the red insert has a smaller inner diameter so it restricts flow a little more than the blue one.
my NT came with the red insert in out of the box, I swapped it out for the blue insert before I ever shot the marker. I have been getting pretty consistent readings at the chrono (285,284,277,281)
ABET Accredited Level II Machinist - CNC machinist/programmer - Mech. Engineering Technologist
Polarst*r Engineering and Machine