The GAU-12 fires 25x137 and the GAU-8 fires 30x173, so it's more than just a slightly larger diameter bullet: http://milpas.cc/rifles/ZFiles/Misc/MILITARY%20CAR

The GAU-12 fires 25x137 and the GAU-8 fires 30x173, so it's more than just a slightly larger diameter bullet: http://milpas.cc/rifles/ZFiles/Misc/MILITARY%20CARTRIDGE%20RELATIONSHIPS/MCRel3.jpg http://milpas.cc/rifles/ZFiles/Misc/MILITARY%20CARTRIDGE%20R... I didn't dig too deeply, but I found recoil force measurements... 4000lbf for the GAU-12 and 10000lbf for the GAU-8.