I have had lots of weird readings from my Chrony Chronographs. The errant readings fall into two categories: 1)off axis bullet travel over the sensors. The chronograph has to be as close to perfect, in line with the bullet travel. And the bullet should go right over the centerline of the chronograph sensors.
And, 2) unburnt powder residue. Messes up a chronograph all the time. For some reason, long barrels have a bad affect on my chrono's. As an example, the Schmidt Rubin M1911, my chronograph registered some great velocities, if they were only true!
7.5 x 55 Swiss Model M1911 29" barrel
174 FMJBT 57.0 WC852 INDEP cases INDEP primers OAL 2.85"
30-Apr-03 T = 86° F
Ave Vel = 2837
Std Dev = 188
ES = 544
Low = 2646
High = 3190
N = 17
I think the velocities were so high because of the muzzle blast. The chronograph really shook after each shot. The next time I took it out, I made sure the chrono was a long distance from the muzzle, and the velocities are radically different, and, close to what I think they should be. There are time when you need to move the chronograph out, away from the muzzle blast. Blackpowder is one of those times.
174 FMJBT
57.0 WC852 Swiss "T" case WLR OAL 2.850"
7-Mar-18 T = 48 ° F
Ave Vel = 2497
Std Dev = 22
ES = 102
Low = 2437
High = 2539
N = 24
So, move your chronograph out. And, shoot over it with something like a 22 lr. I have been blasting a case of Rem Target 22lr this way, use that ammunition as a calibration standard before I start banging away with the big stuff.