Ah, let me add a little bit more confusion to that Rich.
The BC figure used in Chairgun and most other trajectory plotting programs is different for different velocities.
Well, it's not different, but Chairgun uses a table for BC/Drag retardation that has definite steps in velocity to alter the BC figure used in the calculation.
The table it uses is selected by changing the 'profile' used. So G1, G2 etc.. and Chairgun also uses a custom profile for airgun pellets called 'GA' which I can't find the actual figures for, but I've discovered you can use G1 and reduce the drag by 7.5% to give you a very very close answer to the GA profile.
We don't really know what's going on in terms of pellet stability between 40 and 50m though, so perhaps that has a bearing on things.
In the G1 profile - the steps in Velocity are : 810,780,750,700,640,600,550 - so when the velocity hits those numbers you will see a 'step' in the drag coefficient in the calculated trajectory. Not sure what the steps are in Chairgun's GA profile because I don't have the code for that table.
Taking measurements beyond 55 yards is an interesting idea.. I'd definately need to build some sort of perspex front cover for the chrono to do that though