If a 75rnd Drum mag is attached to the AKM, reload ("R") does not work. The mag only allows itself to be replaced by another one if it is completely empty.
If one whises to change the mag before it is dry (in a firefight, unsure about bullet count), this needs to be done via the inventory screen.
This is different for 30rnd mags which can be changed even if only a single bullet has been fired. This is how it should work!
It should be noted that I only had one 75rnd mag and cannot say if it works if you have multiple mags of the same type (Additional 75rnd in Inventory). My 30rnd mags where aftermarket, not original.