Distance between the rifle barrel and the chronograph ?

I've tested this on my Red Shooting Chrony to see if the distance really matters in PCP and spring guns. 
It doesn't.
Air does not cast a shadow.
Besides, the pellet travels across the sensors before any air could have an affect. 
Ideally, you want the muzzle reasonably close (say 6") to the lead sensor to get the truest muzzle velocity. The further back you are, the greater chance you have of the pellet slowing down and the sensors pick it up. 

In Field Target 12 ft/lb competitors are tested with the muzzle at 6 to 8 inches from sensor.
Chrony tip: I put 3M clear packing tape over my sensors. You can easily wipe off any accumulated dust. Without the tape can become a problem with the chrony.

SvilenP did you get a rifle? Sorry I missed it if you did! Bill