I have shot 20 gauge most of my life starting with skeet shooting in the late '50's. I have used 20's for all of my bird hunting and really enjoy the gauge.
Any 2.75" 20 will propell the shot column at the same velocity as a 12 gauge. The only difference is that the 20 will have a little less shot in the column.
I also really enjoy shooting 28 gauge in the field and for skeet and informal sporting clays. I guess that while I really prefer the magnum caliber rifles, I feel that the smaller bore shot guns are just fine.
Anyone that has basic wing shooting skills will be able to shoot the smaller gauges as proficiently as the 12 and will really enjoy it. (except .410).
The first gun I ever fired was a shotgun. I had so much fun that I was hooked on shooting from then on.
In my opinion, the Remington 3200 shotgun was the best over-and-under ever made for less than $2000. I truly love the one I have. I have three sets of barrels for it: 12ga trap, 12ga skeet, and 20ga skeet. I change the buttstocks between the trap and skeet styles. I do all of my target shooting with this gun.
I prefer the 12ga to the 20ga because they reload better for me, but either gauge is fine for breaking clays. I do not hunt with anything other than 12ga.
I shoot competitive Skeet during the warm months of the year..also "dabble" at Trap and Sporting...use either a Winchester 101 O/U w/Skeetmaster sub-gauge tubes or a Remington 3200 O/U w/Purbaugh sub-gauge tubes...I usually just shoot the 20-ga. tubes in the 12 ga. event, but the 28 ga. is REALLY my favorite...in 1985, before Prescription glasses, gettin' old, etc., I had the third-highest 28 ga. avg. in the country...shot at 800 targets, "lost" 5--.9938, I believe...not bad for a "po' boy" from Georgia!!!....mikey357