When you empty a double m4 magazin you get a 60 bullet stack. But you cant combine two 30 bullet stacks to get a 60 bullet stack. Its the same with an 40 bullet magazin when you empty that you get a 40 bullet stack but anyway cant combine it from you own.
It seems you can only create m4 bullet stacks up to 30 bullets for your own. Thats not right cause you can even have 60 bullet stacks when empty the double mag.
So to get 60 bullet stacks you have to fill the double mag an empty it, but cant combine up to 60.