Some barrels are just slower, my 16 J&T has a generous size chamber on the loose side of tolerance and produces less velocity than most reported velocites from 16 6.5 Grendel. My Faxon 16 Monster barrel produces velocities closer to what I see reported.
Verify the spacing is correct on your chronograph sky screens. Most new chronograph sky screens are already mounted to a bracket but not all. Some chronographs have optional sky screen spacing so verify your using the correct distance setting. Lighting conditions can affect the angle of the bullet shadow cast on the sky screen creating less or more than the true velocity.
Shooting at an angle and not square over the chronograph screens can have an effect on recorded velocity.
I know people that use a Magnetospeed chronograph at the same time they are using an optical chronograph to try to avoid erroneous readings or no recorded velocity.
Verify the spacing is correct on your chronograph sky screens. Most new chronograph sky screens are already mounted to a bracket but not all. Some chronographs have optional sky screen spacing so verify your using the correct distance setting. Lighting conditions can affect the angle of the bullet shadow cast on the sky screen creating less or more than the true velocity.
Shooting at an angle and not square over the chronograph screens can have an effect on recorded velocity.
I know people that use a Magnetospeed chronograph at the same time they are using an optical chronograph to try to avoid erroneous readings or no recorded velocity.