I like the old Mk VI, and the .455 rimmed round outperformed the .45 ACP when tried out by the US Military in some tests in a abbatoir, inspite of its low MV which is barey more than a .22 airgun at 12 ft/lbs. Very heavy bullet though. The .38 was adopted because there wasn't sufficient time to train the large numbers of men needed to face the threat in Europe in the 30s.
I didn't know one could own one on the 'Heritage' FAC, thanks for that info.