There should be a gauge stating how much more "xp" or "damage" is needed to level up a weapon. It isnt really a huge problem, just a convenience sort of thing.
I like this a lot because of a lot of reasons. One of them is because I think it's kind of simple to make it, and plus it doesn't take up any room if the gauge is inside the bar underneath the weapon.
I have ^ ^ Lvl 5 in shot, rain, and twinkler I came here looking to post for the exact same thing... Id like to see a "needed xp" for each weapon... just as we have it for Lvling up glad I'm not the only one to think about this... (but Im surprised.. this threat began over a year ago... and no change has been made..maybe we can revive interest in this idea)