You put a rifle on the bags and shoot five shots at a hundred yards... you have five holes to measure and you can evaluate the rifle, scope and cartridge and be happy or go back to the drawing board or reloading bench. If you want to evaluate a shotgun and load, look at all the holes you have to count, all those percentages in a 20" circle, 30" circle, 40" circle.... plus you have to replace your shooting chrony every shot unless you're very, very careful.
