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).
A shot from a 7.62x51
SLR at close range would typically go straight through the target and out the other side, doing minimal damage and
transferring little monentum to the target. A shot from an
7.62x39 AK-47 will tumble inside the target, doing considerable damage and transferring all of its monentum to the target.