Funny, I was just popping by here on the off chance there was new info about these. I bought...I think the M3 when CDNN had these a while back. I used it with some success in .22 indoor bullseye with a Bushnell TRS-25 or Primary Arms micro dot (can't recall which was on which pistol).
Two others that shot there had variants...an M2 and an M4. As long as the bolt in front of the barrel was snug, they seemed to work pretty well.