Weapons exp?

So I noticed that each weapon has a progress bar and it doesn't seem to indicate what I've killed with (for example) pistols per say but THAT pistol.

I know I've killed many things with pistols but THIS pistol says 0/793950. So .. is there some benefit to keeping the same weapon and making this bar progress?

I've been replacing weapons with higher levels ones that I've gotten. Should I not be doing this and leveling up the same one? What is this progress bar really mean to me in game terms and what does it do when it fills up?

http://forums.defiance.com/showthrea...l=1#post225028

Thank you for the link :)