9mm - Plentiful ammo from very cheap to very pricey. Sufficient for self defense and easy enough to shoot that a guy could work up some light loads to teach a new shooter with.
.40 S&W - Can handle heavier bullets than 9mm, plentiful ammo, good enough for self defense.
.357 Sig - 125 gr. .355" bullet @ 1350 fps from a 4" barrel (duplicates supposedly ideal .357 Magnum defense load), uses same platforms as .40 S&W (only needs barrel replacement to switch between the two).
9mm - Some consider it marginal for self defense, +P+ ammo is hard on guns.
.40 S&W - Can't do anything 10mm can't do and can't do some things 10mm can do. Sort of noisy, ammo not quite as plentiful as 9mm.
.357 Sig - Ammo selection still limited, can make .44 Magnums seem quiet, can be hard on the shooter.
That said, I have a 9mm and a .357 Sig, both of which I like and do not wish to part with. I don't have a .40 S&W, and don't really have a desire for one. A 10mm would be good, though...
I have several firearms in all three of the chamberings at my disposal.
Of the three, I have found the 40 to be the least accurate. Since that is an issue caliber for us, I have shot over 150 of them from 4 different manufacturers. The 357 has proven to be the most accurate in duty weapons.
Which one to get rid of depends on his situation. If he only shoots factory ammo, the 9 and the 40 would be the choices to keep. There is plenty of inexpensive 9mm around and the 40 has become popular with big urban PDs that do not want penetration.
If he wants performance, keep the 357. If I were keeping just one, that would be it. It does provide a 357 magnum autoloader.
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