Mid length gas system malfunction

[medicjim]

I recently assembled a mid-length featherweight barrel upper which was to become my carbine class primary. I've assembled a half dozen uppers in the past and I suspect I did this one correctly in terms of parts assembly. When I obstruct the breach and blow air into the muzzle, I can feel air escaping from the breach end of the gas tube, so I know the gas system is unobstructed. The bolt closes and head-spaces properly in this setup and it works fine in proven full length and carbine setups I own, so I suspect the issue is not with aligned gas ring gaps.

When I fire the rifle, the shell does not eject at all and the system does not cycle…. I've had a buddy watch as I fire and there is no detectable movement rear-ward on the bolt.

My gas tube is 'straight'…. no loop or other impulse storage provision.

I suspect my buffer spring and weight might be the problem. Does the mid-length setup need a special spring, or will either carbine or full length setups suffice? Would a gas tube with a loop increase the gas impulse or reduce it…. clearly I either need to increase the gas impulse somehow…or reduce the inertia / resistance of the buffer config. Maybe my gas rings are bleeding gas but it's not sufficient to stop the cycle in a full length?

I'm really just looking for a sanity check here…. before I go ripping into stuff, am I missing something that is right in front of my nose?

 

[rjacobs]

What barrel?

Does the lower function with other uppers?

I helped a guy trouble shoot his "SPR" he got built by some local shop and it was doing something similar. Tried it on 3 of my lowers, tried his lower on a few of my uppers(his lower was new as well). Tore the barrel down to check gas block alignment and to make sure the tube wasnt messed up. Checked gas rings and carrier key. Finally told him to take it back to the dealer that assembled it since I was pretty sure the gas port was way small and I wasnt messing with that.

It ended up that I was right and the gas port in the barrel was WAY under sized. Manufacturer had to re-drill the gas port in the barrel.

 

[ORD]

If you are NOT getting ANY movement of the carrier whatsoever, then its a gas system hiccup SOMEWHERE I suspect.

Check the gas port diameter in the barrel? What size is it? Possibly well undersized? If so, you can open it up slightly as needed in increments until you get it where it needs to be.

Are you getting a good seal at the gas block? Are there signs of issues there when you disassemble the block from the barrel? Is the gas block properly installed in terms of it being tighten down/secured over the gas port in the barrel? Is the gas tube properly installed in the block? Is the tube a mid-length tube and not an intermediate or other one-off tube? Is there any impingement or blockage?

Is the carrier key properly secured in the bolt carrier? Screws tightened down to 55 in/lbs? Staked? Good seal on the key?

Is the rifle correctly lubed up and running pretty wet?

Just a few more things to check from tip to toe that I can think of off hand.

 

[Deadshot2]

With NO movement of the bolt carrier my vote is for either a massive gas leak, gas block or carrier key, or obstruction/misalignment.

You say you feel air escaping from the breech end of the gas tube but "How Much?"

I had a rifle I was building do this. I finally took off the upper and clamped it in a vice. Using a bore guide from a .223 rifle (for cleaning) inserted in the chamber and hooked to a regulated air supply, corking the muzzle, I then used a piece of Windshield wiper tubing to measure the air flow. One end on the breech end of the gas tube and the other in a pan of water. I adjusted the gas block position until I got the maximum amount of air flow (and largest bubbles) through the tube.

The gas block was secured with set screws with cup points so once adjusted I changed them out, using Cone Point set screws and capped them with shorter cup pointed set screws to lock them in place.

Problem solved.

 

[KTDLS7]

medicjim,

You didn't state your barrel length except assembling for a "carbine class". Mid-length gas system ports are as follows:
16": .085 - .0855
18": .076 - .080
If the barrel started life as an 18", then cut down to 16", it is under-gassed. Check the port diameter.

FWIW regarding carrier key screw torque, all my documentation and training material state 35-40 lb/in.

Let us know what you find.

 

[ORD]

FWIW regarding carrier key screw torque, all my documentation and training material state 35-40 lb/in.

Let us know what you find.

I have seen the TM (and other mil-spec sources) re: carrier key torque and frankly...that's one of only a very FEW things I take issue with. I have been torquing them to 55 in/lbs for ~15yrs now and I have found that it is significantly better than the 35-40 in/lbs otherwise recommended by mil-spec. More and more mfgs are going to that value as well, including Armalite, Young and various others. 35-40 may get the job done "ok," along with staking them (use or don't use loctite/permatex...personal pref more than anything) but I for one will continue to torque them down to 55 in/lbs for my builds and any others that I work on for folks, but to each their own.

 

[medicjim]

Problem solved.

When I pulled everything down to measure the gas port size, I spotted a flake of material lodged in the adapter, obstructing gas from entering the gas tube…. when I removed the obstruction, the volume of air that could be moved through the gas system increased dramatically. Test firings resulted in clean cycling.

Thanks to everyone that responded, special thanks go to KTDLS7 who supplied the details that drove me to find the offending debris…..

 

[KTDLS7]

medicjim: Great that you found the problem!

ORD: Good for your success using 55 lb/in on carrier key screws. Certified armorer schools (i.e. Colt & WSTC) are still stating 35-40 lb/in, proper staking, no sealant or Loctite. A torque increase of 37.5% is significant for a fastener. BTW, good luck on a field replacement of a broken carrier key screw that's had Loctite applied. I will agree - to each his own.

 

[Deadshot2]

BTW, good luck on a field replacement of a broken carrier key screw that's had Loctite applied. I will agree - to each his own.

For "field replacement" I prefer to just keep a complete BCG handy for a really fast repair. A properly staked key will also be a bitch if attempting a "field replacement".

Just my choice.