A lot of people on the other side of this debate seem to have difficulty with scale (Greenland melting in four days etc.) probably because they have been telling people for their entire adult life that three inches is actually six inches.
I put together a quiz to help out. Order these from left to right. Some are in metric units and some are English.
.762×39 .762×54 .545 .22LR .556 0.50 .22HMR
Bonus question. What is .223 inches in centimetres?