Short cycling resolved

[Drag]

I just want to share my experience with the forum in case it helps someone out, like you have for me repeatedly.

I changed the stock out on my M&P10 from the oem adjustable carbine stock to a Magpul PRS using the required full length A2 tube (DPMS). I did this prior to firing the rifle at all in its original configuration (my bad). I did use the required aluminum spacer from S&W.

At its first outing the gun would repeatedly short cycle. I was using NATO spec ZQ1 7.62x51 M80 initially. It would fire a round and eject but not pick up the following round. It did also stovepipe and not fully eject a few times as well. The two guys I was shooting with that day were both former Army and one was actually an armorer while in the service. He suggested we try different ammo - this time real mil-surplus 7.62x51 NATO taken off of a linked chain that my buddy got his hands on, but the same results. I also fired the rifle while my buddies observed the bolt cycling from the side, and they stated the bolt was clearly not going back far enough. We concluded the gun was under-gassed.

I went home, and checked everything. Bolt was wet. And I made sure the recently installed Troy low profile gas block was perfectly aligned, and I had even used Rocksett to secure the set screws. I relubed the bolt again.

At the second outing a couple weekends later, same result - short cycling. I was thinking that the rifle just needed to be "broken in" but admittedly this was getting worrisome. My (former armorer) buddy was shooting with me at the range again. We started discussing about my installation steps for the handrail, gas block, and Magpul PRS. When I mentioned it uses a spacer to make up for the longer length extension tube, he suggested that we try it without the spacer. So we removed the spacer, lessening the preload on the buffer spring.

PRESTO! The gun started functioning perfectly after that point. What a relief! We sighted in the scope enough so that we were repeatedly hitting a smaller size steel silhouette at 100yds.

The rifle still only has less than 50 rounds through it at this point, but I was ecstatic that it was finally working like it should. At the next range day, I'm gonna focus on the feel to see if the bolt is slamming/bottoming out. And if so I will put the S&W supplied spacer back in to see if it will function properly once more (after supposedly loosening up a bit). There's a chance I may need a shorter spacer than what is provided by S&W, but as for now the gun seems to be working fine without it installed.

 

[Veeper]

Thanks for the update! Great info here.

 

[ZLDRider]

Funny, I had the exact opposite happen to me when I installed the MagPul PRS buttstock. But then, my AR-15 is a Daniel Defense DDM4.

Perhaps the difference was that I also got a different spring for the buttstock. It was longer and came with a hard plastic spacer.

The rifle cycled fine prior to installing the new buttstock but, afterwards, it wouldn't fire. It felt like it was short cycling but, it looked like it was going through a full cycle.

I looked through the box and found the spacer and added it.

After that, my DDM4 fired fine.

Now that I've got that working, I'm thinking of adding an adjustable gas block to dial down the gas pressure to improve accuracy.

These things are definitely temperamental, aren't they. I try to keep it down to one change at a time then take it out to the range to test before I move onto another change.

I wonder why we had opposite experiences?

 

[bergermeister]

Measure the length of the inside buffer tubes

If'n y'all would notice http://smith-wessonforum.com/smith-wesson-m-p10-rifles/426270-a2-stock-issue.html and measure the inside of the stock S$W folder buffer tube and then the inside of the replacement buffer tube (whatever it is); then space the longer replacement tube with a spacer to make them equal, they will almost have to run the same (as was) with the stock S&W folder, buffer and spring Cheers!

[use the stock buffer and spring]

 

[Drag]

Got that. I am using the stock buffer and spring.

 

[jcddoran]

Short Cycle

I had the close to the same problem with and AR I changed the buffer spring in. And when I did I added whats called a 'Twang Buster" Suppose to stop the rattle the buffer spring can make. I got it from Brownells. I fought with the problem for weeks until my dealer made a comment that got me thinking. Well, I took out the Twang Buster and all problems solved. My bolt finally locked back after the last round was fired, witch it would not do with the $5 Twang Buster.

