9mm Dillon is 10mm Auto necked down to use .355" bullets.

There are several mm of difference between the case lengths of 10mm Auto and 357 sig casings.

357 Sig resembles 40sw necked down. Both are small pistol primed where as standard 10mm Auto casings use large pistol primers. While 357 Sig has a slightly longer case length than 40sw. If one actually necked down a 40sw casing the length would increase as the brass is displaced. (not recommended for novices)

It is note worthy that 357 Sig has a higher pressure ratings (CIP and SAAMI) than both 10mm Auto and 40sw. But also rivaled by SAAMI 9mm +P specs and developing a 9mm +P load can yield comparable ballistics. (not recommended for novices)