I tried fireforming the brass for my 357AR which is 1.595" trim, 1.605" max length. That included the entire shoulder on the 223 case and a touch of the neck. I worked up the wax plug and charge until it was a max charge for a 357mag and quick powder. It didn't open up the case enough after repeated firings. . So I tried necking up the case to .357" and tried the same thing. with pretty much the same result, an undersized case. The web on the cases is thick and takes a near full charge with a bullet to fireform. What I did was to just make an expander die that would open up the 223 cases including all but the bottem 1/4 of the case which takes a near full charge to expand all the way. To address the bump left over from the neck I made a roller tool which removes any hint of the bump. Otherwise the bump will stay there through at least 12 firings at 55k-62k PSI (and according to quick load a couple at 72k psi)
Heres the expander stems I use in a standard Lee powder through die to open up the cases and the roller tool. Any nick in the case neck will result in a split case. By going to 1.455" long and cutting them off at the shoulder you won't have to deal with all of that and still end up with a respectable round.