Borderlands Enhanced how to: Weapon Merge
While dropping a locked weapon, once the model disappears some of its bonus stats are now equipped to your character, and will roll over even when you switch a weapon. This works with critical and melee damage, allowing you to stack these stats of all 4 weapons into one. If you are having issues with this working, make sure your weapon is fully drawn before dropping it. Weapon merging will only function in multiplayer if you previously locked your guns.