I bought my MP40 right after they came out (I think) in July of 2006. As soon as I heard about the 357 Sig barrel I put in a back order at Midway. S & W didn't have the barrels then either, for some reason. I finally gave up and canceled my order from Midway.
They eventually did get them in but I was off in another direction by then. I soon bought a MP9 compact and then a MP9 Pro. I have bought several pistols, rifles and a shotgun since then. Maybe now I will try again to get the 357 barrel. I can't imagine why S & W doesn't stock up on their own brand.
And, by the way, I have seldom had problems with any of my M & P's and no problems, ever, with my 40 caliber with many rounds through it. Great guns!