Short answer - yes. The best candidate is the 257 Roberts. Less necking down involved. The 7mm Mauser may work too. The 8mm Mauser can work, but may be too big a step in one pass from .323" down to .243". If you decide to do this, you need to size your case to your chamber to fire form it. Back your die off and run your case into it. Keep turning it in just a little until it will just chamber in the rifle. There should be a step in the shoulder to head space on. The shoulder angles are not the same for the Roberts and Mauser cases. You will head space on the step to fire form. You may need to neck turn the out side and or ream the inside of the neck. When you neck down a case, the brass has to go some where. Some times it thickens the neck and some times it forms a ring inside the base of the neck. It all depends on the shoulder angle and how much you neck it down. The Roberts may not take any of this (because you are not necking down too far), but you will have to try it to find out for sure. Forming brass is a pain in the neck. I have done it, forming 221 Fireball out of 223 brass and 30-30 out of 375 brass. In both cases, I had to outside turn the necks. I did not have a ring on the inside to contend with because in the 30-30 case the shoulder is a low angle and the 221 Fireball, the shoulder angle is the same. The necks did thicken up, because I was down into the thicker part of the case with the 221 and in the case of the 375, the neck is thicker and I suspect thicken up some through that much necking down. I used a special form die on the 223 and a full length size die on the 375 Winchester. If you can possibly buy 6mm Remington brass, I highly recommend it. Forming brass is a lot of work, care must be taken and details paid attention too. You will more than likely loose a few pieces getting things set up and you may get a wrinkle in the neck or a collapsed shoulder on some, depending on how far you are necking down. If you decide to do this only use new cases forget using up fired cases, you will lose too many in the form/fire forming process. I would not do it unless you just can not get 6mm Brass or you have a large quantity of one of the other brass setting around. One thing I forgot to mention - If you do this, you more than likely need to anneal some where along the process. With new cases, you will need to anneal after forming and before fire forming, if you neck down very much you may have to do it is steps and anneal after each step, but I did not with the 375 down to 308 (I did it in one step, but annealed after forming. Do not even try it unless you have new brass, your fall out of used brass is going to be high and you need to anneal the necks and shoulder before forming. Good Luck and Good Shooting.
