Admittedly the early coiled-brass MH cartridges were loaded with lead bullets of around .452 to .455 diameter, but the later drawn brass cases carried projectiles up to .458 diameter. Bill Fleming of "British Sporting Rifle Cartridges" fame, miked heaps of original cartridges in the various configurations, and in fact the single 'Carbine for BP' cartridge he measured carried a .459 projectile!

Admittedly Henry rifling is a bugger to measure accurately, however slugging the bore of a Martini Henry might surprise you! I believe the idea was that soft lead bullets would 'bump up' a few thou into the rifling at ignition, so were often loaded undersize. Hardened lead or jacketed bullets, on the other hand, will not obturate in the same way as soft lead, so need to be very close to groove diameter to start with.

Brent, when your rifle arrives, slug the bore! It only takes a minute.