I don't know how many times, me or one of my hunting buddies has stumbled across a piece of brass and then we play "name that caliber" without looking at the headstamp. It's typically pretty easy to get close, but sometimes a 25-35 or some other old piece of brass will throw you off.
Well, Here is a picture of a few pieces of brass that you all can try to deduce electronically. They are in ascending order of caliber size from left to right.
If these are too easy, I can dig out the bullet collection and really try to stump you.