Buy loose ammo, not a pile of magazines

Magazines cost money and weight. Buy one or two spare magazines and a stack of loose ammo instead, and refill your magazines from the inventory between fights (the "combine ammo" key, C by default).

• Tap reload for a normal reload, hold reload for a faster combat reload that drops the magazine. Pick it up again or refill another from loose ammo.
• You need a backpack to carry loose ammo; the Pouch is free.
• In the vendor, Ctrl-click buys loose ammo quickly.

The jackfrags video above shows it in practice.