While this does not qualify as a scientific sampling, I just sorted a large box of range brass my brother brought me. At our local range, most just walk off and leave brass everywhere, a violation of the rules but they do it nontheless. We usually carry a broom and dust pan in the truck, and sweep up all the empty lanes and dump them in a box. I just went through about 3,000 pieces of brass, and of the pistol brass, the majority.....a LARGE majority was 40, with 45 ACP and 9mm coming in second and third. Obviously the 40 IS popular, at least here abouts.
By the way, in rifle brass, it was 223 hands down.
Ben