Yeah, AK-47's have probably killed the most people on earth. They weren't around for WW2, but in the years since, think about how many people the Soviets, and anyone who's bought arms from them has killed?
Alternatively, the 5.56x45 round has killed a lot of people. From Vietnam (a hell of a lot of Vietnamese casualties) on, the US, and NATO states have used this round.
The .30-06 killed a few in WW 1&2.
The 8mm Mauser certainly.
The 7.62x54R.
The musket ball...
The list goes on.