Yes, You are right, a loaded gun is serious and there should be accountability. But again, you are relying people without a college education, some without high school educations to clean these cars. You cant expect them to really focus or even care about the importance of such things. You can fire someone everytime an incident like that happens but what good will it really do?? The next guy hired is more likely than not the same. No company can afford to pay someone who would actually care enough to ensure each and every vehicle is propperly prepared...not Hertz, Avis, National, Budget....no one.
Also I have found through internet research that a gun left in a car is not a unique experience to Enterprise alone. People have compalined about simuliar happenings at all of the other major rental companies and two have been sued over this within the last two years. Neither case was tied to Enterprise.
I do understand the seriousness of this situation but until you change basic human behavior (forgetfullness), no company will have a perfect record.
You could always drive your own car.........