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Old 07-28-2018, 07:55 AM
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Default M&P15 short stroking

Hi All,

I bought it last yr. has about 1500 rounds thru it. Last 2 times
out started to short stroke. It will fire, extract and eject, BUT
not recock the hammer or chamber the next fresh round. Fails
the one shot hold open test.

After the first time, I took it down, cleaned and lubed it, took it
out again with the same results. Never had an issue before with
it and nothing appears broken or out of position.

Now here's the best part. Took the complete bolt assembly from a buddy's Ruger AR556, put it in the S&W and it functioned
fine. Cleaned the S&W bolt, put it back in, and got same short
stroke condition.

Anyone run into this?
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Old 07-28-2018, 08:09 AM
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It would seem to be a bolt issue check and make sure the Key has not loosened and replace the gas rings on the bolt they don't generally get that worn at 1500 rds but it's possible if they haven't been keep lubed, it's a cheap replacement. If the bolt hasn't been keep lubed check the bolt carrier to see if the interior is scored.
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Old 07-28-2018, 08:16 AM
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Hi Andy,

Thanks for the input. The rings look good and staggered. Key is secured and staked. Carrier isn't scored, and bolt runs "Wet". S&W wants it back to check it out, but if I can get it going again, I'd rather not send it back.

Looks like the Ruger gas key has a smaller inlet diameter compared to S&W. Wouldn't think this would be a factor but grasping at straws. Never had a problem before with it.
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Old 07-28-2018, 08:36 AM
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Ammunition issues come to mind. What are you shooting? Have you tried different ammo?

When you clean your rifle, you don't use those long pipe cleaners do you? I have seen those break off and stay in the gas tube.
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Old 07-28-2018, 08:40 AM
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With a little searching, all the info you need to solve this problem is out there.

Testing and replacing gas rings.

AR-15 Maintenance - Do You Need New Gas Rings?

Troubleshooting short stroking.

Diagnosing a Failure to Feed *Video* AR-15 or AR-10 – Greatoutdoordinary

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Last edited by Kodiakco; 07-28-2018 at 08:55 AM.
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Old 07-28-2018, 08:43 AM
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Quote:
Originally Posted by MichiganScott View Post
Ammunition issues come to mind. What are you shooting? Have you tried different ammo?

When you clean your rifle, you don't use those long pipe cleaners do you? I have seen those break off and stay in the gas tube.
I considered a possible ammo issue but the OP said it ran fine with a different bolt and I'm assuming it was the same ammo.
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Old 07-28-2018, 09:15 AM
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Hi All,

Thanks for the help so far. No pipe cleaners broken off in gas tube. Used Federal ammo exclusively, but tried PMC and Fiocchi.
All 3 function well with Ruger bolt but not with S&W bolt. Gas rings pass test OK with S&W bolt.


Now, I'm thinking that something in the S&W bolt is out of specs. Since the rifle works fine with the Ruger bolt, I should replace the S&W bolt. However now the costs are involved. I
can replace the bolt at my cost or just send it in and let S&W
work on it under warranty. Somehow the rifle isn't getting enough gas to operate with the S&W bolt, but functions fine with the Ruger bolt.
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Old 07-28-2018, 09:27 AM
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Please Keep us posted on what you find.
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Old 07-28-2018, 09:34 AM
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Hi Vonn,

I sure will, but at this point, really considering sending it back to S&W.
Just curious if anyone else ran into this condition. Just started out of the blue, and did all checks that you tube and internet provide.

Started looking at a new bolt assembly and found some around 70.00, but thinking why should I experiment and take on the cost of new parts when the rifle is basically new and covered under the warranty?
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Old 07-28-2018, 09:44 AM
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I have ran into that condition but was a rifle with many more rounds than you have through yours. I know it sucks to be without the rifle but it seems that if S&W will fix the problem that may be the best route to go, hopefully S&W will pay the shipping cost.
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Old 07-28-2018, 09:55 AM
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Hi Andy,

I talked to S&W yesterday and they want the rifle back for evaluation. I've also heard that some bolt carriers are out of spec
and need to be replaced. I first wanted to see if it was a common problem with the S&W crowd.

The strange thing in this is that it functions fine with another bolt assembly (Ruger in this case) but not with its own S&W factory bolt. Never had a problem before.

