A problem I found with 20 gauge pumps and semi's, is that they opening in the bottom of the receiver, where you have to shove your shells into the mag tube with your thumb, is much smaller than a 12 gauge, and my thumb gets jammed and stuck trying to do it. So, I have to turn my thumb sideways to shove them in. Highly highly annoying, particularly if you've shot a 12 gauge your whole life and are use to being able to shove those shells in really really fast. With a 12, you can actually do it with thin leather gloves on. With a 20 your glove would get stuck in the mechanics of the mag tube.
Anybody else have this problem? Again, so many disadvantages shooting a 20, except for a double.