I dont know about rare. It was certainly common in conjunction with the 30mm outer guns. It is critical to be able to choose to shoot the mg151 and the mk108 seperatly. They dont fly the same at all and hitting with the 108 is much more difficult if the 20s are flying with it. So I want to be able to select to fire the cannons seperatly, dont really care about the mgs. Cant believe it will make that much more difference.
But, if funked is right and cg is an issue. The magazines for the inboard 151s are right bellow the cowl mgs. So just give us equivilent weight in 20mm rounds....

nother topic.
Funked do you know why the 108s where not carried inboard??? where they not syncronizable? Would seem to me to make more sence to remove outer wing guns, cowl guns and replace the iner guns with 108s. You could have like 200rpg that way and the whole thing would probebly weigh less then the 4X20mm a8..
course thats alot of ammo in the fuselage.