I spent a little time looking up the Middlestead and from what I've read it is just a 243 necked to 224 with a 30 degree neck.. How you form the brass has alot to do with how the chamber was cut.
1. If it was cut like a AI chamber where there is a crushfit for necked down 243 brass all you have to do is neck down the brass , load and shoot. It should be a crush fit when you chamber a round.
If it was just cut to where ever and necked down brass is loose in the chamber. You can check this by necking down a piece of necked down factory 243 brass and trying it in the chamber, if it chambers with no effort put a piece of scotch tape on the base of the cartridge an try chambering it again if it chambers with resistance or will not chamber you can use method #1
2. If it is still loose with 1 piece of tape You can try not sizing the neck all the way down and creating a false shoulder to give you a crush fit. A 6mm shoulder might not be not be enough to crush fit, you might have to neck the brass up to 7mm first the neck it down to 224 to creat a false shoulder. OR you can neck down your brass to 224 and seat a bullet long so it jams into the lands and and holds the base of the cartridge base firmly against the face of the bolt. Use the lowest pressure starting load you can find, jaming the bullet in the lands can raise pressure very fast. I would use a fairly light for the caliber bullet for fire forming, this way as the there is less baring surface and less resistance starting the bullet down the barrel.
When fire forming you want to have the cartridge tight against the bolt face so the fireforming just forces the shoulder and neck into the voids in the larger chamber. If the case is loose in the chamber it will stretch the brass in the center to move the base against the bolt face and creat a weak spot in the center of the brass that if it doesnt seperate upon forming will do so shortly.
