BudsGunShop.com - Ammo doesn't fit

357 sig and 357 magnum are two completely different rounds. 357 sig was created to replicate the 357 magnum but be reliable in a semiautomatic. They did this by knecking down a 10mm round and adding a 9mm diameter projectile. So yes 357 sig won't work in a 357 mag. There's alot of firearms made for 357 sig like Glocks, sig saurs, and S&W. So look at it this way, now you have a reason to buy another firearm, lol.

Raymond, unfortunately, you purchased the Semi-Auto .357Sig ammunition instead of .357 Magnum ammunition. They are totally different and not interchangeable as pictured here : flickr.com/photos/itwuzcryptic/2318885470

The.357 SIG is a bottlenecked rimless centerfire handgun cartridge developed by the Swiss-German firearms manufacturer SIG Sauer, in cooperation with ammunition manufacturer Federal Cartridge Co.. The cartridge is used by a number of law enforcement agencies and has a good reputation for accuracy.
Sounds like you need a new gun to go with that ammo.
Glock 31 (357 sig) in stock here
https://gun.deals/search/apachesolr_search/764503702037

Its rimless and made for semi autos Developed in 1994, the new cartridge was named "357" to highlight its purpose: to duplicate the performance of 125-grain (8.1 g) .357 Magnum loads fired from 4-inch (100 mm)-barreled revolvers, in a cartridge designed to be used in a semi-automatic pistol with greater ammunition capacity than a revolver. Performance is similar to the 9×23mm Winchester.