Tough question, i think youll be good with either, power wise, as they are both good rounds, 40 is probably in more widespread use, but with the ammo situation being slightly different in different locations, check the online outlets and local shops before you buy
The 357 SIG imitates the 125 grain 357 mag load in a 4" revolver. Since it is a necked down 40 it has an extremely high reliability rating as it feeds easier. You already have a 10mm so why would you want a 10mm short or is it slow and weak?
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Getting a conversion barrel my Glock 20 (10mm). My options are a .357 sig and a .40, which is more powerful and which is cheaper to shoot? Please help!!!!
Around here the .40 S&W is cheaper to shoot and easier to find. For some reason the .357 Sig never seemed to catch on around here. Even under normal conditions it's very hard to find guns or ammo in that caliber. Most people I know that shoot it ended up going the conversion barrel route.
357. will be more powerful however the 40. is nothing to sneez at.I shoot 357. Sig when I get a wild hair but it's not cheap.For a less expensive "plinking " round I would go with a 40. but if you really want to lower you're cost I would go to a 9mm barrel if they make a drop in for you're gun.