We talking about picking up range brass over in Reloading, but I think this is applicable to everybody.
With brass you can sort it, feel it, check it and weed out the undesirable stuff. BUT with loaded ammo, you don't know. There's no way to know how much or what kind of powder is inside. I've made it a habit long ago, never to trust ammo that I don't know where it came from. Anything loose on the range is suspect. If someone is shooting out of a box that they purchased over the counter and offers you their left overs, that's one thing, but be very wary of anything you find laying around.
HERE's an example of why:
Some shooters have guns built up that will withstand significant pressure loads. (we're talking .357mag or .44 mag levels). Nothing wrong here. BUT, they'll have them chambered in 9mm (as an example). There are a number of reasons for doing this and again nothing wrong. These shooters will then custom load 9mm ammo at major power factor (165), which means they're pushing a 125gr 9mm bullet in excess of 1,320 fps. The pressures will come close to 140% of SAAMI maximums for 9mm. The reasons for doing this are many, including ammo cost is significantly lower for 9mm than for .40SW or .45ACP. I know several shooters who do this. Not my cup of tea, but each to his own. These shooter are very careful about not leaving any of there shells around. When they clear their gun, they won't leave the area until they find the live round.
HERE's the question: When you pick up a stray 9mm round on the range, how do you know it isn't from one of these shooters? YOU DON'T. Found ammo is not a gift. While it hurts to see it go to waste, dispose of this stuff. Don't leave it for somebody else.
Be careful out there. And have a happy thanksgiving.