If you take basic NEW BRASS, run it through the size die that is set up so the cartridge goes easy in the chamber, then you give this case room to move forward when it is fired and forms to the chamber of your rifle. To prevent this from happening, set your size die back up a tad until you feel a slight bump as you chamber the cartridge or case. The shoulder of the cartridge is bumping in the chamber, when fired, it does not move forward, and therefore you get no stretch of the case, preventing case head separation in the future.
Now, with the 416 B&M, there are some exceptions that you can get by with. Most of the first loads used were with ball powders, and specifically AA 2520. The 416 B&M is very good with this powder. But when I started working with IMR 8208, more of a stick powder, I discovered that even if I sized new brass to chamber easy on the first firing, that with IMR 8208 I rarely got any case stretch, and subsequently rarely got case head separation issues. I also found this to be the case with lighter bullets, like the CEB 225 gr #13 and the 180 gr CEB using H-4198 as long as my loads exceeded 50000 PSI. No case head separation at all.
Naturally once the brass was fully fire formed to the chamber, then you could full length resize to chamber easy with no issues from that point forward.
With the 475 B&M I have not found a powder solution that would allow you to cheat case head separation. I still have to bump that shoulder on the first firing with all of these.
The 9.3 B&M uses a stick powder to begin with, IMR 4320 or RL 15, so it has not had any issues with case head separation.
The 458 B&M and the 50 B&M are the easy loaders here, size to chamber easy, trim, load and shoot, new brass, old brass, fired brass, it don't matter what you do with either of these.
50 B&M Alaskan
There are no known issues at all with the 50 B&M Alaskan designed for the lever guns and single shot rifles. Another easy loader.
The mighty 500 MDM..... 2.8 inch RUM case, made best from 375 RUM.
The first design of the 500 MDM was probably actually a better cartridge design in and of itself, than the current cartridge with the shorter neck. The old version had a very long and tapered neck, and I could see a bullet bulge at the bottom edge of every bullet, and I did not like that at all. It was totally aesthetics, and had nothing to do with function, but I changed it anyway, and probably introduced some of the issues we have with it today because of that... OK OK.. My Fault for making the change and I take responsibility!
What we have today really looks pretty good, and it most certainly works just fine, and is a hell of a good cartridge and the biggest cartridge one can "easily" convert a Winchester M70 to. Full .500 caliber. In 20 inches it can run 500 gr bullets to 2400 fps, 450 NonCons to over 2500 fps, and even 300 gr bullets to 3000 fps. It is a power house on still a small package for what it is capable of doing.
But it can be somewhat troublesome with making new brass from 375 RUM. And it can also suffer case head separation just like the 416 B&M and 475 B&M can.
Forming brass is not quite as easy as any of the other B&Ms. With the other B&Ms we are going down in caliber from basic 50 B&M brass or 50 B&M Super Short brass. With the 500 MDM we have to go UP from 375 RUM Brass. I find its always easier to go DOWN in caliber, than to go UP in caliber, especially if you have to go a long way up or down. Going from .375 to .500 is a LONG way!
Nearly from the beginning I have used corn meal to blow the 375 RUM out to 500 MDM, and this method still works great, and so good in some rifles that the finished case looks nearly fire formed and ready to load.
Corn Meal Case Forming
But this is a lot of work and is very time consuming. Sam Rose recently made a set of punches and dies for me to take the brass up in steps, first step from 375 is to around 416, and then to 458 and then final to .500 caliber. I then resize this brass back to spec, load and shoot. Now this method is causing some issues with head separation, and I might be able to solve some of it by not sizing the case as much, which I am going to work on in the near future. But, I can make brass very quickly and much easier than using the corn meal. While it is not as nice and formed as a true fire formed case, it is functional, for the most part, especially if I could reduce the number of cases I loose to head separation, which currently is only 4-5 per 100 cases. See below......