It isn't necessarily a calibration thing. Even when properly calibrated in ideal conditions, they need to make sure they have the right car, and there is still a margin of +/- 2 or 3 mph, sometimes more. They can also be reflected, and there are a number of other situation specific but real potential errors. I was simply suggesting that if there is a cutoff at 15mph (in vt there is for junior operators), that he admits to speeding, but to say that he was under 15 over, which is within the margin of error of the radar gun. This way he would face a smaller ticket, less on insurance, etc. I'm not suggesting that he try to have the ticket voided because the gun was off by a little, but to get it changed to a lesser charge (if applicable).
Eg. You are doing 100 in a 65, only car on the road, not raining etc., and you get caught. Your car might have only displayed 95, the radar might say 102, but the margin of error is so small compared to the violation, that it is quite clear you were speeding. Not really worth arguing about a few mph.
If you are clocked at 67 in a 65, you can and should get it overturned (mostly for insurance purposes), because that margin of error is the difference between guilty and not guilty. Same thing when there are tiers for speeding. In vt, first is 1-15 over, after that it goes into the next category. If you are near the border for one of these, then you should argue.
Hope this helps.