If you open the door to allow the lab to do any mods at all, then it will be impossible to define a limit to those mods. Ergo, no mods to the rifle as seized
. That would be my top line.
Define the ammunition and that could be a matter of debate to get agreement.
Define the test procedure, including specified ambient conditions, including temperature, humidity etc.
Define the testing equipment and specify the traceable trail for calibration etc, and apply calibration corrections.
Allow a tolerance for experimental error.
Standardise the format of the output, to be factual and not opinion.
There's a start.