Are there any restrictions or whatever on 'military' chamberings in Oz, or are you free to choose the best option available? On a related note, is your list the most common chamberings available to you, or just personal preference? I understand certain things (specific bullets, brass, barrels, and dies) are very hard to come by down under, and I assume others are plentiful.
If you want a black powder thumper, I'd say a 45-XX or even 50-XX would be pretty cool. Sort of a buffalo gun, basically.
For more of a hunting/target gun, I'd suggest picking whichever of the identical WWII 30-cal centerfires is most common and rolling with it (7.62x54r, 8mm Mauser, 7mm Mauser, 30-06, 308, 7.5x55, 7.5x54, 7.35x51, 7.7x58, and I think like six or seven others, all with essentially identical performance/ballistics as far as you & your rifle are concerned). Obviously some prudence is required when choosing options for a smokeless conversion, but the Martini-Henry is at least one of the stronger black powder actions (I'm guessing only the true falling block Sharps & Farquharsons are stronger, and they're overkill for practically everything survivable by a human shooter)
What cartridge ruled the ranches in Australia back in wilder times? That might be a practical and historical option. Me, here in the States? I'd do 45-70 or 50-90, but those have special nostalgia that I can only assume is present in different chamberings over on the far side of the Earth
For barrels, you'd be much, much best served by going with a new blank and having a machinist contour, thread, and ream it for you. I'm not sure what sort of options you have, but again, that will have to factor in. In the US, we're blessed with Green Mountain, who somewhat recently began selling unfinished rifled blanks in multitude calibers to individual purchasers (as opposed to manufacturers) and has made projects like yours much more affordable.
TCB
