I saw IBS results and it seems the most used caliber is 6BR and improuved like 6 Dasher in Long Range Bench Rest Shooting. On the other side it seems F Class Open is going on the 7 mm (WSM-SAUM) direction. Why not 6 Dasher. Any reason?
The short answer is wind drift and the amount of time required to shot the number of shots you have to shoot to complete the match.
The long answer is small cases can win F-Class matches in many cases, but when it gets tricky the less wind drift of the larger bullets/cases more than makes up for the less potential accuracy of the large ones. On any given day you can flip a coin and take your chances on what will give you the best score. In my 50 years of shooting there are many more windy days than calm ones.