When I researched the costs of a new bolt assembly, I think I'm better off sending it in to S&W than incurring all the costs and experimenting to get it to work reliably. The rep I spoke to was very knowledgeable about the rifle series and said right off "You definitely have a gas problem. Let us take it back and see what we find."

Last edited by noro; 07-28-2018 at 09:59 AM. Reason: more info.
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Old 07-28-2018, 01:41 PM
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Could be a loose gas key on the bolt, out of spec bolt, or even gas tube. Do you notice any unusual marks inside the upper receiver? If the bolt is out of spec, there could be too much friction occurring to not allow it to cycle correctly.
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Old 07-28-2018, 03:53 PM
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Hi Jagular,

Thanks for the input. The gas key is tight and staked. The gas
tube must be clear since the rifle will function with the Ruger bolt.
Possibly the bolt was on the limit of the specs and 1500 rounds
later, is now out. Really thinking this is a job for S&W since its under warranty. to go further on my own would be buying a new
bolt and guessing at my expense.


Just curious if anyone else ran into this situation.
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Old 07-29-2018, 07:14 AM
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Quote:
Originally Posted by noro View Post
Really thinking this is a job for S&W since its under warranty. to go further on my own would be buying a new
bolt and guessing at my expense.


I think this is the way to go as it could be a combination of things.
S&W is best equipped to deal with your problem and at no cost to you.
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Old 07-29-2018, 07:31 AM
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Thanks for all the input, guys. Will give S&W a call tomorrow.
Will let you know what happens when it comes back.
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Old 07-29-2018, 01:06 PM
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I've run into this situation myself, but it wasn't an S&W AR. You did your due diligence and used good troubleshooting techniques. All you lack is experience interpreting the data.

noro, in a nutshell, your problem lies with the BCG. Either the rings are bad or the key leaks or both. Start by replacing the rings and don't worry about staggering gaps.

It's possible for the gas key to leak even when the gas key screws are tight. Sometimes, the mating surfaces are uneven or simply don't seal. You won't know until you actually pull the gas key off and check the carbon pattern. If there is carbon on the surfaces between gas key and carrier, there is a leak.

I had a carrier that started short stroking. The source of the problem turned out to be two fold-
1) Although the bolt passed the gas ring test, they were worn and leaking
2) Gas key was leaking. Screws were tight and there was no external signs of a gas leak. However, removing the gas key showed carbon build up between key and carrier. This is a sure sign of a leak.

It wasn't until I addressed both issues that I was able to get the carrier working again.
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Old 07-29-2018, 01:37 PM
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I've never had this problem, but I'm glad you told us about it.

Sending it back to S&W is the right thing to do. Not only will you get it fixed for free, but S&W can then use what they find with your rifle to improve future rifles. I'm not saying they will, but at least they have the opportunity. This makes things better for all of us.

Let us know what happens.
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Old 07-30-2018, 08:16 AM
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MistWolf,

Thanks for your input. You're right in that I am inexperienced in interpreting the data. This is my first AR, and I'm new to the platform. From all I've found, the problem is in the Bolt Assembly. Not to be redundant, but the rifle functions fine with the Ruger bolt.

I could order parts and throw new ones in to experiment with,
but its not cost effective for me. For example, if I obtain a new complete bolt assembly for roughly 80.00 and it solves the problem, I still wont know which component failed or which combination of components failed. Yes I could get it back in operation much more quickly, but i'd be out 80.00 when its under warranty, and from what I understand, S&W frowns on modifications and non S&W parts.

I love the rifle and have no complaints at all with S&W. Stuff
happens and I get that. I just wanted to see if any other Sport
owners ran into this and what they did to correct it. In most cases the MP15 Sport 2 is a relatively trouble free platform.
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Old 07-31-2018, 07:20 PM
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noro, i'm telling you that your problem is either the gas rings or the gas key. We know this because your rifle works with a good BCG.
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Old 08-01-2018, 11:50 AM
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Mistwolf,


And I agree with you. Either the gas rings have failed or the gas key is leaking. Now neither look bad to me but what do I know?

S&W has sent a ticket today for return and the rifle is under warranty, so I'm going to let them take care of it. I did ask them
to note their findings when they get done with it and let me know
what they found. Will keep you all posted. If this was a home built AR, I'd be much more inclined to
throw parts at it, but being its a stock S&W with a warranty and a great reputation, my thinking is to
let them deal with it. I can always use my buddy's Ruger AR 556

Last edited by noro; 08-01-2018 at 12:06 PM. Reason: add info
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