Yeah, it doesn't make much sense. Everyone knows there are plenty of carbines that shoot pistol ammo. But I have had to show my ID every time I bought 9mm at Dick's Sporting Goods, and I assumed they were trying to verify I was 21 (because I'm pretty sure I don't look young enough that anyone would think I'm under 18.
) I don't remember if there's a law against selling pistol ammo to people under 21.
If you have friends who are over 21, best thing to do is ask them to come with you. Or just ask somebody you know to grab you some .357 the next time they happen to be in Cabela's.