This was tested and verified by myself in Vanilla Arma. In the virtual arsenal. you can overload the backpacks, using the following method:
1. Fill the backpack with Titan AP (takes less space). Takes 8 AP missiles for the Bergen.
2. The backpack cannot take anymore of any type of ammo.
3. Once out of the arsenal, switch the AT in the launcher with AP.
4. The AT goes into the backpack, when it shouldnt since there is not enough space for it.
5. Switch the AP out with an AT on the ground.
6. You now have 7 AP and 2 AT, which isnt possible normally.

Inventory showed the Bergen with 7 AP & 1 AT.