Smith & Wesson M&P10 - turned mine into a bolt action

potentpoefie · Apr 19, 2015

Hi guys

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. Both 20 rounders. Give 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.

****Edit****
Was just over at the S&W forum. So the gun is short cycling. (Got the technical term now). Ammo used was Norinco surplus 7.62x51 147gr ammo. Could the ammo be under powered? Not like the Chinese to load weak loads. Also I would like the rifle to work with all kinds of ammo.
*************

What do you guys think could be the problem?

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.

Wildcat · Apr 20, 2015

Does the barrel have dimples for aligning the gas port? If not it can be a bit tricky. Some barrels the gas block has to be 0.030 off the shoulder, not right up against it. If it uses set screws, did you lock tight them? Look for signs of fouling around the block too. Sometimes you can see where the gas is leaking.

drmitchgibson · Apr 20, 2015

Gas block misaligned

potentpoefie · Apr 21, 2015

We measured the width of what the old rail's end cap would have been and placed the block with that offset from the shoulder.
The gas block is of the grub screw type not the clamp on type and yes it was lock tight'ed.

Will go take it apart and have a look.

drmitchgibson · Apr 21, 2015

See if there's a blockage then. You can plug the chamber end of the barrel and blow down the muzzle, and should be able to feel your airflow coming through the end of the gas tube in the upper receiver. It will have to be taken off the lower and the BCG removed, obviously, to do this. Muzzle device will need to be removed as well.

I have used some gas blocks that are sized to fit to the shoulder of the gas block bearing surface, and some need the gap that would be provided by the standard handguard cap. Your problem does sound exactly like a short-stroking gas flow issue.

potentpoefie · Apr 24, 2015

Thanks for the tip. Will definitely check that out.

Unfortunately I've not had time, as planned, to work on the rifle but we do have a 3 day weekend coming up in SA. So I should have some time to work on the rifle.

Really appreciate all the advice.

potentpoefie · May 4, 2015

Update:
The rifle has not been touched since it last gave issues. That includes no lube, no cleaning.
I got to the range with some PMC Bulk Ammo. Loaded up the first mag. Shot it slowly and no issues.

Wondering if maybe some people "limp wristed" (or whatever you will call it for a rifle) I gave it to my friend to shoot. This would be his second time shooting a firearm and figured if a noob error would be induced he would be the man for the job. Functioned just fine.

Then I ran the gun fast and hard. Still no issues. The PMC bulk ammo isn't exactly top quality ammo but it still worked. So conclusion is that the fault was definitely ammo related. So the M&P 10 issue needing to be run in for a couple of hundred rounds with full power loads seems to have been my issue.

Now that that is sorted out I figure I have enough PMC ammo left to just run our first ever 3 Gun Nation South Africa nationals with Heavy Optics division (only because I don't feel like putting down my Versamax for a 870). Happiness is a functioning gun.

P.S. Of course the guy that worked on the gun originally was on hand and ready with an "I told you so". He was always sure he did a good job on the alignment. For him it was a matter of pride.