309coug is right, it is a 357 max necked down to 7mm. Redding is the only source (that I know of) for dies and they make 2 versions, 7mm Super Mag and 7mm Super Mag TC. TC bbls have a tighter neck IIRC. I have used 7mm TCU dies with good results as well. Case forming for mine (2 TC bbls) has been fairly easy. I just anneal new 357 max cases and then FL size in either the TCU or SM die. Loading data for the TCU can be used as well but start low as there are lots of variations from source to source.
HTH,
Tim
Last edited by trcuda; 10/08/08.