How do I measure pellet fit and head size?

A PelletGage is a precision laser-cut sheet with calibrated holes (usually in 0.01 mm steps) for sorting pellets by head diameter.

Drop pellets through and they fall through the smallest hole that fits the head, indicating its size. JSB, H&N, and Air Arms publish target head sizes on their tins (e.g., 4.51, 4.52, 4.53 mm in .177). Match the size your barrel likes; many guns prefer 4.52 mm in .177 or 5.52 mm in .22, but it is barrel-specific. Calipers work too but are slower and less repeatable.