My 10.5" AR is short stroking

I built this rifle about 6 years ago and up until now it has never given me any issues. During my last to trips to the range my otherwise trusty rifle has been giving me trouble. The trip before yesterday I was shooting the PMC Bronze ammo, and I would occasionally have a round extract but not feed another round. Yesterday with Remington ammo it would short stroke almost 90% of the time. 3 times the bolt over rode the round and jammed it onto the feed ramps. I have shot all manner of cheaper ammo with no issues, and the rifle was well lubed and reasonably clean (there is hardly any carbon on the bolt).

I figured that the PMC was just crap ammo, but now the Remington is doing the same thing (worse actually) so I am starting to wonder if there is something wrong with the rifle.

Pertinent info:
Upper: LMT
BCG: LMT FA (checked the gas key and it is solid)
Barrel: WOA 10.5" Wylde chamber
Buffer: LMT "H"
Mags: 3 different Pmags 30 & 20 rounders

Any advice would be appreciated.

See if you can pull the gas block and make sure the port is clear. Also check the tube and gas block/FSB. I'd run some hotter ammo through it to see if that cures the short strokes. It might be a bad batch of ammo, but you never know. HTH and good luck.

Edit: the gas block might be loose if it is a setscrew type and not pinned in place.

Do you have a forearm over it or is it a FSB/clamp type gas block?

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How many rounds do you have through it? Could the recoil spring be wearing out? If so, it's a simple fix.

It totally slipped my mind that my gas block is set screwed and it hides under my rail, so I wouldn't notice if it is loose. I will have to check that (ideally I need to get it pinned at some point).



I don't think there are that many rounds on the gun, but I may replace the spring for good measure.

Thanks!

Couple things. For sure check the gas block. I recently checked mine and it wasn't loose but the screws were easily turned. Could have led to a problem. Gonna try rocksett on the screws or get the GB pinned.

I have had bad luck with Remington UMC ammo, seemed under powered. Never a prob with PMC.

Long story short. I had a small piece of debris floating around in my buffer tube and was having similar problems. Got it out and had no more problems. Same mags, same ammo etc. The piece was tiny and like a flake of metal. COULD have been blocking the tiny hole in the end of the buffer tube.

I did not see any mention of this in the above posts, but did you happen to check the gas rings on your bolt to see if they have spun around and lined up the gaps so gas is getting past them?

just check your gas rings in general. replace if they are worn.

Tag for reference.

Checked that... gas rings are staggered and the bolt will support the weight of the carrier, so I am led to believe the gas rings are good.

I unscrewed my rail and the gas block was not loose, but a got an 1/8th of a turn on the screws. Took a pic of the gas block, and barrel to show the carbon build up from the gas tube/block. I have never cleaned under here and this upper has seen some full auto time on a firend's FA lower about 500 rnds), but this seems like a lot of carbon.



Any thoughts?

Try another carrier from a friends or one of your ar's. Give a standard buffer a shot. Take off the gas block and inspect it for proper gas port alighnment. If all else fails enlarge you gas port. A 10.25" barrel port should be no larger than .093" and no smaller than .078"

That amount of carbon on the barrel looks normal and I wouldn't sweat it. As for your short stroking issue, I'd blame the ammo. Keep it in the 5.56 realm.

You stated you were using a H buffer most with 10.5 including myself run an H2.

Going to a heavier buffer will not fix short stroking. Going lighter will. Usually the first indicator that you have too heavy of a buffer in your carbine is that it short strokes. There are other issues at work here.

Gas rings being staggered or not wouldn't affect the way your rifle is cycling or failing to cycle.

Check the screws on the gas key on your carrier for looseness.

It's either undergassed or something is slowing down the BCG. Gas tube itself could be the culprit.

I disassembled the gas block and cleaned the port and gas tube. The carrier key seems tight and properly staked.

I borrowed a regular buffer and I have a spare BCG to swap if I need to. Been waiting for my local store to get some PMC and XM193 to see which helps.

Thanks for the suggestions guys.

Just because I went back and was able to see the picture. A little bit of gas leaking from the gas block is normal. It shouldn't be a lot, but a little bit of carbon is fine. If it isn't leaking, the gun isn't running right. That's actually a pretty good indicator of a gas port obstruction if it DOESN'T leak.

From the picture though, It doesn't look like it's leaking enough at the gas block to affect operation. Was this a factory upper or a build? My first guess is anemic ammo and/or a too small gas port. A 10.5" should run fine with an H buffer. It should ideally run with most ammo with an H2 buffer.

Federal XM193 is full power that may help too. The gun was designed to run on 5.56 not 223.

Did you check your gas port size when you cleaned the port? What was the gas port size?

Yeah, I had made a mental note to get the size of the gas port, then promptly used that memory for looking at a shiny thing.

OP. Did you test to see if you gas rings are worn?



From my research and exerience it is the exact opposite of what you said dealing with SBR's. Going to a heaver buffer does fix short stroking. In an SBR it is usually over gassed which will cause the bolt to unlock early before the chamber pressue drops. The carrier will not travel fully to the rear (short stroke). The heaver buffer holdes the bolt closed longer which increases dwell time.

