Well, I just tried fireforming a handful of new brass into 6mm/30-30 Imp with results that are just what others have posted here have come up with.  I started with virgin 25-35 brass, neck sized to 24 caliber after trimming and used 6 grains of shotgun powder with creme of wheat.  I lost all ten cases to splits just as others have said.  I then tried 15 new 30-30 cases shortened, necked to 25 cal in a 250 Savage die then necked to 6mm.  Using the same COW load not a single case split.  Now to prep the rest of the 30-30 brass so I can begin working up a load for the Stevens 44 1/2 this winter.  Thanks to those who forewarned me about starting with 25-35 brass so I didn't waste my time!  Tom

George, a few years ago I was hunting on Steens Mtn in Oregon when I sat down under a small tree overlooking an open hillside to rest.  I noticed an old piece of brass and picked it up.  It was a 25-35.  I sat and pondered for quite awhile if an old sheep herder let a coyote have it or if he collected a deer for dinner!  In my golden years I have switched to single shot rifles for all my hunting and also whenever possible to the old classic rounds.  It puts the "hunt" back in hunting.  My next rifle will be either a 30-40 Krag or 38-55 if I can find a good single shot.  In the meantime I promise not to neck 25-35 up but I will have to try going all the way to 22 caliber before I promise not to go down.   

Also forgot to say that I think using 25-35 brass would work OK for this if the case was held back by bringing the 6mm neck sizer down as little as possible until the action would close snug
Good 25-35 brass is available as 6.5X 52R (European designation for 25-35 as sold by Sellior - Bellot.117 gr. soft point ) Loaded cartridges were accurate and flat shooting in my Win 94 rifle . Also crimped very tightly , as I could not pull a bullet ( to measure Diameter for curiosity ) with my kinetic hammer

Phil