From my experiences you need to attack
red tagged events to effectively have a chance of getting famed weapons.
If you are aiming for a specific type of weapon lets take a blunderbuss for example, you need to train your blunderbuss within 3 levels of the requirement for the specific blunderbuss. I have NEVER gotten a rare weapon that their requirement has been 5+ levels away from my current level.
I have also found that attacking bosses such as Darkheart in a group gives you a better drop quality.
Most of the good weapons such as El Patron's Lost Sword/Famed are achieved from 30+ undead/human NPC's.
Also I have found that Loot Skull Chests occur more frequently if everybody in your crew is using the same type of weapon.
I hope this helped you,
-Moonwalk
If you would like to add/prove me wrong feel free.. I'll gladly add it in!