Currently, I don't recall exactly when I decided to proceed with an in between cartridge, between 500 MDM and 50 B&M, but based on the same RUM case. This case is 2.5 inches, and resembles a shortened 500 MDM.
But for what purpose you ask? 500 MDM and 50 B&M have proven themselves over and over in the field, and both these .500s have desirable traits in the field, so how could it be possible to improve on either? In all reality the new 500 B&M cannot improve on either 500 MDM or 50 B&M within their respected domains. It only offers an interesting project, with some attributes that might be desirable in some cases.
At 2.5 inches, the 500 B&M does in fact give some higher performance than the 50 B&M, but of course not as much as the 500 MDM, so it does indeed fall in between. As a stand alone cartridge it has proven to be as good and valuable as the other .500 caliber B&Ms. The difference is in the action. For this cartridge I use the longer Winchester M70 RUM action, same exact action that the 500 MDM is designed for. At 2.5 inches one can load the various .500 caliber, long nose projection Raptors, and add the Talon Tips. These loaded rounds, fit, feed and function from the magazine. The Talon Tips added to these bullets buys one an additional 100-125 fps IMPACT at 50 yards, a very distinct advantage over either the 50 B&M or 500 MDM. While the 500 MDM might start out a bit faster than the 500 B&M does, by the time they reach 50 yards that gap has narrowed, simply because of the added Talon Tip increasing the BC. Naturally the 500 B&M is an increase over the 50 B&M.
The 500 B&M that I built uses the larger Winchester RUM action, but the same barrel contour as the 50 B&M, and I did go with 18 inches of barrel as well. This makes the 500 B&M just slightly longer than the 50 B&M, and about the same weight, maybe a few oz more.
I did not, and do not consider these two cartridges standard B&M cartridges, with their own menu pages. More of a speciality cartridge, and something for me to explore.