Previously reliable AR suddenly not cycling.

[TomMcC]

Roll pin in the buffer isn't protruding and doesn't seem to be loose.

No interference that I can detect while hand-cycling the bolt. I can ride the charging handle and slowly ease the bolt home and it will still go into battery without issue.

I did brace myself for potential cycling issues right after changing the gas rings out, but it functioned fine. It seems odd that now after at least a couple hundred rounds that they would suddenly cause the gun to choke. Again it's not like I've been having intermittent short stroking that's been getting progressively worse. I went from 100% function to having to hand cycle every cartridge into the chamber within a matter of a few rounds.

I'm wondering if there's some obstruction in the gas key/carrier that I'm not seeing, however I was able to feed a twist-tie (I didn't have any pipe cleaners on hand) from the gas key down into the carrier, it's pretty flexible so anything offering resistance in there probably would have stopped it.

Could there be some sort of obstruction in the gas tube itself?

While you're looking for a spare BCG, you could see if there is an obstruction in the gas key. There have been cases of popped primers winding up in the gas key.

[Hmac]

Gas tube wear? It is hard to say from the last picture, but it looks like the mushroom at the end could have worn down a bit. That could be a cause for the symptoms you are experiencing. Slightly misaligned gas tube is often missed in regular cleaning and inspection, and it causes accelerated wear to the gas tube mushroom.

I had this happen....previously reliable rifle suddenly not cycling. Ultimately, after reinstalling the gas block and trying a different BCG, I changed out the gas tube even though it looked OK. That fixed it.

[TomMcC]

I don't know how many rounds the OP has on his gun, but after looking at the pictures of his gas tube, do you think it's worn out? Honest question. It looked ok to me.

[Circle_10]

A couple thousand probably. Most of that in the last three months.
I'm not using an A5, it has a standard rifle receiver extension and a standard rifle buffer, 16" barrel, mid length gas system.

This is the most logical place to start. Until the BCG is eliminated as the source of trouble, anything else is a waste of time.

Intellectually, I know this is correct, however it could be as long as another week before I can get to the range again (it gets dark at like 4pm currently) so I feel compelled to pick over the gun piece by piece in the meantime.

[Circle_10]

While you're looking for a spare BCG, you could see if there is an obstruction in the gas key. There have been cases of popped primers winding up in the gas key.

Seems like there isn't anything in there but I'll have to try it again with an actual pipe cleaner instead of the twist tie I tried.
Luckily I have a spare carrier that I can drop in, it's seen some use (200-300 rounds or so) so I know it works.

[ssc]

I agree with Mistwolf. Run a different BCG and see it the problem is cured. If so, then investigate the BCG. I assume you have completely disassembled the BCG and the bolt and checked it for issues? Let me preface my next comment; I just got new glasses and it is hell for a week trying to get use to new glasses for me. If I am seeing the picture correctly of the gas tube as it enters the upper receiver, it looks like there may be a gas leak. I could very well be seeing it incorrectly or misinterpreting the picture. It appears to look like a bit of blow-by, but I may be misinterpreting the picture.

Cheers, Steve

[Hmac]

I don't know how many rounds the OP has on his gun, but after looking at the pictures of his gas tube, do you think it's worn out? Honest question. It looked ok to me.

Mine looked OK too. Swapping it out fixed the problem.

[TomMcC]

Mine looked OK too. Swapping it out fixed the problem.

Gotcha. If nothing else comes to light, it would be something to try. I'll be keeping this for future use... "don't be fooled by appearances".

[Circle_10]

I agree with Mistwolf. Run a different BCG and see it the problem is cured. If so, then investigate the BCG. I assume you have completely disassembled the BCG and the bolt and checked it for issues? Let me preface my next comment; I just got new glasses and it is hell for a week trying to get use to new glasses for me. If I am seeing the picture correctly of the gas tube as it enters the upper receiver, it looks like there may be a gas leak. I could very well be seeing it incorrectly or misinterpreting the picture. It appears to look like a bit of blow-by, but I may be misinterpreting the picture.

Cheers, Steve

Yes, BCG was taken apart and examined, nothing jumped out at me as seeming awry, but I'm no armorer. I thought the area around where the gas tube enters the receiver looked somewhat strangely gunked up as well, which is why I included a pic of that spot. I'm wondering if the tube could have ruptured or something just inside the receiver, but I think I'm going to try a bolt carrier swap before I go yanking the tube out because it could also be just excess grease and crap that's gotten in there and gotten cooked by the gas tube.

[Titan74]

I don't know how many rounds the OP has on his gun, but after looking at the pictures of his gas tube, do you think it's worn out? Honest question. It looked ok to me.

I don’t remember the exact specs for the gas tube, but I think the mushroom for a new tube should measure approximately .180 and problems will start at around .170 or thereabouts. Visual inspection is going to be difficult. The tube can also erode at the gas port, which is not easy to diagnose without removing it.