Once the weapons itself reaches max XP, its suppose to be swapped out for another weapon, so you can keep leveling your skills. Max XP weapons, no longer level your weapon skills so there's no point in keeping it, unless you want it for the bonus stats it gained.
As far as I know, you cannot reset a weapon once it reaches max XP, this encourages people to always change up their main weapons. I know it may pee off some people but its an MMO, not an FPS, so keep the weapon if you want to use it for battles and so on but just remember, no skill increases if your at max XP with it. There's plenty of weapons to go around and no need to stick with one type, just swap it out for another class and level that while you search for a new drop.

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Double barrel Shotgun will be a pain to get to skill 20 -.- @ 2 shots per reload + 1hr to reload the gun (sarcasm)