I had the same problem with a different brand of chronograph, one that was known accurate with rifle loads. In my case it was muzzle blast & sonic wave at right about mach 1.
The cure for me was a cut piece of cardboard about 12" in front of the START screen with a 2" hole in it to allow the bullet through but to trap & disperse muzzle blast. I just cut pieces out of a box & built a Bubba gizmo with duck tape.
That dropped the errors to 25 FPS without anything else being done. Give it a try & see how it works for you.
Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”