This trick will let you have up to 15 rounds in any gun.

1. Make sure that you have a Beretta 92 in your inventory.
2. Select the gun you want to have 15 rounds in, and make sure that is the ONLY thing you are holding.
3. Drop the gun on the floor.
4. Take out your Beretta 92 and pick up the gun you just dropped.
5. You will be holding both guns at the same time, and you will have 15 rounds in both guns since they always shoot together.