I had written a response a couple days ago, and it disappeared. I know that my Alpha chrony is not top of the line and light can cause false readings.
I was surprised that there was a 200fps difference from one day to next. I thought that light shifts would give false or Errors, not consistent solid readings (only 1 shot missed).

Originally Posted by Muffin
I have used a Chrony before, and found one day that it had gotten hung up on something on the tripod and was not fully opened, only by a very small amout.............
Given the velocities it would not take much shortening of the distance between sky screens to effect velocity readings...............


This above may the reason, as it was open and prompted on a box, not locked on tripod.

Temp wasn't much different from 1 afternoon to following day, hot both days, low 80s one day to mid-high 80s the following, so I dont think RL22 temp sensitivity would be a problem. I have shot that same lot powder at higher temps in the previous barrel (different loads)

I do not think muzzle blast is culprit, as chrono was 15ft away both days, and shot through suppressor, unless shooting prone and closer to floor causes gases to expand differently?

I will retry this weekend, loading in 0.5gr increments from 64,5 upwards as long as velocities are in the 2900-3000fps and no pressure signs appear.