And remember, the M&P .40 was designed as a .40 to begin with. Glock made their 9mm 17 into a .40 with the 22. Not a lot of room to spare in the barrel area. SIG could not get .40 to work in their folded steel slide 9's and stopped and designed the 229 series just for the .40. They were, IIRC, about 2-3 years late in offering a .40 cal pistol, just for that reason. BtW, the 229 was where SIG begat the machined slide for their pistols, and now build all of them that way.
Also recall that the .40 is already at +P pressures, it is not a weak round by any means.
Also, as previously mention...lead bullets thru a glock are a no no. Jacket round on top of built up lead deposits overpressure the fail limit on the barrel and BOOM! This is true of ANY tolerance fit projectile weapon using high pressure as a force.
I also tend to think that we are seeing some "detonation" issue that blew several .357 magnum revolvers into scrap metal in the 60's. That was, for the most part, traced to reloads w/ very little powder content in them. But not always, there were some cases where no one ever figured out why the revolver detonated.
This "KB" or "Detonation" issue will always be with use, due to the dynamics of pressure projectile weapons, and materials limitations.
You can't get something for nothing, ya know.
Just my $.02
Thanx,
Ofc.JL