I've got a couple of .50 BMMs and like them. Excellent conical shooters. Tough as nails.
They have 1-in-28 twist (the .50s do anyway) and were marketed as a sidelock that offers the same level of performance as an inline. They're supposedly able to shoot Pyrodex pellets in 150 gr. loads and the fast twist gave them good performance with sabots. They came with both a musket nipple and the standard #11 nipple. T/C recommended musket caps with the Pyrodex pellets.
I can think of a few reasons why they didn't takeoff. First of all, the black Rynite stocks are ugly and give the impression of a cheap gun. The fact that they are nearly indestructible doesn't matter to most people. Blued steel in a black plastic stock will turn off anyone who values tradition in firearms.
Secondly, ignition with the Pyrodex pellets wasn't all that reliable. I've tried pellets in mine and seem to get varying amounts of ignition delay. With loose powder, ignition is fine. A year or two later, T/C brought out the Firestorm percussion sidelock with a radically different breech design to shoot pellets. This was a tacit admission that the BMM/pellet combination wasn't all that great.
Third, the gun was really aimed at a narrow niche where inline performance was desired by the hunter but the sidelock design was mandated by law (don't ask me where, just repeating what I've heard). To most people wanting inline performance, it made more sense to simply get an inline. Inlines are easy to put a scope on; BMMs are not and look funny when you do.
I like mine for their conical shooting ability, their toughness and the ability to take really heavy loads. Though I rarely load above 100grs. of RS, the extra strength gives an added margin of safety....and that rubber recoil pad is nice too
. Why don't I just shoot an inline? Well, I do at times, but the hooked breech BMM is easier to clean and maintain.
If the Loading Supplement booklet T/C packed with the rifle has any basis in truth, the BMM is a ballistic holy terror when fully utilized.
FWIW
Bob