In this case the crime would be purchasing a handgun in a state in which you are not a resident.
Handguns can only be purchased in your home state. It’s why California’s handgun roster is such a big deal. They simply refuse to allow new models of handguns to be sold there regardless of whether or not their features are illegal, and buying them out of state is prohibited.
With long guns, the gun’s features must be legal in both the state where the sale is taking place and the state in which the buyer lives. With pistols it’s simply illegal to transfer the firearm to someone from out of state.
If I was from Arizona and wanted to give a single-shot 22 plinking pistol to my Dad in Texas, I’d have to sell it to a local firearms dealer, have them transfer it to a Texas dealer, and have my Dad buy it there.