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.