With a 2 window unit, the front window "STARTS" the timer and the rear window "ENDS" the timer. With the more expensive units, 3 windows are used as a "CHECK AND BALANCE". It's all a matter of START/STOP time as measured against a known distance between the timers.

Length of the object does not effect the START/END of the timing session. I have a very cheap "CHRONY" that I have used for years with firearms, shotguns, and bows with no problems.