I have a Mk VI, and so does my son. The chambers are not stepped, but tapered. I would feel comfortable loading any diameter of cast bullet that will chamber. However i usually load them at about .452. My most accurate load is the 450229 Hollowbase. the RCBS 201KT does well as does the 452490.
Getting the load right is a bit tricky, but easier if you have Stan Waters' PET LOADS article on the .455. I have had success with 3.8 gr. BULLSEYE with any of those bullets. About 21 grains of FFF Goex also works in Mk I cases.
Cases are .45LC cut to .45acp length and rims thinned to .035. I also have some HORNADY Mk II cases, but haven't loaded any yet. My son has a box of FIOCCHI .450 CORTO, which is their name for .450 Adams, aka .450 Revolver. The cases are shorter than Mk II cases, but otherwise the same as .455
A friend rented ZULU for her kids. The 8 year old was facinated with Michael Cains' Mk VI. I'll have to take him shooting. This is the kid who shot 5 straight on trap at family Day at our range, and then lectured grown men on the finer points of the game!
A famous Sidearm, and great shooting.