I agree that the unique shape of the bullet on a 9mm "ball round" is a dead give away (which this guy was using). Speer Gold Dot and other hollow point ammon OTOH could be rather tricky when you're moving fast and the rounds aren't side by side. I did a full inspection of the firearm and found it to be safe to shoot. The guy had a magazine ful of 9mm rounds. He fired one shot and got what looked like a double feed. He locked back the slide and manually extracted the case that formed t the .40 barrel and one round, chambered and fired again. After the second time, I suspected that something was up and stepped in to check out the situation. I inspected the first case that he extracted and immediately noted the hard hit on the primer. After reading the headstamp, I asked him what caliber he was shooting (to see if he knew the proper ammo for the G22 in his hand), and he told me .40 s&w. I handed him the case and asked if there was anything wrong with this particular case. That's when he turned several shade of red
and realized what he did. I inspected all of his magazines and discovered only one of them (the one he was shooting) was loaded imporperly. After fixing the situation and giving him a brief lesson on how to tell the difference between the two cartridges, he resumed shooting with his friends. It was a great "teachable moment" as his friends were first time shooters. Didn't think it warranted tearing him a new one in front of all of his buddies. He got the message "Lima Charlie" and quietly thanked me on the way out for not dressing him down in front of his buddies.