View Full Version : Thoughts on this malfunction?
BrigandTwoFour
10-19-17, 20:17
This one is a bit of a head scratcher to me. I’m on day one of a four day training course, rifle is running great for most of the day. Throwing a nice neat pile of brass to the 4:30, running like a sewing machine from 0800 until around 1:30.
I’m estimate about 300-350 rounds fired to that point.
First malfunction is a failure to go into battery. Did not strip the next round from the mag and just got stuck about midway through the cycle. I do immediate action and keep going through the rest of the drill just fine. Then I get the same malfunction again twice in a row in the next drill.
Bang-failure to feed-immediate action-bang-failure to feed-drop magazine and replace with a new one. Gun runs fine for another magazine.
I pulled the original magazine out of the rotation and chalk it up to being an old mag.
Next two mags run fine, and then the fun begins.
Failure to feed, same way, two more times. And then I start seeing the bolt has partially stripped the cartridge, but impacted it against the feed ramp.
At that point, the trainer checks the rifle. Had block (pinned) looks ok. Bolt is shiny with lube. He does immediate action and fires the rest of the mag without issue.
I set that gun down and grabbed my backup rifle to finish out the day. I just got doing cleaning everything g up and can’t seem to find anything that would cause something like that. Any thiughts?
Edit: I did not lube the gun at all during the day. I topped it off with some slip2000 the night before and just left it alone.
It is short stroking. Details about the rifle would be helpful.
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BrigandTwoFour
10-19-17, 20:57
Centurion 16” barrel, mid gas with pinned gas block
BCM bolt carrier
BCM upper
BCM KMR-A
Vitor A5 standard buffer
Standard rifle spring
It’s the only “frankengun” I own, but it has run pretty flawlessly for years. This is the first course I’ve taken it too. I plan on running my BCM rifles for the rest of the course, but I do want to have this one fixed.
BrigandTwoFour
10-19-17, 21:02
Also, the trainer and I both noticed it looks like the bolt lugs were rubbing against the feed lips on a few mags. It might be that the friction of that rubbing and the lube working itself out were enough to short stroke.
I forgot to mention the ammo. I’ve been shooting Magtech 62gr FMJ
5.56 Bonded SP
10-19-17, 21:25
What weight is the buffer? What kind of mags are you using? I see the A5 has everything from H to H4 weights on their website.
The two times I had similar experiences where this
1. Piston gun set on suppressed gas setting with no suppressor on
2. USGI metal/aluminum mags that became out of spec. ( I have trashed many USGI type mags and now run Lancers ).
I have played with suppressor gas setting with no suppressor on purpose and have noticed, if you hold the gun very tight in your shoulder it will cycle more reliably than with a loose grip. With a loose grip and suppressor setting with no suppressor, I had identical malfunctions as you describe.
But these things I mentioned may have nothing to do with your problem.
BrigandTwoFour
10-19-17, 21:38
All Pmag M2s that were gently and then stored disassembled while I was stationed for four years in California.
It’s the standard buffer that comes with A5 kits. H2, I believe.
5.56 Bonded SP
10-20-17, 00:53
I would try a lighter buffer.
Test with another one of your BCG's. Take a close look at your gas key. Just spitballing here for what appears to be a short stroking issue.
Did it ever fail to lock back to the rear on an empty mag?
Are the feed ramps on the barrel extension and upper receiver correctly aligned/configured?
Action spring in spec/newer, and not worn out?
Test with another one of your BCG's. Take a close look at your gas key.
This is a good suggestion.
Sounds like you have a gas block that is clogged, shifted or lost seal. You need to check you gas system thoroughly, tube and all.
Hammer_Man
10-20-17, 07:53
Check the area around your gas key to make sure there is no gas leaking past it. Also, check the gas rings on your bolt, they may be worn out. Check your gas block to ensure it is aligned properly, and ensure it is not clogged with soot.
Test with another one of your BCG's. Take a close look at your gas key. Just spitballing here for what appears to be a short stroking issue.
This. I had the same problem, turned out to be a slightly loose gas key.
turnburglar
10-20-17, 11:34
idk why but to me it sounds like once friction started to build (350+ round count, rubbing mags) your action spring lost its oomph. I replace my action spring about every 3k rounds.
idk why but to me it sounds like once friction started to build (350+ round count, rubbing mags) your action spring lost its oomph. I replace my action spring about every 3k rounds.
+1
That's where I'd start for a fix. You stated above that it is a "standard rifle spring" and that it had run fine for years. Did you change the spring in that time?
BrigandTwoFour
10-20-17, 17:42
Yes, rifle spring was swapped for a Sprinco green last year.
I appreciate the input, guys. I’ll have to troubleshoot more once I get home from this course. I ran my backup rifle flawlessly all day today. I probably won’t make any changes for the next couple days.
Based on rifle running well for a long time, pinned gb, quality barrel/gb/gas tube, I'd bet the problem is within the bcg.
The action spring and buffer have nothing to do with this short stroking issue because these components have already been vetted in this rifle. And a a5h2 will run in anything, it isn't on the heavy side of this equation. If the spring was worn, the symptom would not be short stroking. Rifle action springs last much longer than carbine springs.
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BufordTJustice
10-21-17, 15:36
If you're seeing wear on the bolt from the magazine feed lips, those two things should NEVER touch.
Either the lower is out of spec, the mags were deformed, or the mag catch is deformed.
I would try those mags and that lower on the new upper.
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If the bcg is fine, you could ask a gunsmith with a borescope to try and look at your gas port without removing the GB.
A youtuber named Nolan's Mindset is having a similar issue with his AR pistol -it's stove-piping. He replaced the gas rings and extractor internals, but didn't mess with the buffer system yet (which is what I think it was).
I'm very interested in how this works out!
Magazines aren't supposed to rub against the bolt or carrier -- that friction (along with lube blowing off) is compounding energy loss. 300-350 rounds is a warm-up and one or two drills.
Lube on friction surfaces is required, not just oil on non-contact surfaces.
You seem to have a good build with quality parts, but your timing seems close and perhaps tempermental. Try aluminum GI mags.
Was a lock back check performed? A lock back check should be the first step when trouble shooting a problem of this nature.
Primer in the gas key if key is not loose. My short stroke was also a slightly loose gas key.
Was a lock back check performed? A lock back check should be the first step when trouble shooting a problem of this nature.
Yep. I asked this on the first page with no reply... :confused:
Lock back in my case was intermittent. 16" centurion midlength and bcg 5.56 pressure ammo.
BrigandTwoFour
10-24-17, 06:21
Sorry about dropping off replies, connection was intermittent until I got home.
It was locking back just fine on empty mags prior to the malfunctions starting. However, I did not test it again after switching to the backup rifles. I didn't want to suck up class time by messing with it. On disassembly and inspection, I didn't see anything that would obviously be a problem (loose gas keys, leaks, etc.). Until I can get back to another range and do more troubleshooting, I'm afraid this problem is just going to have to wait.
Of note, I did see this same issue years ago in a prior configuration. With the Centurion barrel, BCM BCG, carbine tube and ST-T2 buffer, I would see it do the same malfunction in very cold weather and underpowered ammo.
It's going to take some time to swap between lowers/bcgs and narrow down the problem- but I appreciate all the input here and I'll test it when I have time again.
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