I am a Class III FFL and Slide Fire dealer . They will work on the M&P 15-22
But takes some mods and patience. once mounted on the rifle,
There is a distinct "sweet spot" at which the Slide Fire will work but firing It repetitively takes a bit of practice
The new Slide Fire SBS will fit with absolutely NO modification...the Gen
I and II will take relieving the left side of the SFS to clear the trigger guard.
Also, use a good grease on the buffer tube and inside the SFS stock.
You will also need to drill a 7/16 very small dimple on the SFS mounting block as the S&W uses 2 springs instead of one...Safety spring and rear
Takedown pin.Finally, and Most essential replace stock trigger group with a
3lb trigger...plus, it as to have a much faster trigger reset and lock time.
Best I have found is the American Trigger Co. 'Gold' 3lb trigger...truly drop in.Comes with 2 new trigger and hammer screws..drops right in
You will have to probably relieve the safety 'flat' with a file to properly
clear the lower trigger group.That's it. The trigger I get from Brownells
And other excellent triggers for this is the Timney and Jard respectively
Followed by Gesselle.Depends if you want a drop in one piece modular trigger group (Retail is about $279 ,expensive, but incredible,)
or build your own trigger using lighter springs,etc to lighten trigger to 3 lbs
I like the American Trigger Gold because it has a 70% faster reset time than
Stock and No creep or take up..trigger pull is phenomenal.Timney is closest in quality IMO, followed by the Jard.
Takes some work and it can be frustrating but it does work.After the mods are complete it's all about technique aand practice.Much cheaper alternative to .223/5.56
The Fun Factor is off the boards
By the way, You Tube. Has dozens of videos of these modifications.
I didn't invent this!
PS...Any good 2 stage 3 # trigger will work fine
And Yes, the 'conversion ' to the S&W is not cheap.
The mods will cost more than the original gun.