SBR cycling problems.

[LouBoyd]

Originally Posted by Recusance

\ I am really leaning toward the gas block being the culprit.

The way I align gas blocks is to plug the chamber and blow air into the muzzle. Just listen to the air flow coming out of the gas tube while moving the block to find the position with maximum airflow. The method is quick, reliable, and requires no special tools. I use a fired case for the chamber plug. The method is also a check for obstructions of the gas ports and gas tube at the same time without disassembling anything.

[Recusance]

that's the method that I use. maybe my ears are not tuned.

[Alleycat]

I use a pencil and mark the center of the port on the barrel and the center of the port on the block. Then I line them up. The problem you run into is the port location. The distance from the shoulder of the barrel to the port and the distance from the back of the block to the center of the port on the block need to be checked. Either one could have been made on Monday or Friday. Blowing through the barrel will allow you to get maximum flow for the setup, but you may only have half of the barrel port exposed to the gas block port.

[LouBoyd]

Originally Posted by Alleycat

Blowing through the barrel will allow you to get maximum flow for the setup, but you may only have half of the barrel port exposed to the gas block port.

It would take some pretty weird fluid dynamics to get maximum flow with "only half of the barrel port exposed to the gas block port. If they're different sized being exactly concentric isn't critical as long as the smaller hole isn't blocked at all. The larger hole will always be blocked some. I turn the bock side to side and slide it end to end till i hear an equal change then split the difference each way.

Diagnosing a rifle though snips of symptoms on an Internet forum isn't nearly as easy has having the rifle in hand to inspect and shoot.. I saw one AR-15 at friend's gunshop which had similar symptoms to this one. The problem only became apparent when the gunsmith attempted to remove the gas tube from the receiver and it wouldn't come out. The reason was fairly obvious and the owner of the rifle admitted that he'd checked the torque of the barrel nut He had not removed the gas tube first.

[Alleycat]

I don’t think you’re getting what I’m saying. My ear can’t calculate CFM by sound. Yes someone may be able to rock the gas block back and forth until you hear the maximum flow, but unless you measured the ports you’re guessing. If the center of the barrel port is 0.300 from the shoulder and the block port is 0.310 from the shoulder you may get flow, but how much? Ports get drilled in the wrong place. I’ve bought used blocks that were full of copper. They had flow, but caused the gun to short stroke. If the ports are in the right place your method works just fine. It’s a “Trust but verify” kind of thing.

[tp555]

Gas port is .295" from the edge.That is the spec.I have seen a gas block very nicely machined pretty and all that was .250 from the edge.Customer supplied it.I do machine a dimple exactly 180 degrees from the gas port for the screw on my barrels.That makes it easy for block alignment.The gas hole going up in the block is usually .125" so you have some slack.

[Recusance]

Originally Posted by tp555

Gas port is .295" from the edge.That is the spec.I have seen a gas block very nicely machined pretty and all that was .250 from the edge.Customer supplied it.I do machine a dimple exactly 180 degrees from the gas port for the screw on my barrels.That makes it easy for block alignment.The gas hole going up in the block is usually .125" so you have some slack.

that does sound like the ticket. I'll "improve" my gas port to .125.

thanks gentlemen.

[dick]

Check to see if your carrier key is bent and double check to see if the carrier key screws are not backing out.

[Recusance]

Originally Posted by dick

Check to see if your carrier key is bent and double check to see if the carrier key screws are not backing out.

checked, staked properly, all is good.

[Sicarius]

I currently have a registered M6A2 LWRC SBR with a 10.5 556 upper. I have an AAC M4-2000, I got a chance to actually go shooting this weekend and I brought a long a 10" 9mm Upper to try out. The 9mm fired fine for about 5 rounds and then I started getting dual feeds but I knew it could be problematic and well didn't give it much thought.
I put the 556 upper back on and fired a couple mags with the canon no problems. Took the can off just to go louder because I could and no one was there to complain about the sound. The gun would fire one or two rounds before I had to manually cycle it would extract and eject but wouldn't strip another off the top of the mag. I tried a different mag even though I was using pmags. A buddy put the mag in his 16" AR no problem. I got really worried and stuck the can back on wham bam 25 rounds down range no problem. Has anyone seen this before? Does it take the can to build up the sufficient back pressure? My buddy ex cop and military says he has heard of this but never seen it we checked everything in the field. The gun has less than 300 rounds through it but I have always ran the can because my neighbors fuss and call the cops every time I shoot even though they have all been told I am legal. I just stopped shooting at my place because the local government started talking about a possible statute about shooting in the county. I didn't want to screw it all up for everyone else.
Sorry so long and thanks for any help or suggestions.