seeing options for this on many firearm brands who changes their 9 to a 40 or vise versa or 10 to a 45 or opposite? having extra barrels in the same brand of gun you've already own is great as you can shoot the same cartridge in another w/o buying a complete gun? well..............i'm 1 to change barrels for what i want..................having a m&p 40smith on order and a kkm barrel in 357sig. i only bought the 40 as there's no m&p in 357sig available.