I would buy a cheap cone shaped bore gauge, they always come in handy. You can use an inside micrometer (caliper), but they are more expensive. .52 cal. is 52 hundreds of an inch and .50 cal. is 50 hundreds of an inch or is a half inch. The difference between .52 cal. and .50 cal. is too small to measure accurately with a ruler. Also watch how you measure, the riflings being worn or not worn can come into play. Take several readings with the gauge or calipers by moving them around in the bore. This should get you close to your gun's caliber.