300 round ammo box either takes more or less ammo than the description says.
Depending on the ammo you use you will either be able to put more or less than 300 rounds in the ammo box.
Mosin rounds stack 20 but the box only takes about 200 mosin rounds because of the stack size limit, however with stanag you have exactly 30 rounds but then with the pistol rounds it takes up to 400.

This is not game breaking, however if you say that it takes 300 rounds I expect it to take 300 rounds, regardless of what ammo type I use. ( other wise please specify)