I started shooting the GP's just to see more courses and harder targets and I saw some very clever courses last year. If I think about the hardest of last years targets, there were quite a few where they had created an optical illusion and made targets look further away than they were. You would expect all the top scopes to read the distance correctly and hit the target but the illusion put enough doubt in peoples minds for them to quietly ignore their scope and miss!
Also the local ground conditions were put to full use, if you lit a bomfire on the course you might see some strange wind effects caused by the topography and vegetation and this can cause one target to take an inch of wind and the next (seemingly similar angle and distance) to take none.
If the course can be set out in a fan shape then you could have left to right wind at the start and right to left at the end so you never have a fixed idea of how much wind the targets need. And cutting channels through the undergrowth perpendicular to the lanes (and out of sight of the firing line) causes almost invisble wind, which has been employed by the Dartmoor marksmen to full effect. The only downside to these course building tricks is that it gives quite a lot of advantage to the locals, who soon get to know the landscape and its effect on the wind.
Anschutz 2013 MEC1 .22LR
Miroku 3800 12bore