Went to the range today with a built upper that has never been shot. Equipment is a rainier upper, sig mp400 14.5" barrel, stock mp15 BCG, and a lower profile gas block. My lower is a mp15 sport lower with stock everything. When I was able to get a shot off the brass ejected fine (or enough to not get stuck in the chamber) but after one or two shots it would fail to chamber the next round After I chamber the round manually it will shoot that round fine but then the bolt would go half way and not chamber the next round fully. I suspected a gas issue so I took the gas tube off and checked to see if it was crimped in anyway which it was not. Then checked the alignment of the gas block via the paper clip method and it clears into the hole in the barrel. I haven't tried compressed air into the gas tube yet but it seems to clear the other side when I blow into it by mouth. I can't seem to figure this out. My BCG worked with my old upper and I haven't touched it since I shot it last. Could something have gone wrong with the BCG not cleaning it the last time I shot? I hope its not that fragile. I did the stand the bcg on the bolt thing and it stayed up.

Have you checked that your gas rings are properly aligned? The gaps in the rings staggered?

yep checked that, checked the gas key, and block alignment and all check out. Now I didn't lub the upper and barrel so the fact that it was never shot and brand new can that have something to do with it cycling slower? i would see how it would effect it that dramatically but i could be wrong.

What ammo were you using?

PMC 5.56 55gr. Turns out it was a lube problem. Shot 75 rounds without a hitch yesterday. Lesson learned that like a women guns like it wet!

Giggity.

Glad you've diagnosed the issue.

I'm delighted that you solved your problem.

Indeed, ARs absolutely love running wet ... and it amazing HOW WET. It seems like the more light oil a fella uses, the better the M&P-15 runs.

I guess, some lessons we have to learn over and over. Happily, this is a simple one.

God Bless,

Steve Timm

well turns out i still might have a gas issue... went to the range friday and same thing happened but not as bad as before. keep in mind I went to the range 3 times with about 175 rounds put through and didn't relube so maybe lube is still the issue but I doubt it. Fri it was shooting fine until about 20 rounds in and it started to short stroke again where the bolt wouldn't chamber the round all the way. Its not a clearance issue because when it happens I pull back the charging handle and let go and the bullet chambered half way chambers correctly. Only problems I can think of is #1 gas hole in the barrel isn't big enough causing restricted gas flow (doubt it but who knows), #2 the upper is still brand new and only has 175 rounds through it and needs to be broken in, #3 gas leak, #4 gun doesn't like the ammo (which i hope isn't the case since I want a gun to be able to handle anything)

btw: bought a niboron BCG to hopefully avoid the lube issue if it really is that sensitive to hopefully get a smoother cycle but I'm going from a ar15 semi bolt to a m16 bolt which is heavier. hopefully it would make matters worse.

I have found that the short barrel weapons do function betterwith 5.56x45 ammo instead of .223. The shorter barrel allows for less gas forcing back the bolt and the stronger powered ammo helps with that. Gas rings on the other hand do not have to be staggered as they move constantly during operation and will become aligned at some point anyway. Even when directly lined up they will not leak enough gas to matter and wont stay alligned. The most common cause of short stroking that I have found among shooters is not having the weapon securely locked into the shooters shoulder. While the 16" variants of the AR-15 will operate well with a light hold, the short barrel weapons wont, they need to be held securely in the shoulder. I have found this while instructing other LEOs in the use of an 11.5 inch barreled weapon.

i've only shot the 5.56 pmc but from what i hear the 55gr is pretty light on this ammo. I shot again today and first shot jammed but then the next 3-4 mags went flawless. then occational jam again. this time though the jam is from the empty casing still being in the chamber. its not a lube problem as i just bought a niboron bcg and lubed that up. the one constant though was that the bolt doesn't lock back on the last shot which makes me think that maybe the bolt goes back just enough on most shots but has the occasional jam where it doesn't go back enough. that would explain why i can go through some mags without jamming. so without enough force bringing the bolt back what can it be? gas tube looks fine, gas block is tight. could it be a defective barrel in any way?

What magazines are you using? I had some from asc that wouldn't lock the bolt open on the last round.

pmags gen 3- 15/20

Do you have any other magazines to try besides gen3? usgi style? I'm just trying to eliminate the obvious things before looking for other ideas.

yep tried the stock 10mag and same...

not sure if this is normal but there seems to be a lot of carbon build up around the gas tube by the gas block and around the roll pin area...

just decided to test the gas flow and blew compressed air into the gas tube from the upper side. air definitely was coming out of the connection between the gas tube and gas block. is that normal or is there an issue with either the seal of the gas tube or alignment of the gas block?

it looks excessive to me. mine shows some carbon during a day of shooting. but not that much. you could be losing pressure to your bolt. losing pressure is probably why it's not cycling. send the rifle in or try replacing the gas tube.

What length gas system do you have?

carbine... i thought it was a gas tube issue so i changed it and still no go. i gave up and ended up dropping it off at the gunsmith by me. cross my fingers he can figure this out.

Maybe a defective gas block. Keep us posted.

Check the gas block and gas port next

how common is it to have a gas port thats not large enough on barrels?

If its a sig sauer barrel Im sure that it is to spec especially for a carbine length gas system, but you never know things always slip through the cracks at quality control, thats why I would have it checked, I dont think its very common but like I said it can happen did you put the upper together yourself or buy it from someone reputable? Just check it and the gas block, you just have to find the the source of the problem by process of elimination. I just looked at teh Sig sauer website and it seems that all of there sig sauer barrels of 16", so did you chop this barrel down yourself or did it come in that length from Sig Sauer? If it was cut down it is possible that the gas port would have been sized for a 16" barrel (needing a smaller gas port) that for a 14.5 inch barrel, EITHER WAY CHECK IT

turns out it was the gas port... compared it to another barrel and was def smaller. gunsmith drilled it open more and all is good!

A note to all AR based platform shooters on this:
When you clean your bore, I recommend you DO NOT use bore foam type cleaners in these rifles. It tends to be sticky as heck and WILL clog up gas ports and tubes.
Also, I recommend you use a dedicated upper or a dedicated barrel for the .22LR conversion thing. While it is nice to save on ammo, the 22lr tend to load gas tubes with powder and lead/ copper gilding. I have had to really go thru several guys' rifles to get them to run right after a lot of 22 has gone down the tube on their rifles.

Chrisfc- we have found that after 15,000 rounds or so, the gas tube tends to open itself up a little bit, almost guaranteeing the port size is no longer a problem!!!