M&P 15 Short Stroking. Warranty Question.

[glide]

I have a S&W M&P15FT that recently began short stroking. It only happens with FN 5.56 ammo, Remington UMC .223 runs fine.

I have changed out the original stock on the rifle and one of the nuts on my bolt carrier key is stripped from a previous attempt to disassemble; the rifle was functioning properly long after this happened.

My questions is; will S&W have any issues with this as far as warranty work is concerned? I have heard short-stroking was a major issue on early M&P 'T' models and the problem was with the gas tube. Thoughts?

 

[vellnueve]

They generally don't have a problem with it unless it's directly affecting the malfunction.

 

[andy t]

Call them up. Chances are they will take care of you.
Why would you try to remove the key from the carrier?

 

[mac1911]

you changed the stock and messed with the key? S&W may void your warrenty for messing with it?

 

[MassMark]

Not sure why you tried to remove a staked key, but if you tweaked it, that may be your problem Also curious what you changed your stock from/to and if this involved a buffer/spring/tube change....

 

[glide]

The stock was changed from the standard pinned one to a Magpul CTR because I needed a shorter LOP.

I tried to remove the key because I was stripping the entire rifle just to kind of familiarize myself with it (this was my first AR).

 

[vellnueve]

There is no reason to ever remove the key. If any metal is protruding from the stripped screw, file it down.

Are you sure you have the correct buffer? I'm not sure if S&W's fixed M4 stocks are rifle-length tubes or carbine-length.

 

[MassMark]

There is no reason to ever remove the key. If any metal is protruding from the stripped screw, file it down.

Are you sure you have the correct buffer? I'm not sure if S&W's fixed M4 stocks are rifle-length tubes or carbine-length.

Good point (s).... I seem to remember that the tube is not a standard carbine tube on some models and neither is the buffer. Glide - please provide a little more detail. Specifically: What buffer/spring/tube combo are you using? (you can get good comparisons on what buffer you are running by checking the buffer head for a stamp and also the length. If it's a long buffer, it's an AR-15/M1-16 buffer. If it's short, it's a carbine buffer. If it's a carbine buffer and is unmarked, it's a satndard - 'H' is Heavy, 'H2' is heavier...Did you do any further disassembly on this rifle you have not mentioned? (barrel, gas block, etc). The M&P15 is a super-reliable rifle. If you're running weak underpowered UMC fine, but not higher pressure 5.56, I think you may have tweaked something. This rifle should run both .223 and 5.56 just fine....

Just so you're clear, if you look at your bolt carrier group, you'll see that your key was staked. hence, the reason why you stripped the bolt...There is no reason to ever remove the key - especially if it's properly staked...

 

[glide]

Good point (s).... I seem to remember that the tube is not a standard carbine tube on some models and neither is the buffer. Glide - please provide a little more detail. Specifically: What buffer/spring/tube combo are you using? (you can get good comparisons on what buffer you are running by checking the buffer head for a stamp and also the length. If it's a long buffer, it's an AR-15/M1-16 buffer. If it's short, it's a carbine buffer. If it's a carbine buffer and is unmarked, it's a satndard - 'H' is Heavy, 'H2' is heavier...Did you do any further disassembly on this rifle you have not mentioned? (barrel, gas block, etc). The M&P15 is a super-reliable rifle. If you're running weak underpowered UMC fine, but not higher pressure 5.56, I think you may have tweaked something. This rifle should run both .223 and 5.56 just fine....

Just so you're clear, if you look at your bolt carrier group, you'll see that your key was staked. hence, the reason why you stripped the bolt...There is no reason to ever remove the key - especially if it's properly staked...

It is a carbine length DPMS buffer/spring. I am using a Magpul CTR commercial tube (yes I know it originally has a mil-spec one, but that should only matter as far as diameter is concerned right?)

 

[glide]

So I'm an idiot. I just looked back through my Midway orders and I ordered a rifle length buffer spring with a carbine buffer by mistake.

Just ordered a new spring and that should fix the problem. Thanks everyone.

 

[vellnueve]

So I'm an idiot. I just looked back through my Midway orders and I ordered a rifle length buffer spring with a carbine buffer by mistake.

Just ordered a new spring and that should fix the problem. Thanks everyone.

It happens... I'd say your problem should be resolved with this fix.

 

[glide]

So as it turns out, I am not the idiot i thought I was. I did have the correct buffer spring installed. The setup is as follows...

M&P15FT
Commercial tube w magpul commercial stock
Carbine buffer
Carbine spring

The bolt does not lock back after a single round is loaded and fired from a mag.

Any ideas why this is short stroking or where to look next?

 

[MassMark]

Is your buffer and H, H2, Standard, (unmarked)? I still think you tweaked your gas key....Anyone you know have an AR you can toss their bolt in your rifle?

 

[glide]

It is a standard (unmarked) buffer.

I'm inclined to replace the tube as it is only a $12 part. My only concern is there appears to be a small roll pin holding it into the block that looks like a PITA to remove. How easy is it to change out a tube?

 

[mac1911]

how about reinstalling all the original parts? we have a saying here in the shop. "look see what man touched last."

 

[glide]

how about reinstalling all the original parts? we have a saying here in the shop. "look see what man touched last."

A good idea indeed. I did reinstall the factory stock and tube, replaced the buffer and spring, still no dice.

 

[MassMark]

Anyone around you with an AR-15...Swap bolts and let it rip...I'm not rich, but I'll bet you a buck, either your gas key, or your rings are tweaked...i'm not sure where you are, but if you're close to The Berkshires, I'll drop a BCM in there and will bet you that buck that your gun will run like a top....