I am sure it's been asked but I am new. I bought a police trade in
full size M&P. It shoots fine, but i bought a stock barrel and mags for 9mm. I wanted to shot 9 and 40. I know the back slide does have some play, but I have noticed no problems. What are your experiences?
Thanks

Stock barrel works in the 1.0 version. Can’t speak about the 2.0 version. I fired a LOT of 9mm ammo through it. That said I recently bought a 40 to 9 threaded conversion barrel on close out and am very happy with it.

Although many people buy a regular 9mm barrel and drop it in a M&P 40 and never have any problems, I have always preferred to buy a dedicated 40 to 9 conversion barrel. I have a M&P 40 1.0 and I recommend a 40 to 9 conversion barrel such as one from KKM Precision.

How about a .40 barrel in a 1.0 FS9? Will the extractor handle the bigger case?

You can change the .40 barrel for a 9mm stock barrel, at least on the 1.0. They are the same size, except for the bore, of course. You cannot change the 9mm into a .40, though, mainly because the breechface won’t accommodate the larger cartridge.

I can't speak for the "regular" 9mm barrel but I do have a 9mm "conversion" barrel for my M&P .40 1.0 and it works pretty slick! Now I just need a .357 bbl. and I'll be GTG.

I just swapped my 2.0 9mm barrel into my.40 2.0, and it worked fine. The 1.0 and 2.0 barrels are different lengths, so you’ll have to stay within the model. The ones I have are the compacts, don’t know about the full size.