7.62

As stated, the AK-47 and FN-FAL/SLR both have barrels of calibre 7.62mm. However, the 2 weapons fire totally incompatible cartridges. The FN-FAL/SLR used the 7.62x51 NATO cartridge whereas the AK-47 used the shorter 7.62x39 Soviet cartridge. The NATO cartidge is more powerful and has greater range (hence it has often been used in machine guns such as the BREN and GPMG/FN-MAG, and in sniper rifles sucn as the SR-25), however the less powerful Soviet cartridge is more effective at wounding/killing a human at typical combat ranges (less than 400m).