If you're looking to measure the speed of projectiles firing out of your airgun, you'll need a chronograph. Should you measure the speed of every single shot you fire? No, but you should know what speed a particular pellet comes flying out of your airgun, so you can monitor your gun's performance at different intervals. This will help determine if your gun is not performing correctly.
Competition Electronics ProChrono DLX ChronographMSRP: $188.99:page_builder.pdp-sale-price-label $161.99
Competition Electronics Replacement Diffuser Pack, Fits ProChrono ChronographsMSRP: $15.39:page_builder.pdp-sale-price-label $13.19
Shooting Chrony USB to 9-Pin RS232 Serial AdapterMSRP: $44.00:page_builder.pdp-sale-price-label $10.99