I don't know about you. But keeping track of ammo is not that interesting to do.
It can be cool to send out the Marines on a mission with limited resources where they have to make them count but for most missions the rule of 'they have enough ammo for the mission' seems to be the best way to go.
For the OP. As the Backpack got 200 rounds, that means 20 rounds of continuous shooting. Where the other marines will have 18 rounds of fire with their reloads of 3. So you could rule that he can either jug around 3 clips of 60 ammo or 1 backpack with 200.