Lupus brass used and re temped, fed match primers. Should I try magnum primers?
The problem is obvious...your brass has lupus.
I purchased one of the early Magnetospeed chronographs and used it for years. I was/am very satisfied with it. The only thing I didn't care for was the change in POI and group size/shape caused by muzzle blast off of the sensor deck, which rendered its use more difficult when firing longer practice strings at increased distance. Not saying it can't be done, but it's more work than it needs to be. In any event, I eventually purchased a LabRadar and have also tested them side-by-side. All the velocity trends were similar between the two, but there was a discrepancy of a few fps (not accounted for by distance to the muzzle). So I never worried about it.
If you or a friend have another rifle with a proven and velocity-stable load, it might be worth trying the Sporter with one of those and see what the results are. Alternatively, if you have a friend that has another chronograph, you could try both at the same time. As an internal control, the trends in velocity you listed above generally seem to increase with respect to increasing charge weight, as would be expected. Although that is not proof the unit is working properly, neither does it support a conclusion that it is
not working properly. This is the real problem with diagnosing chronograph issues; without a "second opinion", meaning a second set of simultaneous chronograph results, the velocity data need to be really bad, or the unit must fail to take measurements in order to conclude with any certainty the problem lies with the unit itself. Although perhaps they are not acceptable to you, the velocity ES values merely seem a bit high, which may also be caused by other factors such as the load itself.
Trying a different primer might be beneficial as you suggested. Alternatively, if the the issue really is load-related, it may work itself out as you develop it further. Finally, be certain the bayonet is not moving while measurements are being taken; it can definitely happen depending on how it is attached. Regardless, some kind of test to determine the functionality of the unit is probably in order, if only to provide peace of mind.
