How can I put bullets in a magazine

To put bullets into a magazine, you first have to holster your weapon. The complete sequence is:

• Put the magazine in your left hand by either taking the current one out of your gun (E) or selecting one from the inventory (0-9).
• Press ~ to holster your gun.
• Now that you have your right hand free, you can add bullets to the magazine with Z
• Take your gun out again with ~
• Put the magazine into the gun (Z) or put it into the inventory (0-9)