As others said, since it was "yours" to begin with, if you wanted to sell it to her when she turns 18, that would be legal. Or, you could just give it to her. Also legal. She just cant front you the money to go buy her one, nor can you buy it with the intent of selling it to her. Dumbest law ever.
I was thinking of the retail purchase age not private. She still won't be able to purchase hand gun ammo at 18, unless the counter person buys the its for a rifle/carbine bit. Besides I like the idea of my daughter seeing me to go practice shooting anyway!