I have 2 m&p one 9 other 40. 9 has all the good stuff in it 40 has better sights. Any reason I couldn't just swap slides and mags?

M&P 9 has different dimensions than the MP 40/357

You can put a 9mm mag and 9mm barrel in an MP 40, but not the other way around. SW says not to put the 9mm barrel in the MP40 FWIW

I bought a 9 mm barrel and magazine for my MP40, and I have been getting some FTF shooting 9mm ammo. I think the spring might be a bit too heavy

Is there any difference in the frames at all? I've got two guns just trying to save myself the hassle of taking both all the way apart to put into one that I'm keeping

There is an in depth discussion on this on the Smith and Wesson Forum that I stumbled on when I was looking at buying the 9mm barrel. There is slight dimensional differences in the slide, but if I recall the main difference was the face and ejector. One gentelmen even posted pictures/dimensions measured with calibers

Got a buddy that works for Walther and they use same frames for 9 and 40 but he's not sure on the m&p. May just be better off taking apart and swapping or ordering another trigger.

I would be the 9/40/357 frames and slides are all machined to the same outside dimensions, but I don't know about the inside dimensions. I was just searching trying to find that thread ont he SW forum, and I am reading conflicting info on wether you can swap slides on 9 and 40 frames. Some folks say yes its the same, and others say there is a few thousandths off and the ejectors are different

Other idea may just buy another trigger and have two good ones.