I just want to share my experience with the forum in case it helps someone out, like you have for me repeatedly.

I changed the stock out on my M&P10 from the oem adjustable carbine stock to a Magpul PRS using the required full length A2 tube (DPMS). I did this prior to firing the rifle at all in its original configuration (my bad). I did use the required aluminum spacer from S&W.

At its first outing the gun would repeatedly short cycle. I was using NATO spec ZQ1 7.62x51 M80 initially. It would fire a round and eject but not pick up the following round. It did also stovepipe and not fully eject a few times as well. The two guys I was shooting with that day were both former Army and one was actually an armorer while in the service. He suggested we try different ammo - this time real mil-surplus 7.62x51 NATO taken off of a linked chain that my buddy got his hands on, but the same results. I also fired the rifle while my buddies observed the bolt cycling from the side, and they stated the bolt was clearly not going back far enough. We concluded the gun was under-gassed.

I went home, and checked everything. Bolt was wet. And I made sure the recently installed Troy low profile gas block was perfectly aligned, and I had even used Rocksett to secure the set screws. I relubed the bolt again.

At the second outing a couple weekends later, same result - short cycling. I was thinking that the rifle just needed to be "broken in" but admittedly this was getting worrisome. My (former armorer) buddy was shooting with me at the range again. We started discussing about my installation steps for the handrail, gas block, and Magpul PRS. When I mentioned it uses a spacer to make up for the longer length extension tube, he suggested that we try it without the spacer. So we removed the spacer, lessening the preload on the buffer spring.

PRESTO! The gun started functioning perfectly after that point. What a relief! We sighted in the scope enough so that we were repeatedly hitting a smaller size steel silhouette at 100yds.

The rifle still only has less than 50 rounds through it at this point, but I was ecstatic that it was finally working like it should. At the next range day, I'm gonna focus on the feel to see if the bolt is slamming/bottoming out. And if so I will put the S&W supplied spacer back in to see if it will function properly once more (after supposedly loosening up a bit). There's a chance I may need a shorter spacer than what is provided by S&W, but as for now the gun seems to be working fine without it installed.