The Catbird is a 270 Winchester necked down to .243. Off hand I do not have the dimensions. I do know that it is pushing an 85gr bullet to over 4400fps. It is hard on barrels with only a 200 round life expectantcy, but if you slow it up to 4150fps it lasts alot longer. There are several fella's using them here in the dakota's.
Yes I thought about one for a few years now. One 6MM barrel a factory take off and the custom barrel chambered with the same reamer. Use the take off and fire form a couple hundred cases then switch barrels.
I've seen one a very nice custom that shot like a laser and very small groups.
The info I quoted is from several shooters who shoot this cartridge. If you are claiming I am giving false info, don't because it is what these guy's are shooting. I will get the (real info) from them and post it.