Do I need a chronograph and how do I use one?

A chronograph is the single most useful airgun tool after the gun itself.

It reveals your real velocity (often different from advertised), your pellet's actual energy, and the gun's shot-to-shot consistency (extreme spread and standard deviation). Use it to find a regulator's plateau, the sweet spot of an unregulated PCP, your usable shot count, and the ideal hammer spring or pump count. Optical units (Caldwell Ballistic Precision, Competition Electronics) start around $100; modern radar units (LabRadar, Garmin Xero C1) are $200 to $600 and do not need lighting setup.