I wouldn't exactly say its an exploit. Monsters all have a limited amount of exp to give. Each shot you are empty that exp pool. You'll notice right away. The ammo cost is more of a deal.
Oh I see, that is another interesting mechanic, so this xp pool is drained from each hit (dispersing xp) and then once killed the rest of the pool goes to the killer?
I will delete this post if I get no reply by tomorrow or so, as it is useless so I wont be surprised if it isnt looked at again, but im interested to know how it works exactly, if someone knows?