I've hunted quail with more shotguns than I can remember.
My favorite is a Winchester Model 12 16 gauge 28" modified. I've shot more quail than any other with this combo. They're the right weight for me (6 5/8 pounds or so for a field gun with plain barrel) and with normal, lead loads, have the same range as a 12 gauge in my experience.
Honorable mentions include the wonderful Benelli Montefeltro 20 gauge. I killed a ton of black throated bobs down in Mexico one trip with one of those and have hunted with them in the US also. I also have done really well with a Benelli M2 12 gauge with 26" barrel that I hunted for 15 years or so. I have a really clean, older Winchester 101 20 gauge I want to try. It's lightweight with 28" M/F barrels and handles perfectly. I think it may work really well. I've hunted with various side by sides over the years but always migrated back to pumps, semi-autos and O/Us. I love the thought of a nice SxS but seem to shoot better with other guns.
I've primarily hunted wild flushing blues with the occasional covey of bobs thrown in and mostly on public land. Most of this hunting has been done without a dog so the birds can be hard to find. I found long ago that when I switched to modified chokes with quality 6 shot loads, I considerably increased my odds of finding downed birds.
