The most common chambering for so-called 'kangaroo rifles' was Eley's .450 No.1, which fired a 270gr lead bullet at modest velocity from a 1 1/2-inch long brass case. Some were chambered for the .442 Carbine cartridge firing a 220gr bullet, the case barely an inch long. I have one, a Westley Richards, chambered for the bottle-neck .450 No.1 Carbine.

Occasionally we find kangaroo rifles chambered for the .425 Webley, a straight 1 5/16 case. IMHO this is most likely the .42 Kangaroo referred to in 88Mau's first post.

From memory, there was an article on these cartridges in an Australian Cartridge Collector's Association journal a few years back, if I get time I'll try to locate it.

Hope this helps meantime.