The American M61 and M39 cannon fire a 20x102mm cartridge, with a rim diameter of 29.5mm. This cartridge actually started life as a WW2 .60" anti-tank rifle round (the gun was never adopted). During WW2, the cartridge was also used in an experimental US aircraft gun, and the tests included necking it down to .50 cal. The .50/60 was capable of firing a 42.8g bullet at 1,200 m/s or a 32.4g one at 1,340 m/s (that's 660 grains at 3,940 fps or 500 grains at 4,400 fps). After the war, the case was necked up to 20mm. There is a photo of all three cartridges on my website; see the article on 'Military Cartridge Relationships'.
The USA also necked down the 20x110 Hispano round to .50 cal in WW2; there's a photo of that in the same article.
Other 20mm currently in service include the 20x128 Oerlikon KAA, the 20x139 Oerlikon KAD/HS 820 (not the same as the 20x138B used in the WW2 Lahti and Solothurn S18-1000 anti-tank rifles), and the 20x82 in the South African Vektor GA-1 MG (the venerable MG 151/20, still being made).
Rifles currently offered in 20mm include the South African Mechem NTW (available in 20x82 or the 14.5x114 Russian KPV round), the Finnish Helenius APH RK97 in 20x99R (the WW2 Soviet ShVAK aircraft cannon round) and the Croatian RT20 AMR, in 20x110 Hispano. Take your pick, and insure your collarbone :)
Tony Williams
Military gun and ammunition website: http://www.quarry.nildram.co.uk
Discussion forum at: http://forums.delphiforums.com/autogun/messages/