Glitch: More Ammo - This works in single and multiplayer.
For just testing this glitch it's easiest at the start of the first level because of all the guns lying around. First you need to find four one-handed weapons, a few of which must be the same type of gun.
To save confusion I'll just say the 4 guns are 3 SMG's and a Pistol. Pick up the Pistol and 1 of the SMG's (NOT dual-wielding) Your left hand should be empty.
Have the Pistol in your right hand and pick up one of the SMG's off the ground with your left hand. You should now have a pistol in your right hand, an SMG in your left hand, and another SMG in reserve, leaving only one SMG left on the ground.
Stand over it and even though you already have an SMG in reserve it will say "Hold X to pick up SMG" Hold X to swap your Pistol for the SMG on the ground. If you did it properly you will now be carrying all 3 SMG's, 1 in each hand and the third in reserve, giving you a maximum of 540 extra rounds rather than the usual 180, (or 360 if you dual-wield) This will work with any one-handed weapon.