What else do you expect to fit in the assault clips, bananas?
Let's assume that the customer and his family were new to firearms and were not going to take a class. It seems the most likely scenario anyway. Now let's forward to 4 years later. The gun is laying there in the closet, untouched since that last trip to the range a month after it was purchased. Bad guys enter the property with larceny in their hearts and crack in their veins (Yeah, that's right - I can do cheesy Guy Noir narration). Pop is off somewhere and Ma is looking to keep the bad guys away from Junior. What type of gun is in the closet? Press A for AR or B for 870 (No, I've never been a dungeon master).
A: Sorry, the little oil left in your AR since that last range session dried up a couple of years ago, assuming that Pop ever learned how to clean the gun in the first place. Chances are slim that it can fire once (it has to be loaded after all), and way less that it will feed after that. Ma has never actually heard the term "charging handle" before, and has long ago forgotten how to find it, let alone operate it. Moot point, since she's managed to drop the magazine while poking at the controls. May as well climb into that closet and hope the bad guys want the TV and not you. Maybe they just want directions to the nearest church.
B: While the oil in the 870 has probably dried up as well, it will run with little or none for a few hundred rounds. Not that this is any good for the gun, but seeing as it's only had a couple of boxes through it ever, it's still fully functional. Since Ma is operating under the delusion that it will spray instant death down the hallway without even aiming, her confidence level is high. This is a good thing today. Assuming that she can locate the slide release, somebody is about to trade their next crack fix for a colostomy bag or maybe a toe tag. Recoil? Not on Ma's mind right now. Nobody puts Baby in the corner. Now does this have the shoulder thing that goes up? I hear that makes it deadlier.
C: Nobody told me there was a "C". What is this? I'll tell you what it is - it's a revolver. It's still in perfect running order, loaded and ready to go, locked in that quick open safe that Ma can still open with ease. Manual of arms: 1) Point at bad guy. 2) Pull trigger. 3) Repeat as necessary. Too bad there was no option C. Darn.
Now before you start yelling, an AR or 870 is a perfectly legitimate choice for most of us on this forum, since by the time we forget how to use one we'll be unfit to operate a can opener. But for the typical person who would ask a Wallmart employee for firearm advice, neither is that great as a first gun.
So what have we learned? Training. Next time tell the guy not to buy a gun yet. Take a class. Take your family to a class. Try a lot of different guns and see what suits you. It's great that you want to defend yourself - we need more people like you - but learn first. It will save you a lot of money and frustration. If you have to buy something right now get a revolver. Then take a class. Then take your family to a class. You see where I'm going here.