 

[yqtszhj]

I had the close to the same problem with and AR I changed the buffer spring in. And when I did I added whats called a 'Twang Buster" Suppose to stop the rattle the buffer spring can make. I got it from Brownells. I fought with the problem for weeks until my dealer made a comment that got me thinking. Well, I took out the Twang Buster and all problems solved. My bolt finally locked back after the last round was fired, witch it would not do with the $5 Twang Buster.

I use white lithium grease as my twang buster. My AR with an A2 stock came with it in there and I cleaned it out. After a few trips to the range and getting tired of hearing the ring after everg shot i put it back in. Twang gone.

 

[syzorax]

Thanks for that info! As I've just picked up my MP10! can you tell me what type of forearm or hand guard you have installed ?and you say a Troy low profile gas block? Is that adjustable? Not alot of LR308 compatible parts out there! At least that I can find. Anyone that can help I would greatly appreciate!

 

[copemech]

Got that. I am using the stock buffer and spring.

So if I understand correctly, you have removed the required spacer to relieve the spring tension to get the gun to cycle and REMOVED THE MECHANICAL STOP THAT PROTECTS THE GUN AND YOU!

The Nato ammo you are trying to run is lower pressure thatn the gun is gassed for! The guns are gassed for US made Brass Hunting and match ammo at full pressures. They do not like the lower pressure rounds. If thet bolt overtravels with normal pressure rounds, bad things can happen! You are talking the bolt moving back two inches farther than it is supposed to!

That buffer had a hard rubber stop on it for a reason, a mechanical stop!

You are running stock spring and buffer, correct? Longer tube requires spacer!

Now if you want the gun to run lower pressure ammo, you will likely have to take the gas port from the stock .068 diameter to .075 to run cheap **** reliably.

Now take that as you wish before you break something!

 

[Drag]

Thanks copemech. You make a valid point. I'll reinsert the spacer and try some 175gr match ammo I have on hand for my 308 bolt. Apparently this saga isnt quite yet over.

 

[bergermeister]

Check out SLR adjustables

Thanks for that info! As I've just picked up my MP10! can you tell me what type of forearm or hand guard you have installed ?and you say a Troy low profile gas block? Is that adjustable? Not alot of LR308 compatible parts out there! At least that I can find. Anyone that can help I would greatly appreciate!

SLR adjustable Gas Blocks Good people, great adjustable gas blocks

 

[yqtszhj]

True what copemech said, that ZQ1 ammo is the only ammo i have had a problem with In my m&p10. Everything else including xm80 works fine. ZQ1 groups terrable too.

 

[Tbat]

My MP10 is in stock form and has ran ZQ1 ammo reliably so far.

 

[copemech]

After 1.5 years of fighting this gun, and much input from many, I could write a book on them! Not a good book! But not necessarily a bad gun!

If yours works and you are happy, great!

I want mine to eat anything, Tula , Wolf, foreign mil spec, us brass or whatever I have( or can get) if need be.

That magic number I threw out there on gas port size did not come out of my butt! Fact is my gun is at .073 right now. It works though. Is it ultimately reliable? Who knows. It was an experiment at the time.

.308 mid length, ok, I did not even get to finish my story speaking to this expert! He finished it for me.

Years of AR development, he knew immediately.

I am not gonna name names, but suffice to say he just runs a little website that begins with AR15.

You can always gas down with an adjustable gas block, mandatory if you are gonna run a suppressor. But you cannot gas up for lower pressure ammo the gun was not set up for, and from what I hear, if you do send the gun in for repair, all they will tell you is it worked fine using Remington 150 SP!

Be your own judge, look at your ejection pattern, needs to be about 3-4 oclock prolly, but this a bit different than the .223 round, yet same principals apply.

 

[syzorax]

LR308 Upgrades!

SLR adjustable Gas Blocks Good people, great adjustable gas blocks

Thank you! Bergermiester.

 

[potentpoefie]

OK, I have a problem with short cycling then as well. Now I know what to call it.

My story:

Thursday I finally got my M&P10 (sku 811308) and Friday night I headed over to a friend's house to get some parts fitted that I've had for awhile now. He has the workshop and all the AR tools a man can ask for.

