In 20g, I've used IC/M but found them to be a bit tight for Calif Vly Quail in the chapparal. I've used a 12g with Cyl/Skt and did very well. I've also used a Parker 12g with Skt/Full to very good results. For my quail hunting in my favorite area, between Cyl and Skt is perfect for the first barrel. The second barrel should be at least Mod IMO, tighter is o.k. too, for my uses. But I wouldn't feel particularly disadvantaged with a Cyl/IC gun in 20g. The first barrel takes most of the meat and the second usually is either not fired or seldom fired in my quail hunting.
