There are some questions and I have had this problem from day one of bringing the M&P-10 home. Failure to feed from the short Mags 4 or 5 round mags!
Fix found
The 5 round S&W that came with the rifle and the 4 round DPMS I use with my POF; I had the thought, look at this step by step *just what is taking place*??
The bolt is simply sliding over the round in the Mag, do I replace the follower ?? NO
Do I add power to the spring?? No
Then looking at the Pmag (20 round) that works, but not with reliability, its follower, is built as a mirror image to both the other Mag. followers.

In short there is no need to take apart, switch anything around or buy new mags.
I took a piece of Velcro just the soft half; cut it to fit the top of the follower. It has its own adhesive so for the testing the adhesive was good enough, with the Velcro; a half inch wide and long enough to cover the entire follower.
The followers configuration is entirely changed; the Velcro picks up the round just enough for the bolt to catch the rear of the round chambering it perfectly!

Yesterday 60 rounds fired only one FTF with a jam, (one round over riding the next).
The problem, the Velcro adhesive giving way making a hump in the Velcro.
The next step; remove the adhesive from all the Velcro, use something like J-B weld
to keep the Velcro permanently in place and try it on the Pmag (20 round) also to see if this will make it function each and every time with total reliability?



PS; I had this rifle up on the *For Sale* wall at my gunsmith’s, I took it down to give it just one more try. This fixed headache gives me room to try and fix the others.

As takes pace a few times with rifles and feed problems I thought I had found something that works. Wrong!!

In this case; yes it worked, but it was not the proper fix for the problem. The mag or mags did not have any effect on the * failures to feed*!
Because I had added an adjustable gas block; added to the rifle for use with my suppressor. It was the miss adjustment of the gas block that was the start and finish of the problem. Proper adjustment of the gas block; problem solved .It now feeds from DMPS 4 round mag, S&W 5 round mag and PMAF 20 round mag. All with no feed problems.

If my first post about Velcro on the mag follower seemed to fix anyone’s feed problems?? Remove the stuff and look for another fix
Please disregard any thoughts about a fix by adding something on to the follower.

R&D in the field is a darn important skill to undertake and learn. Don't be sorry for seeking solutions. And thanks for the heads up on the fix.

Seems S&W has a whole lot of Derp on the bench, and they are adding it to these rifles liberally.

Very frustrating for new rifle owners who are excited to have a new target tacker, only to find there are half a dozen senseless glitches possibly waiting for them.