In Mass Effect 3 for the PC, PS3, and Xbox 360, unlocking weapon capacity upgrades to use in multiplayer games is based on a card system. You earn points to buy card packs (or you can use real money if you’d like), open the packs, and you get to use whatever you get from those packs when you play online.
Weapon capacity upgrades only come in common and rare form, so you’ll need to purchase recruit and veteran packs to get all of them.
Unlockable Weapon Capacity Upgrades from Card Packs
Common Capacity Upgrades
Cobra Missile Launcher
Ops Survival Pack
Thermal Clip Pack
Rare Capacity Upgrades
Cobra Missile Launcher Capacity Upgrade +1
Medi-Gel Capacity Upgrade +1
Ops Survival Pack Capacity Upgrade +1
Thermal Clip Pack Capacity Upgrade +1