Anyone who owns a firearm knows what caliber it fires, nobody buys a gun but not ammo for it. It's similar to driving a car, you don't have to be a car enthusiast to know that you drive a Honda Civic. Just like how you don't have to be a gun enthusiast to be a firearms carrier. But at the absolute minimum, people will know the caliber of their firearm. These are all basic things a gun store owner will ask you when purchasing. Not to mention, many handguns have the caliber they fire stamped onto the barrel or the receiver.