Case Forming with Corn Meal
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.
Fill the case to the top with Corn Meal, or some other similar filler. Corn meal has always worked perfectly for me, but I have used flour in the past without a problem. As you see below I use a funnel that has had the hole enlarged some, and tamp the corn meal down into the case. Light tamping is all that is required.
Place a wad of toilet tissue, even wax would do, or a card board wad that will fit over the top of the corn meal to hold the filler in the case. For the 500 MDM I happen to have a surplus of these card board wads and have been using them.
Now it's simple, go outside, well ventilated areas, put your ear muffs on, point downrange in a "SAFE" direction and start fire forming the brass. If you are forming something like a 458 Lott, or 470 Capstick, a straight case, then you get nearly a PERFECT case, ready to trim to length and start shooting. A case like the 500 MDM that has that little ghost shoulder, then it is not quite as ironed out as firing a full load, but it is ready to trim, size and shoot at this point.
Now I have been of late using the Hornady form die to take up the 375 RUM brass and then using the corn meal to form them further. You do not have to do this, you can fire form these straight from 375 RUM and blow out the shoulder/neck to spec also. You must trim the 375 RUM brass before you load them with the corn meal loads to 2.790 or so as they will stretch as they are blown.
I have heard all sorts of BS about pressures going beserk, blowing rifles up, and all sorts of nutty things concerning this method. If you follow these instructions you can't go wrong. I had a pressure trace hooked up on the 500 MDM for some of these corn meal forming loads and I could not even get a reading on 3 of the 4 tested, I finally got one load to register 18000 PSI. I have done hundreds of these for 458 Lott and 470 Capstick from 375 HH and 416 Remington, zero issues. Have done hundreds of them for the 500 MDM, no issues or problems at all.
Now this does not work so well if you are trying to blow out a shoulder or neck. For instance you wanted to blow out a 375 to a 416 with a shoulder, don't waste your time, it does not work well enough. But going to a straight, or near straight cartridge this method works like a charm. Other than being a little messy there is not any downside I know of.