Well Chaz, there are 12-bore rifles and there are 12-bore rifles!

Single or double? ...and all-up weight will give a good indication of the intended charge.
Nitro or black powder? Rifling type and twist-rate will tell you a lot too, ie whether designed for round ball or conical.

Is there a specific rifle for sale, or are you going looking for something in particular?

Sorry to answer your question with more questions, but that's how it is.

Marrakai
http://www.hotkey.net.au/~orrs/Marrakai