[Kframe]

I've just bought both the M&P 15-22 and a Slide Fire.
First day out, got it to work very well, was able to do bursts and entire 25-rd mag dumps.

A couple tips: Aguila SSS 60gr. ammo worked the absolute best, and I also had good results with an old box of CCI Mini-Mag+V (I think they now call it the Velocitor, but I may be wrong). Keep the trigger finger as rigid as possible, and I also rotated my finger slightly so that the trigger contacted the lower edge a bit more than the fleshy pad, if that makes sense.
Believe it or not, I was using the stock trigger!

Last night I pulled the 3.5# JP trigger out of my M4 and swapped it into the M&P, I expect that will make functioning even easier.

Others have reported success by lightly sanding the 'buffer tube' with 1000-1500 grit paper, and some also have added some dry graphite lube to the tube/stock interface. But, really, I think that's probably unnecessary if you get your left-hand technique down.

With the stock trigger, I had no luck with Rem Thunderbolts, Golden Bullets nor CCI Stingers, but I have a hunch that with the JP trigger I may have better results.

I had originally planned on putting the SF on the M4, but man, I don't have a thick enough wallet for that kind of fun! Plus, the .22 is much quieter and less likely to irritate the muggles.

Good luck!