Guns With Different Attachments..:

If you want more ammo for your gun you might have to pick up another person's gun of the same kind into your secondary because it has different attachments. When you pick up the weapon, you will get any ammo that is in the current clip.

ex...
You have an m4 with silencer with only 15 rounds left, you run over to another m4 and get only 10 rounds. However, the m4 on the floor has an acog scope, so you have the ability to pick it up. Switch to secondary and pick up the m4 and then pick up your secondary again. You now have 30 more rounds.