I don't know if this is a glitch or not, but once you can afford the laser, overkill and reload upgrade, buy them and you'll make over 10 times the amount of xp than with any other weapon. I made 3000 xp one level, then I got the laser and made 55000. Just kill as many zombies with one shot as possible and you can get up to 15000 xp per shot. I was making 200,000+ xp per level before I got bored. It would take hours for me to die with all the health I bought since there's not anything else to do with it.