A pump action would not be my choice for skeet or sporting clays, but I have shot clean rounds of skeet with a 20 gauge Wingmaster, and I have seen some impressive sporting clays shooting with a pump action. I have also seen some very good sporting clays shooters using semi autos, even though I prefer an O/U.
I knew that my answer would get that response eventually. You are absolutely right: you can, of course, shoot clays on report easy enough with a pump gun. The only thing is, they are rubbish at pairs as you can't reload fast enough and you loose a target every time you pump. In general, pump guns are lighter and therefore produce more felt recoil. The added step in having to pump the gun doesn't help to shoot well skeet or sporting (where the majority of the targets is pairs).
I have seen folks shoot pump guns as fast as I can pull the trigger twice on my o/u. These folks have been shooting pump guns all their lives. But! If you want to do well at clays with a pump, you have to be ready to waste a lot of money on clays with the one that gets away each time while you are learning. By the end of the day, the amount of wasted money might be sufficient to purchase a semi-auto or o/u from the beginning.


















































