If they’re such important measurements, why do they differ from one gun to another?
Imagine a straight line, projected backwards from the line of the rib along the top of the stock.
?Drop at comb? is the vertical distance between this line and the front of the comb.
?Drop at heel? is the vertical distance between this line and the heel of the stock.
Distances differ from gun to gun for the simple reason that shooters come in different shapes and sizes.
Some have long necks and sloping shoulders, so they generally require a lot of drop: others with shorter necks and straighter shoulders require less drop in order to keep their eye in correct alignment with the rib.
Gun actions also differ in depth and form, and this factor affects drops, too.
This is a generalisation, and the regulation of drops is one of the basic elements of gunfitting.