While in use, heavy magazines increases the magazine size of the weapon by the listed amount of rounds, however decreases the weapon's hip accuracy by 20%.
In exchange for this harsher downside, heavy magazines are located in the ammo slot rather than fit as a rail attachment as most Extended Magazine variations are.
Ammunition Belt takes into account the weapon's current magazine size, increasing the benefit provided by the suit slot enhancement in the case that a Heavy Magazine is used.