Fair, however still shocking. If they are working that counter, they need to be trained, period. Tactical shotguns have pistol grips. Does that make them a handgun, or is it still a shotgun? Training will teach the differences. The customer can always still lie. Buy as you pointed out, buying .40 S&W for a carbine would/should bring up a red flag to someone whose trained.
Yes, it's Wal-Mart so you can't expect a lot from them. Not to bash the employees (not my intent at all), but Wal-Mart is not exactly known for there training in anything. The 17 year old kid running the paint department barely knows the difference between a brush and a roller. The guy in home and garden has never mowed or fertilized a lawn in his life. So the odds of anyone in the gun department knowing the difference between rifle and pistol ammo is not expected to be high.
On a side note, do you have to be 18 to sell ammo? I know you have to be 18 to serve liquor, but I've never seen anything that states the same thing for ammo sales.
If it is designed or redisigned to be fired from one hand, it is a handgun...
Bring up a red flag? Why?