Short Stroke After Hand Guard Change

[dog guy]

I took an old cheese grater hand guard off of my factory built BCM 16" upper and replaced it with a BCM MCMR-15. The upper is mated to a Bushmaster lower, with an Vltor A5 recoil system. The original configuration worked fine for several thousand rounds. Test firing after the change resulted in consistent short stroking and failure to feed with two previously reliable mags and two different commercial loads (PMC and Black Hills). I'm basing the short stroking diagnosis on the marking left on the cartridges after each failure to feed. The gas tube seems to be aligned properly with no binding when the bolt is moved forward and back. The gas block is a BCM low profile. It put it back on using good Mcmaster-Carr screws, torqued to 25 inch pounds. I re-used the gas tube, new roll pin. I'm expecting that my problem is in the gas system. The gas block had to be tapped off with a nylon mallet (not too difficult) but did not need to be tapped back on, although it needed some effort to slide it into position. I did not use any other treatment under the gas block.
The lower also had the SSA trigger replaced by an ACT trigger when I did the upper work.
Does anyone see any glaring mistakes here? I plan to pull the hand guard and double check the torque on the set screws. Should I replace the gas tube? Is it possible/likely that the trigger change is impacting this?
Also, back when I used to read a lot more, I seem to recall that some people suggested a coating of green Loctite between the gas block and barrel. What's the current thinking on this? Seems like it would make any future work rather difficult.
Thank you for any suggestions.

[markm]

Two possible issues that I'd guess...

1. The gas block didn't go back in the same position... partially occluding the port into the block? or...

2. The gas tube is worn at the gas key end. I've had this happen on middies to where there's gas bleeding out of the key forward back through the front of the upper receiver.

Does the gas tube still have the larger outer diameter, bulb like shape at the key end in the upper??

[Iraqgunz]

Pictures would be great. My first feeling is that you did something wrong regarding the gas block. I WOULD NOT USE green Loctite on the barrel and block. It's not needed.

[dog guy]

"Does the gas tube still have the larger outer diameter, bulb like shape at the key end in the upper??"

I think so based on a quick visual look. I'll get a good look when I take it apart.

[dog guy]

IG, thanks. I'll see if I can post pics although I'm not sure how soon that will happen. I'll need to pull the new hand guard.,, which I need to do anyhow to check the gas block. I like the way they lock up but they're a bit of a pain to work with.

[JerDerv]

Although an unlikely coincidence, may be worth inspecting the gas key screws and gas rings on the carrier.

[MistWolf]

Test firing after the change resulted in consistent short stroking and failure to feed with two previously reliable mags and two different commercial loads (PMC and Black Hills). I'm basing the short stroking diagnosis on the marking left on the cartridges after each failure to feed.

Chances are that your AR has a partial blockage due to some misalignment during re-assembly, but the the only way to be certain is to perform a lock-back check. A failure to feed can have its roots in other problems and you'll just be chasing your tail until you positively verify or eliminate short stroking with a lock-back check.

[dog guy]

UPDATE, now that I've recovered from dealing with too much snow and actually had time to work on the rifle.
I removed the hand guard, removed the gas block and gas tube. I cleaned everything up, including blowing out the gas tube and ensuring that the gas key was not obstructed. Gas key screws were still tight.
I also probed the gas port in the barrel. It felt like something popped loose when I stuck the probe through, but I'm uncertain.
I reassembled with special attention to aligning the gas block set screws directly into the barrel dimple. I'm certain that it's aligned the same as when I got it from BCM. Interesting note: with the rear set screw indexed into the dimple, there is no gap between the gas block and the mating surface on the barrel behind where a hand guard cap would fit. That's a poor description but I don't know my barrel nomenclature well enough to use the proper term.
I test fired today. The rifle ran perfectly: no failures of any kind over about 80 rounds of three commercial loads and 6 different magazines. Locked back on empty every time. I'm confident that the problem is solved, and I'll report further if this turns out to be wrong.
I think that I either had the gas block slightly misaligned, or that there was a partial obstruction of the gas port that popped free when I probed it.
I don't know what to make of the lack of gap behind the gas block. It's the same block that BCM mounted when I bought the assembled upper, and it functioned fine for a long time before this hiccup. Would the port in the gas block normally be large enough to make up for the block being slightly too far back due to no hand guard gap space? And, what the heck is the proper term for that mating surface behind the hand guard cap groove??
Thank you all for your help and feedback.

[MQ105]

Being that BCM assembled the upper with a FF hand guard, there probably wasn't a gap for a hand guard cap. Some LPGBs are made to fit with a gap, others without. One way to determine what you have is to rotate the GB 180, so the set screw hole aligns with the gas port. You'll be able to see how the GB should fit relative to the shoulder.
Now that the gun is running, this is theoretical...

[Iraqgunz]

Did you put new red Loctite on the screws for the gas block?

UPDATE, now that I've recovered from dealing with too much snow and actually had time to work on the rifle.
I removed the hand guard, removed the gas block and gas tube. I cleaned everything up, including blowing out the gas tube and ensuring that the gas key was not obstructed. Gas key screws were still tight.
I also probed the gas port in the barrel. It felt like something popped loose when I stuck the probe through, but I'm uncertain.
I reassembled with special attention to aligning the gas block set screws directly into the barrel dimple. I'm certain that it's aligned the same as when I got it from BCM. Interesting note: with the rear set screw indexed into the dimple, there is no gap between the gas block and the mating surface on the barrel behind where a hand guard cap would fit. That's a poor description but I don't know my barrel nomenclature well enough to use the proper term.
I test fired today. The rifle ran perfectly: no failures of any kind over about 80 rounds of three commercial loads and 6 different magazines. Locked back on empty every time. I'm confident that the problem is solved, and I'll report further if this turns out to be wrong.
I think that I either had the gas block slightly misaligned, or that there was a partial obstruction of the gas port that popped free when I probed it.
I don't know what to make of the lack of gap behind the gas block. It's the same block that BCM mounted when I bought the assembled upper, and it functioned fine for a long time before this hiccup. Would the port in the gas block normally be large enough to make up for the block being slightly too far back due to no hand guard gap space? And, what the heck is the proper term for that mating surface behind the hand guard cap groove??
Thank you all for your help and feedback.