Good example are your fatigues as they don't typically hold much to begin with.
1. Drop your fatigues on the floor and open it up into the inventory menu.
2. Unload your weapon directly into it by right clicking the weapon's magazine.
3. Reload your weapon from another container (such as your backpack or a crate) and unload again directly into your fatigues.
4. You can repeat this with however much ammunition you can find, far exceeding the load capacity of your fatigues. Indeed, you can fit as many rockets as you can find into your fatigues alone.