I had bough an AR 9mm carbine when was 18.
When I needed ammo I just brought it with me in a soft case to a small gun shop and said in my most innocent voice, "I need ammo for this rifle my dad gave me".
Then I unzipped the case and they looked at it and told me that it shot 9mm and that it was $100 a thousand or $6.50 a box of 50.
I successfully purchased ammo from them so I just kept going to the same place.
I usually tried to get 1,000 rnds at a time, if I had the money.
In AZ it is illegal to purchase, from an ffl, a handgun or ammo intended for use in a handgun while under the age of 21.
By explaining it was to be used in my rifle the ammo became intended for rifle use.
Funny thing was it worked just fine in my 9mm pistol too.
My father purchased my pistol and I got it for my 18th b-day.