Here's something else. Like Canada, in the US, anything over 12.7mm is a cannon. Which means the US .50 cal is considered a rifle cartridge, but the British .55 Boyes round is a cannon.
However, this doesn't apply to large cartridges used for hunting because of special permission from the ATF. There are several hunting rounds that fall into this catagory such as the .505 Gibbs, .550 magnum, .600 Nitro, .700 Nitro Express, etc.
BTW, here is pic of the.550 Magnum.
550 mag, 460 weatherby, 458 lott, 30-06
The specs on the .550 are: 700 grain bullet doing 2420 fps. Over 9000 ft lbs of energy. Compare that to the standard .30-06 load of a 160grn bullet doing around 2700fps (ballpark figures).