Short cycling resolved

[Rjin316]

You need to ck some lengths on components. That spacer is there for a reason. It is a mechanical stop for bolt travel. Just sayin,

I definitely need to check lengths. Maybe some of you can help me out.... I first want to know what the weight of the buffer is. Is it a standard ar15 buffer? I was under the impression it is a "carbine" buffer, but proprietary to s&w? Theres some confusion to that. Anyway, my buffer seems to be heavier than my ar15 carbine buffer, but havent weighed it. Next, the length of the buffer spring. Next the length of the s&w spacer.

Are your guy's actions really stiff (difficult to charge)? I mean even after lots of lubing, when that spacer was in there, it was pretty darn stiff. Like i think i need to work out more stiff. Even doing simple things like press-checking was unusually difficult. After spacer removal, while still stiffer than a standard ar15, charging the gun felt much smoother. I can do press checks w/o holding it against a body part.

I did check gas alignment with compressed air and it was gtg.

I will say that the only ammo that runs well is black hills 168. Out of 60 rds, i only had 1 or 2 short stroke failures. Whereas, federal gmm 168 had 3 ss failures in only 20 rds. Ppu 168gr does no better than ppu 145gr as far as failure rate. Whats really weird is that sometimes i only get a couple failures the entire range sessions even shooting the 145gr's. Then the next day, they fail almost every other round it seems like. What really put it over the edge was with the freedom munitions 168gr rds as they failed constantly with a-max's ftf rate 1/1.

Again, even when its not short stroking, the round fires and i can feel a slight delay in the cycling even when it does pick up the next round. When it short strokes, the delay is even more noticeable. Kind of like "BANG-CYCLE" when its supposed to be like "BA-cycle-NG".

It really does look and feel like the buffer system is dicked up in some way. Either the spring is too stiff, too long, or the buffer is way heavier than it is supposed to be. Every other person who experienced short stroking issues in their m&p that i read short strokes only with nato rds or pretty much any round 150grs or less. As soon as they run heavier gr bullets, their problem goes away. Not mine...

 

[Rjin316]

After closer inspection, the buffer weight is the same as my carbine buffer from my ar15. And the bottom part of the bcg where the bottom rails are, made some nice gouges on my lower receiver extension threads. Luckily the gouges dont affect the buffer tube placement but...phew. I also noticed that my bcg was bone dry which tells me it soaks up lube like crazy....i only shot maybe just over 60 rds last time out and i just lubed before i started shooting, and the entire bcg was bone dry afterward.

So i can only think of 2 problems....the bcg just soaks up lube way too fast, hence it getting progressively worse the last time out. Maybe i need to run some grease or froglube or something (using break-free LP now). Or, its under-gassed due to the gas port being too small. I did install a .750 LP gas block and the port size on the block was bigger than the gas port on the barrel which should be normal. Again, alignment is good. I really cant think of anything else. Im gonna try to go out today and im gonna drench the bcg in lube to see if that helps. Ill update...

 

[copemech]

I definitely need to check lengths. Maybe some of you can help me out.... I first want to know what the weight of the buffer is. Is it a standard ar15 buffer? I was under the impression it is a "carbine" buffer, but proprietary to s&w? Theres some confusion to that. Anyway, my buffer seems to be heavier than my ar15 carbine buffer, but havent weighed it. Next, the length of the buffer spring. Next the length of the s&w spacer.

Are your guy's actions really stiff (difficult to charge)? I mean even after lots of lubing, when that spacer was in there, it was pretty darn stiff. Like i think i need to work out more stiff. Even doing simple things like press-checking was unusually difficult. After spacer removal, while still stiffer than a standard ar15, charging the gun felt much smoother. I can do press checks w/o holding it against a body part.

I did check gas alignment with compressed air and it was gtg.

I will say that the only ammo that runs well is black hills 168. Out of 60 rds, i only had 1 or 2 short stroke failures. Whereas, federal gmm 168 had 3 ss failures in only 20 rds. Ppu 168gr does no better than ppu 145gr as far as failure rate. Whats really weird is that sometimes i only get a couple failures the entire range sessions even shooting the 145gr's. Then the next day, they fail almost every other round it seems like. What really put it over the edge was with the freedom munitions 168gr rds as they failed constantly with a-max's ftf rate 1/1.

Again, even when its not short stroking, the round fires and i can feel a slight delay in the cycling even when it does pick up the next round. When it short strokes, the delay is even more noticeable. Kind of like "BANG-CYCLE" when its supposed to be like "BA-cycle-NG".

It really does look and feel like the buffer system is dicked up in some way. Either the spring is too stiff, too long, or the buffer is way heavier than it is supposed to be. Every other person who experienced short stroking issues in their m&p that i read short strokes only with nato rds or pretty much any round 150grs or less. As soon as they run heavier gr bullets, their problem goes away. Not mine...

Read my post #9 above. Yes, I use a light coating of grease on my bolt slides, moly or hi quality wheel bearing waterproof grease.

Not sure about the DPMS tube you have,,or if theirs differ between standard AR and .308, but I think the 2" spacer is designed to put a standard AR rifle tube back to the necessary dimension for the stock and standard carbine buffer and spring so your working length does not change from stock. Therefore inside depth is 2" longer than the stock tube. Although I have not changed mine, others have without issue.

And yes, they are stiff compared to an AR.

 

[Rjin316]

Well i feel like a dumb ***. I found out what the problem was. Not enough lube.... Im so used to my ar15 running 200 plus rounds with only a few drops of oil on each rail that i thought the same would hold true with this gun. Too optimistic... The few drops weren't enough so it really did soak it up real quick....it started choking right away. This thing was bone-dry at around 60 rds. It really Ran flawless today after i applied LP on the rails like mustard on a hot dog. The freedom 168 a-max was the only fail today...pretty much too light of a load and wouldn't cycle the gun. Also noticed the groups opening way up when the lighter loads fire, which were most of them. I ran some 150gr and fed gmm right after to confirm and it ran them both fine. I emailed freedom about it already so we'll see what happens.

 

[fluxican]

Copemech, I found your advice in this thread very helpful. My M&P 10 would not cycle Lehigh Defense 145g Controlled Chaos ammo (March 2016), but works flawlessly now after I enlarged the gas port per your recommendations A local gunsmith charged me only $20 for the modification. I would suggest anyone wanting to use Lehigh's controlled chaos line enlarge the gas port first (I've read posts elsewhere talking about Lehigh's factory load of the 145g being a bit light). My rifle was completely stock when I first experienced this problem, so although I would say it is actually more of an ammo problem, I wanted to be able to use this ammo & enlarged the gas port.

 

[copemech]

Copemech, I found your advice in this thread very helpful. My M&P 10 would not cycle Lehigh Defense 145g Controlled Chaos ammo (March 2016), but works flawlessly now after I enlarged the gas port per your recommendations A local gunsmith charged me only $20 for the modification. I would suggest anyone wanting to use Lehigh's controlled chaos line enlarge the gas port first (I've read posts elsewhere talking about Lehigh's factory load of the 145g being a bit light). My rifle was completely stock when I first experienced this problem, so although I would say it is actually more of an ammo problem, I wanted to be able to use this ammo & enlarged the gas port.

Thanks for the feedback, as I recommend it for even mil-spec loads. That port size is still conservative by many standards.

 

[Zelicur]

I bought a ton of ZQ1 online for near nothing. I find that they run as long as the bolt is SOAKED. Once it dries up the pressure isn't enough to fully stroke. I plink it, and the brass is ok to save. On a side not, my 10 LOVES Tul 150gr.