I’m trying to troubleshoot a no-name 16" mid length upper. Every so often the bolt does not hold back after last round. Also I’d say every 20-30 shots the firearm cycles ok (spent case ejects), click, no bang, and there is no cartridge in the chamber. I’ve played with buffers, tried different ammo, bolt, entire lowers and magazines.
Do the pictures mean anything? I am assuming (very novicely) that the gas block is somehow not in spec and is leaking gas and/or was not installed properly…
2nd picture is the same as the 1st, but I suspect there is gas leaking in the highlighted area? I am not technical savvy to decipher them. Also does the gas port look ok?
I see nothing obviously wrong with the gas port. The carbon fouling almost centered around the port is a good sign because it indicates that the hole on the gas block is completely over the port.
I know you said you had tried different ammo and magazines, but what specific ammo and magazines have you tried?
What does the gas block itself look like? Is it a sight tower block and pinned, or low profile? What does the gas tube look like? Have you looked for any damage, cracks or holes in the gas tube?
In addition to my post above, the gas tube is covered in dark sooth. Is the only way it can get that dark is gas coming back from here (see below pic)? Or am I really looking at a bad/leaky gas system here?
Thanks SomeOtherGuy. I tried HK, Magpul and lancer mags. The Magpul is gen 2 & 3 mixed. Ammo is tula, brown bear and American Eagle… The gas block is low profile, not pinned but is the screw to the barrel (dimple) type. Here is a pic of where its screws meet the barrel (not too deep)… You can see the gas tube too…
From your description, the gun is appears to be getting just enough gas to work, most of the time.
Doesn’t look like the gun is losing gas from the port, all the barrels I’ve ever seen look just like that.
There is some gas loss from where the gas tube enters the block. Sometimes, this resolves when the gas tube gets enough fouling around it to seal off.
Your gas port could be too small, but I think that’s on the less likely end of probability. It’s more likely that the gas block was drilled out too large where the tube enters.
How easy was it to remove the gas tube from the gas block?
Something looks wrong with the gas tube. They would normally have a little soot at each end, but not much (or zero) in between. I would inspect the tube closely and also look at its fit within the gas block. If you have access to a new gas tube, I might just replace it as the next thing to try. They don’t cost much. But if the gas block is too loose around the tube, that could be a problem also. I suspect your problem is either the gas tube or the fit of the tube into the gas block.
Has this gun been abused, used for repeated mag dumps or that sort of thing? Or maybe the gas tube got hit or bent somehow?
FYI, Tula ammo is reputed to be fairly poor. However, Brown Bear and American Eagle should be OK. Pmags and Lancers usually work fine, so it sounds unlikely that magazines are the problem.
First thing that sticks out, since we can’t see what it looked like before disassembly. Barrel is not dimpled. I suspect that the gas block could have been seated incorrectly. I would also venture to guess it is not very snug on the barrel.
I also wouldn’t be surprised if the gas tube orifice is out of spec and causing gas to leak as well.
I finally had a chance to reassemble the upper and took it to the range. I’ve installed a better gas block, new gas tube and had the barrel dimpled by a professional to accommodate the new gas block. Not sure if blue was the right way to go but I loctited the gas block screws pretty tight.
I am happy to say that after 60 rounds earlier today (40 American Eagle + 20 Brown Bear) I had zero extraction, ejection, cycling problems. Tried with a single round in the magazine about a dozen times and the bolt locked back every single time. Tried with 2 magazines to be on the safe side - a 2nd Gen Magpul & a H&K one. Zero fails…
I also noticed something else on the inside of the handguard when I took the upper apart to replace the gas block & tube. See the areas marked with the arrows? They were in the vicinity of the gas block and just accumulation of what I guess to be burnt powder. So unless a little bit of gas is really supposed to seep out of that region, it was definitely a leak / poor seal…
Blue thread locker won’t hurt anything, but it won’t help much either. The gas block gets way past hot enough to liquify it. If it’s an aluminum block, be extremely careful if you ever remove those screws, or you’re likely to take a few threads with you. In any case, don’t spare the heat gun if you ever remove it.
Depending on how hot you get your barrel, red thread locker will most likely work, but this will pretty much be permanent in the case of aluminum, semi permanent in the case of steel. The dimple should be plenty to keep it in place, though. If you’re really paranoid about it, then staking the screws is about the only way to make it foolproof.
I repaired an aluminum gas block setup at a recent carbine course at LMS Defense. The student started seeing holes in his gloves! Surprisingly, the gun continued to function, however the hold on the rifle became painful after using it for a few drills. We took the gun down, and noticed that the aluminum gas block had eroded out the side from use, and formed a plasma torch of sorts. It actually melted a slot into the troy rail, and started to chew threw the students gloves. We broke the rifle down, replaced block with a titanium unit off my backup gun and continued on.
