If they can keep purchasing it to "rent" it just to test it out, it shouldn't be able to count kills. This is just my view because you're using a high damage gun that you didn't pay fully for. It's only to test it out anyways.
So maybe only have it limited to 1 "purchase" per person for that 10 minute use limit, because to me it would ruin the whole point of buying the gun anyways.
I know there's gun game, which does help out by showing you how the gun does while PKing, but of course that's rarely done.