BigJ,
I think I've managed to confuse the issue. The customer wanted a 450 Marlin but wanted to use standard magnum brass so he wanted it to headspace at .220" but wanted the 2.1" chamber. For a 458 American with a 450 reamer one could indeed just run it in short and end up with a slightly long chamber or run it in 2" and cut the belt recess separately. GD
Spot on ... the hybrid chamber made by running a 450Marlin reamer in short is what I'd build. Lots of brass as parent for the wildcat, standard 450Marlin dies will work.
Cheers...
Con
