The 9mm G17 doesnt doulbe as anything...nor does the .45 (G22 i think)

THe only reson why the .357 doubles as a 40 is due to the ammo itself. The 357 sig round is the same diameter as a .40 cal for half the casing. THe only difference is the 357 sig round tapers to a point starting at about half up the casing. The tapering stops and the projectile is .357 of an inch. THe 40 cal stays the same diameter all the way to the projectile.

Hope that made sense and answered your questions.

ps. if it is a home defense piece I would stay with the 40. Not cause its cheaper but the safety. A 357 sig is really hot and can easily travel through interior walls of homes (even with hollow points). NOt really what you want when other people you care about are sleeping in other rooms. THe 40 is slower/heavier and tends to stop when hitting a 2x4 or something.