Unusual problem with my M&P 15T

[KLR_Redux]

Greetings!

I apologize in advance for my first post being a problem. If you have already read my post on this topic on Lightfighter, please disregard.

I have an M&P 15T that is having a weird problem. The initial symptom was that when I would lock the slide back manually (or sometimes after the last round in the magazine) I would insert a loaded magazine, close the bolt and would find (after hearing a click) that the chamber was empty. The weapon functions just fine otherwise it just has problems after the last round in a mag or when locking the bolt back.

I sent the rifle back to S&W where they replaced the BCG and fixed an unrelated misalignment in the quad rail.

I shot the rifle today and it exhibited the same problem. Now, I should have noticed it earlier, but when I benched the rifle after a string of firing, I noticed that the bolt was locked back, but about 1 inch forward of it's full rearward travel. After some manipulation, I found that I could cause the bolt to lock in this position by pushing in on the bolt release while pulling back on the charging handle. If I pull all the way back, the bolt locks in the proper position. Here is a picture of the bolt in the wrong position:










Anybody know what could cause such an odd malfunction?

[R Moran]

It is possible to lock the BCG back on the bolt carrier and not the bolt itself. This would put the bolt to far forward to strip a round from the mag.

You say it happens when the bolt locks back on an empty mag also?

Bob

[KLR_Redux]

You say it happens when the bolt locks back on an empty mag also?

Not consistently, but yes. I benched the rifle after firing the last round and that is when I noticed the problem.

[AR15barrels]

This is known as short stroking.

Is the rifle properly lubed?
Is the gas key tight or leaking?
Are there obvious gas leaks at the front sight base?

Are you running an extra heavy buffer?

[Shihan]

I had a short stroking problem on my CMT made M&PT. They replaced the upper and I had them put in a LMT BCG. Runs fine now. Send your back and bitch. Also tell them you want it next day and not ground shipping.

[KLR_Redux]

[QUOTE]Is the rifle properly lubed?
QUOTE]

Yes it is. Was thoroughly cleaned and lubed after the previous session and I still added lube to the BCG before this 150 round session.

Is the gas key tight or leaking?

I would go with tight. Here is the thing, the rifle is exhibiting the same problem with two different BCGs. The original LMT one, and the CMT one they replaced it with. The replacement did not appear to be staked properly so I sent it to Ned Christiansen and he staked it (for a very reasonable cost I might add) .

Are there obvious gas leaks at the front sight base?

No tell-tale brown stain if that is what I am supposed to look for (from reading the FAQ).

Are you running an extra heavy buffer?

I honestly don't know, the M&P is as it came from S&W the day I bought it.

[KLR_Redux]

I had a short stroking problem on my CMT made M&PT. They replaced the upper and I had them put in a LMT BCG. Runs fine now. Send your back and bitch. Also tell them you want it next day and not ground shipping.

Sounds like a plan. I am gettting tired of waiting. They had the rifle back to me two weeks after I sent it UPS, but I took another week to send the BCG to Ned Christiansen for proper staking. Thankfully this isn't a duty weapon.

The rifle originally came with a quad rail that was out of alignment. I didn't bitch about it at the time, but I have to give the repair folks credit, they noticed the problem and fixed it without my asking.

[Hawkeye]

Are you using the same kind of ammo each session? Have you tried different ammo?

[KLR_Redux]

Mostly American Eagle 55gr .223 FMJ the first run, Mostly Remington 55gr .223 FMJ the second run.

[AR15barrels]

Check the gas tube alignment in the reciever.
You should be able to move the tube about 0.015" left-to-right with your finger tip.
This lets the gas tube "float" in the receiver so that the gas key aligns properly on the tube as the carrier moves forward.

Next check for proper carrier movement within the receiver.
Remove upper from lower.
Charging handle latched into place.
Holding the upper top-side-up, point the muzzle 45 degrees down angle.
Withdraw the bolt carrier group until it stops against the charging handle that's left latched into place.
Release the bolt carrier.
The bolt carrier SHOULD (under it's own weight) travel all the way forward and hit the barrel extension, locking the bolt into place as it goes.
You will hear a "tink" when the two hardened pieces contact each other.
The carrier MAY bounce back 1/8" to 3/16" as well.
As long as you heard the "tink", you are fine and should not be concerned with this.

If the carrier will not close on it's own, remove the bolt from the carrier and try the test again.