Damn, there must be a problem with the XP algorithm. I did not really paid attention to this during my testing (i generally close the game before death when i test xD). Will be fixed soon :p
About the AA12 : yes, at the moment it's intended to be the best weapon. I love AA12 in Killing Floor 2 and wanted the same feeling in my game : shredding everything on the screen !! :p
But actually, you the same chance to find any weapon in the world (once you unlocked it).
How do you feel about it ? Should it stay like this, or best weapons should be rarer ? Or only on later levels ?