Installed the Troy VTAC Alpha 308 LP 13" rail. Mostly because I've been more than happy with my M&P15 VTAC II. So big brother must look exactly like little brother. Of course to get this rail to fit a low profile gas block was needed. So installed the Troy .750 1" low profile gas block. Lubed the firearm and put it back together. Looked pretty sweet.

Got to the range Saturday and the first 20 rounds or so ran fine. Then it started to give failures to feed. Ejects the round just fine but fails to feed the next one from the magazine. Of course the first place I looked for the fault was the magazine. Had 4 there. 2 x Gen 3 PMags and 2 x Gen unknown PMags. All 20 rounders. Gave issues with all mags but it was intermittent. Sometimes it would work and other times 4 out of 5 shots you would have to rack the next round in yourself. Let's just say my charging handle and I really got to know each other on Saturday. The bolt would also fail to lock back after the last round, intermittently.

Now I'm not a gunsmith at all and figured it was just a new gun that needed running in but then again "we" did mess with it. I've looked at the bolt key and it is perfectly fine. Locked and pinned in. Didn't mess with that. My first guess, and I'm no gunsmith, is that the action isn't getting enough gas and thus doesn't have enough energy to take the bolt far enough back to chamber the next round. Could it be that the gas block is misaligned with the gas port on the barrel and thus not enough gas is coming through? Or is my choice of low profile gas block poor? Didn't see any mention of the gas block being only for AR15 platform.

What do you guys think could be the problem?

I see here the mentioning of ammo. We used the rifle at an open day so members of public could come and shoot various rifles. The ammo was the Norinco steeled cased surplus ammo. Don't have the name/specs of the ammo. Normally the Chinese ammo is not underpowered but then again I did not chrony it. Could it be the ammo?

P.S. The positive from the day:

1. Loved the Vortex Spitfire. Took it from the box. Mounted it on the gun and it was zeroed. Took it off during the day and put it back and it held it's zero.

2. Even though my M&P10 was a bolt gun most of the time the gun hits everything I put the sights on. Fine shooter.

 

[sgtsandman]

Did you run that ammo before and not have any problems?

My guess is that the port in the block is misaligned or the ammo.

 

[yqtszhj]

Try some different ammo I would say first.

 

[copemech]

That gas block works fine as long as it is aligned. The ammo is your culprit, but to make the gun run cheap ammo or mil spec, you would be advised to read my post above on gas port size in the barrel.

This really has nothing to do with the chrono speed of the bullet, but how fast the burn rate of the powder is and the associated pressure curves. The guns come gassed for slower burning, US made hunting ammo as used commonly in a 24" or so barrel to achieve full push!

Look at your ejection pattern and last round lock back as well.

The specific sizes I mentioned at .073- .075 do require specific number or letter drills in most cases, but they are cheap.

Enlarging the hole is easy enough with a quality sharp drill, just install a stop so you do not disturb the other side of the barrel bore. The metal is soft enough to drill easily without breaking a bit. If you do, you did not heed advise given by me and you likely have two thumbs in each hand, get help!

 

[potentpoefie]

Enlarging the hole is easy enough with a quality sharp drill, just install a stop so you do not disturb the other side of the barrel bore. The metal is soft enough to drill easily without breaking a bit. If you do, you did not heed advise given by me and you likely have two thumbs in each hand, get help!

Hahaha....why do you think I took it to my friend with the tools and the workshop.

I'll try some other ammo and see what that does.

As for another question about the ejection pattern and if it locks back. With the open day it was too busy to really look at ejection patterns and it doesn't consistently lock back after the last shot.

As for how it worked with the factory setup. I never shot it in factory configuration. Yes, in hind sight that was a bad idea but time wasn't on my side. We needed the rifle Saturday.

I'll have to go to the gun shop and see what 308 ammo they have in stock. At the moment I have 1050 of the Norinco ammo and 200 rounds of PMC ammo. Would the PMC ammo be considered good ammo to test the rifle with?