I ordered a conversion 357 sig barrel for my full sized 40 caliber M & P through MidwayUSA. Finally got to try it out a couple weeks ago. Shot great! Ran a few mags through it with no problems. Recoil seems actually a little milder than the 40 caliber. It's a drop in barrel that's a little tight at first.
I bought the S & W barrel, not the Storm Lake, which is much more expensive. Very happy with it! If you have a MP 40 I highly recommend the 357 Sig drop in barrel by S & W.

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NRA & TSRA
Texas CHL Holder

M&P is sold with both barrels as a kit.

Some still contend that if you don't pay double to get a 'conversion barrel' you are foolish.

I bought my MP 40 on 7/22/2006. That was right after they came out. At that time there was only one caliber, the 40 caliber. Got it at the Denton, Tx gun show for $409.00 +tax. Since then I bought a 9mm compact and a 9mm Pro. Love the M & P's!!
I paid MidwayUSA $73.00 total for my S & W 357 Sig barrel. The Storm Lake barrel was quite a bit higher.

__________________
NRA & TSRA
Texas CHL Holder