I don't have an M&P 15-22 but I was hoping some experts on here could help me out. I have a dedicated CMMG .22 upper on a Colt lower and much prefer the S&W magazines. I got a boonie packer mag adapter so I could use them. Works flawless with the 25 rounders but the short 10 round mags short stroke on only the second round in the mag. The bolt goes back far enough to eject the empty and pick up and chamber a new round but not far enough back to cock the hammer. It only does it on the second round in the mag, when I pull the bolt back and chamber a new round it will work flawless the rest of the mag. I just tried a Black Dog 50rd drum mag and it too works flawless, it's only with the short 10rd S&W mag. Anyone have any idea what could be causing this? Thanks in advance.

No idea but sounds like something to address with the mag adapter manufacturer. Have you don't any research other than this forum (i.e. Google) to see if it's a common problem? Have you tried a different 10-round mag to compare?