Jk

EDIT: Actually, I missed a couple details initially. Reading comprehension probably helps.

OP, what's the approximate round count with this rifle? When was the last time it was taken out to fire before it started having this issue? Did it give you any problems then? Explain exactly what is happening and be as detailed as possible. Give information about whether you're getting excess gas to the face, etc.

If I had to guess, I would say around 2K if I try and error on the high side. I don't have as much disposable income as I used too, so range trips with the AR's are less often, say 6 months or so since my last range trip with her. That was a a 3 gun match and she ran like a top with no issues what so ever (don't really remember what ammo I was using). I have ran a good amount of Silver Bear ammo through it and no problems either.

I am not arms room anal about how I clean my rifles, but I keep carbon off the bolt and carrier, and use CLP on the moving parts.

My local store has XM193 in stock, so hopefully I will be making a range trip this weekend. I'll grab another box of the Wally World Remington to see if it is the culprit. I will see if I can task the Wife to find me a H2 buffer in Tucson tomorrow.

I think you need a slightly heavier buffer (H2). I wouldnt go opening up the gas port until you've, without a doubt, narrowed it down to that. The heavier buffer will increase the dwell time and allow the bolt to travel all the way to the rear and allow the next round inline time to line up with the bolt. I would bet that your actually not short stroking or are under gassed. Sounds to me like the lighter buffer is allowing the bolt to close too quickly, therefore not allowing enough time for the mag spring to push up the next round in time.

Have you noticed the issues worsening towards the end of an empty mag?

I assure you taking a drill to anything on my rifle will be the step right before buying a whole new upper. I have seen what a well intentioned chuckle head can do to a gun with power tools.

The H2 buffer was a NOGO as apparently they are rare as hens teeth in Southern Arizona.

As long as the weather is tolerable I will be testing it out tomorrow.

Tag as this is pertinent to my interests in a 10.5" upper.

Update please. Hows it cycling?

I posted this in another thread. Not an SBR, but similar issues.

My Bushmaster lower and 16" Noveske upper hasn't run right since I've owned it. Upon the advice of a very fine person who just happens to be a member here, we changed the buffer spring, replaced the buffer with an H2, replaced the ejector spring, and replaced the extractor spring (which was weak as hell). It how runs 100% over the last 200. I had trouble running 30 without a malf before.

Small update:

Took her to the range yesterday, and I was not able to fully accomplish everything I had set out to do. Between ammo availability and the weather this was not as productive as I had hoped, but the rifle appears to be running better that before I stripped it down.

What I tried:
I did run 40 rounds of the PMC Bronze that had caused me some issues with this rifle. I had no malfunctions this time, and all the brass ejected into a neat pile about 2ft away from me.

The Federal XM 855 ran flawless as was expected by most. The brass ejected into a nice neat pile also, but it was about 1ft farther than the PMC.

I swapped out the buffer and buffer spring and there was no change in performance with the rifle. I plan on ordering a H2 buffer and seeing if that will allow me to shoot some of the less powerful ammo.

I did encounter one short stroke, it was some 50gn PMC varmint ammo I found in my misc ammo box. It happened on one round and the rest of the 20 rnd box was fine. I didn't get a good idea of how this ammo was ejecting because my range helper (6yr old son) was doing me a favor and policing my brass.

I used a little more CLP on the rifle that I usually do, and that may have also been a contributing factor.

I didn't notice any gas in my face, but I tend to be oblivious to things like that. The EOTech tends to keep my head up more than using the NTCH.

Open action items:
I wanted to set up a chrony and see the velocity difference in the ammo, but it was too damn windy.

Walmart didn't have the Remington ammo I used last time, so I was't able to verify that it was the culprit.

I know that I changed too many things at once to identify any variables as to what is causing the issues, but I plan on making another trip where I have some of the weaker ammo out there, and seeing what difference in velocity and extraction each brand has. On the bright side, this has been the most I have made it out to the range in years.

I dont want to be a negative nancy here, but I fell that this is ALLOT of trouble to go through. Why not just order an H2 buffer and get it over with? Shoot once with 55grn ammo, shoot once with 62grn ammo, and if it all works, call it a day.

Maybe I'm UNDER analyzing the problem. I've had problems like this before in 14.5 and even 20" rifles. I just did a simple switch of some buffer and spring combos and the problem was fixed in that same range session...

I wanted to try a H2 buffer, but none were available locally. I am trying some of the things other members suggested, and providing results of what I tried, mainlt for informative purposes.

Short stroking generally is related to gas problems or ammunition withnot enough power. If it is not cycling far enough back to even strip another round, the recoil spring or buffer will not have anything to do with this. It sounds like you have checked about everything. Here would be my list of checks.
1. Check gas block for proper fit
2. Replace the gas tube
3. Replace gas rings (they are cheap and get worn, loose over time)
4. Check bolt carrier key, make sure the carrier key bolts are properly torqued and staked

SBR direct impingment guns can be kind of picky with ammo, etc, depending on the manufacturer.

Good luck

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