally, you will likely also need to play with the handloads, to generate more pressure or move the pressure curve out a bit.

 

[Greywolf]

Tried 3 different mags with same problems.

Loads are actually pretty weak, I think - they were 120 grain SP bullets, virgin 7.62x39 IMI brass so as to fire-form for more volume. I recall not having these issues with my Wolf 6.5 Grendel ammo in the past.

Buffer weight and buffer spring appear to be correct. Gas block appears to be lined up properly.

It is entirely possible I'm just using some underpowered handloads. I saw no pressure signs, so maybe I'll go from 27 grains of TAC to 28 grains and see what happens. Also will try playing with the adjustable gas tube as well as making sure everything is lined up properly.

 

[TonyAngel]

Grey, i think you answered your own question inthe original post. I'd look at the ammunition first. I don't think it's the gas tube. Many talk about cleaning it, but I've never seen one that needed to be cleaned.

Is this a new rifle or does it have a track record, like was it working before, but not now?

 

[taliv]

ammo would be my prime suspect. i've seen several semis that won't run on weak ammo.

fwiw, i found out two weeks ago that my KAC SR-25 won't reliably run on 175g silver state armory as it is VERY weak.

 

[TonyAngel]

It might not have been so much that the ammo was weak. It could have been that the powder used had too fast a burn rate. In any case, try different ammo.

 

[Greywolf]

I have used different ammo in the past (Wolf) and it worked fine. Rifle hasn't been shot in forever, though, so I may be wrong about just how well it worked.

I've ordered some Hornady 123 grain ammo from Midway and will try that through it - the load was 27 grains of Ramshot TAC with a 120 grain SP - so perhaps it is just a little too lightweight of a load to do what is needed.

 

[Onmilo]

I think you answered your question with your "I will play with the adjustable gas tube" post,,,,

Load two rounds each in several magazines and fire the rifle and adjust the gas tube until it ejects first round and feeds second round flawlessly.

 

[madcratebuilder]

Loads are actually pretty weak, I think - they were 120 grain SP bullets, virgin 7.62x39 IMI brass so as to fire-form for more volume.

I recall a friend that had this same issue when fire forming with light loads.

 

[jad0110]

I'd lean towards this being ammo related, though my rifle started short stroking because the gas key on the bolt carrier came loose (it was NOT staked in place) and it was letting enough gas escape that not enough was left to fully cycle the BCG. It has since been tightened and staked and the problem is gone.

Check you gas key to see if it has any play at all in it. Don't be rough with it, just some normal hand pressure is all that's needed to check it.