Tusker,
I agree with 22HP. Score some Starline 357 brass and go slow when forming. Huntington's sells RCBS form dies, but they're a trifle pricey. I use 2 or 3 partial passes that increase in depth until the brass will chamber in my contender. You can either fireform with a light cast load, or I use 5 gr of unique with a small piece of TP over it. I pour the case full of malt-o-meal and that will give your brass nice sharp corners.
My original 256 brass is all starting to show incipient head separations, so I've been forced to form. I have a couple boxes of yellow/red WW Super factory 256, but the stuff is getting too expensive to shoot. SW