>>394569667
Make sure to hold your gun steady when doing the AK bump. I find holding the mag sideways is the most consistent but the thing that matters most is moving the mag and not your gun. 
Unlike the HK slap it actually has some tactical value; If an enemy spooks you when starting to reload from a partial mag you retain the option to shoot more than just the one round in the chamber. It's also just as fast as a normal reload for me.
It works on both the mini AK and AS Val too.