If I had a dollar for each time someone told me they thoroughly checked their bag for loose ammo and other prohibited items as I showed them the loose round of ammo (or in about 10% of cases a full box of ammo) I'd have been able to retire from the TSA after only about a year.
Of course they could have been lying. There was also that family that liked to prank each other by putting live ammo in each other's bags (what a great family!). You'd be surprised where a round of ammo can wedge itself. They can be nearly impossible to find without the help of an X-Ray machine. Which is why I always recommended getting different bags for travel or range/hunting, or at least check (as in, checked luggage) your range/hunting bag.