Hmm.. Just seems odd. So does the .40 barrel not put any spin on a .357 round? The only thing a .40 and .357 sig have in common is the casing start size. The bullet is the same as a 9mm, right?
Huh? What in the world are you thinking here? are you thinking he'd shoot 357 SIG out of the 40 barrel?
Broken down Barney-style - if he wants to shoot 40, he just needs a plain ol' G22 barrel. If he wants to shoot 9mm, he need a conversion barrel cut to the G31/22 dimensions.