Collecting Antique, Vintage, and Modern Firearms and Ammunition.
A Couple of Simple Ammo Comparison Charts
Sometimes it helps to have a visual comparison of different ammo calibers and sizes, especially when you're looking at purchasing a firearm in a caliber you're not familiar with. These pictures should be helpful.
More ammo caliber and size comparison charts
Rimfire Ammo Cartridges ID and Comparison Rim-Fire