Aguila has it right. Gun dealers used to fix folks under 21 with a steely gaze and say, "so what kind of RIFLE is this for? No, what kind of RIFLE
? Could that be for a Camp carbine, perhaps?"
A couple of years ago, the ATF put out a memo telling us to stop doing that.
As long as the buyer volunteers the information it's for a rifle, the sale is OK. At least, under federal law.