I don't really even see this Kentucky tragedy as, necessarily, a case for locking up guns. This was just gross negligence that could have been avoided with far less parental care than investing in a gun safe. Maybe (?) we all accept that and some are just not willing to come out and admit it, I don't know.
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However, on the question of keeping guns locked up:
I must say it does seem that for many of us the argument that we grew up with guns and ammo were stored in an unlocked closet, or otherwise easily available, and our strong desire not to have to spend the money or go to the trouble (or give up the "freedom", I guess, though I don't get that one) to get a gun cabinet or safe or good locks on a closet (etc.), appears to really equate to acceptance that if our kids are too foolish, or too dumb, or too sneaky, (or their friends are) and they do something horribly tragic, well, that's just life and these things happen and that's an acceptable price to pay for...well, whatever. The "freedom" not to lock things up, I guess.
I'm not sure why we'd be so very resistant. Shoot, when most of us grew up no parents used car safety seats. These days many fewer kids die in car accidents. Same with safety precautions on many other items. From pill bottle tops to safety rails around stairways, and on and on. Nothing's perfect, of course, and a very determined young person, as they get older, may indeed defeat even a heightened level of security. But does that mean we should not make the effort for the benefit of our families?
It is a very cavalier way of thinking, indeed, that says a little money spent and a little effort exerted to very significantly reduce the chances that our kids will DIE this way just aren't worth it.
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Paraphrasing an old MADD advertisement I remember from childhood, if you won't do it to save your kids' lives, do it because guns are valuable and you don't want them stolen!