I have done this test several times using chrono but also a "fired" EH for artillery which triggers at shot. It's possible to hint or display elapsed time from shot (i.e. with diag_ticktime comparison in script). Results are same.
I haven't found why M4 ballistic time could be so different from calculated ETA when shooting at Stratis airfield (regardless of light or heavy scenario).
If you aim and shoot at Agia Marina or surrounding hills, there is no problem.