Since I'm under 21...
Ill usually have an adult go with me to get handgun ammo (which is really embarrassing when your 19, btw). There was this one time that I went into Wal-Mart to get some 9x19 for my P99. I was asked the question and I answered "rifle" in hopes to aviod the whole "must be 21" thing. Lady looked at me and smartly remarked, "really, because this is handgun ammunition". I politely informed her that while 9mm is a very popular handgun round, there are many different carbines chambered for the round. She just looked at me, asked for ID and rung me up.
Oh yeah... quit bagging on the Wal-Mart employees. If you want knowledgeable staff and top-notch service go to your local shop (where you may get 1 out of the 2). Im working retail while going through college and let me tell you that its not real fun. You are constantly moved around and you may be put in an area that you know nothing about because someone called out. Most people dont seem to realize that when you just barely make over minimum wage and get treated like trash all day, you tend not to want to take pride in your work. I have gone so far out of my way to help customers that it isnt even funny, only to get cussed out by the next customer. Realize that most Wal-Mart employees dont work there because they want to.