1. The offense of carrying a concealed weapon applies in a car, unless you are not ineligible for a GFL or unless you have a GFL. Because you are ineligible for a GFL (under 21), you cannot conceal a weapon in a car. The statute specifically says you can carry openly in a car if you aren't ineligilbe for a GFL, it is silent on whether you can carry openly in a car if you are ineligible for a GFL. It is logical to conclude that you can carry openly in a car, but I would not be surprised to hear you are hassled about it if pulled over.
2. The offense of carrying a pistol without a license only applies outside your home or car (e.g., at the range). Once at the range, this statute would kick in, but it has an exception for "sport shooting." I'm assuming, without researching, that shooting at a range is sport shooting. The gun must be openly carried when loaded for the exception to apply (but, it has to be carried openly for #1 not to get you).
3. You can't legally buy a handgun at 18, at least from a gun store/dealer. I haven't really thought about whether it is a problem for the seller or you to buy one in a private sale. You can rent one at a range, and the cost to do so tends to be pretty low.
While it may be legal, some indoor ranges will not allow those under 21 to rent or even shoot on the range as a matter of policy. I looked into this when my brother was planning to visit as he was 16 or 17 at the time. They wouldn't even allow it with "adult supervision". Not sure if this is just a metro Atlanta thing.
Was this at hotshots range in Marietta? They didn't allow me to shoot either because I was only 20 at the time. Ed's does over past Barrett Parkway on I believe Carolton Rd or something like that. I think it's and Insurance Policy thing.
Sure was. I had a membership there, which was nice (discounts, guests shot free in same lane, unlimited time). Freiendly staff even though they tried to charge me an outgageous amount to do a FFL transfer. I live much closer to Ed's so I think a change may be in order.