There are many ways to form cases to make brass for your rifle from other like cases. For most all of the B&M Cartridges the methods used are rather simple, take basic cut brass and run it into the size die. Take all the 2.25 inch B&Ms made from RUM brass. Cut and trim any RUM case to 2.240 inches, this basic brass is ready to go for the 50 B&M, size, prime, charge and seat the bullet and you are set. Take the same piece of basic brass and just run it in the 458, 416, 375, and 9.3 size die, and you have formed brass ready to load and shoot. It is a bit rounded, but not by much and when fired will iron out nicely to sharper edges.
This holds true for the Super Short B&M's as well, only one uses cut WSM brass to 1.640 inches, 50 Super Short, ready to go brass. For 475 and 458 B&M Super Short, run it through the sizer and load. Except for the cutting, it's very easy, very simple.
For the 50 B&M Alaskan it is even more simple, by 50 Alaskan brass (.510) and size it in the 50 B&M Alaskan size die.
But what about the mighty 500 MDM? The 500 MDM is a 375 RUM case necked "UP" to .500 caliber. All the other B&Ms use a basic case that is necked "DOWN", and most of the time, going down is much easier than going up. Hornady makes a step up die for the 500 MDM that is adequate to take a 375 RUM up to 500 MDM, but it is not quite as clean as I would like it to be. So I have been "Ironing" them out a bit with a corn meal filler.
Back before Hornady made 458 Lott brass I would take 375 H&H brass, and later 416 Remington Brass and blow out the necks with corn meal. The process is as follows;
Size and prime the case.
Measure and charge case with 15-18 grains of Bullseye Pistol Powder.
Place a bit of toilet tissue over the powder charge to keep the powder in place at the bottom of the case.