Had a chance to put a suppressor on my M&P 15 5.56 this past weekend. Used a homemade custom reflex suppressor design I'd used on my Colt. (yes, I have the tax stamp ).

Found out there was a little gas blow back while using the suppressor on the M&P 15 that wasn't present on the Colt. Solved the gas blowback issue with a little Permatex silicone gasket stuff available for about $3.00. I used the high temperature red because my buddy just happened to have some in his tool box, but you don't need to use the red and the black will work fine as there really aren't any temperature issues at this point on the weapon.

Applied per the instructions here > gasbuster

Before you apply the permatex, make sure you clean the charging handle semi circular shape area well with a little alcohol and a cotton swab. On the receiver portion at the semi circular part put a very thin coat of vaseline to keep the permatex from sticking to it as it forms and sets up, you only want it sticking to the charging handle part. Let the permatex set up and cure overnight. After its set up, clean off the vaseline, and trim up with a razor blade.

Just went out and fired the M&P 15 with suppressor again and the new permatex - no more gas blow back to speak of.

Yes, I could have paid the approximately $90.00 for one of those gas buster charging handles, but for the $3.00 I can achieve the same end result.

There is also another mod to the charging handle that involves drilling a small hole but I didn't want to do this. I could have and just ordered another charging handle but even then a new charging handle would cost more than the permatex trick.

So for you folks using suppressors....might help you out some.

Is your M&P a piston model?

nope, 16" barrel and gas tube.

.....Pics?