To add a little information; the cartridges were exactly the same except for the primer. You can tell the difference because the primer has an inner, insulating ring. Electric and percussion primers were not interchangeable between guns. The key advantage of electric priming is that it is more precise, so better suited to being synchronised to fire through the propeller disc. Only Germany produced electric-primed aircraft guns in WW2, although they are common now.
The MG 151 was the only WW2 aircraft gun which was simultaneously available in electric and percussion versions in the same air force (although the cartridges for the MK 101 and MK 103 guns were identical except for the priming; the MK 103 was electric). The electric version of the MG 151 was only used on the FW 190 family as far as I know, the rest used percussion. I don't know why the Japanese had the electric one as the Ki 61 guns weren't synchronised.
PS I should add that although the German MG 131 was electric primed, the Japanese version of it - the Navy 13mm Type 2 - used percussion priming.
Tony Williams: Military gun and ammunition
website and Discussion
forum