I bought a used Red Label 12 gauge about 20 years ago. It was a 26" Skeet gun, that had been sent out for thin wall choke tubes. I wanted it for Sport Clays, too light and "whippy". So, I bought a used Browning Citori 30" GTI model (discontinued for around 20 years now) and learned Sporting Clays on that. I traded it for a used Citori 30" 425, and still have it, but I wish I still had my GTI. In Browning, all those different models mean , is the gun has different features. There are only 3 different Browning actions; Superpose, Citori,and The Synergy. They are all production guns, which means the features are standardized to a model, and the the fanciness is described in grades, I to VI (nowdays) or with fancy names (in the Superpose) When buying a used gun, don't assume it is factory original in the stock. Browning tries to keep the Sporting Clay guns a little long and traditional shooters have them cut short to fit their shooting and clothing styles!
I own 2 mid 60's Superpose and 6 Citori Brownings and only bought 2 new but on discontinued closeout. Of all those 3 are 20 gauge and 2 of those have 28" barrels. And that is what I would recommend for a well made all round Sporting, Skeet,and Hunting gun. Many or most have 3" chambers, some have porting, palm swells, and different fore ends. Spring is a good time to shop for used shotguns. Spend time in as many gun stores as you can, heft the gun to you shoulder, swing it in odd arcs including up if it comes away from your shoulder it is most likely too short. Plan on shooting lots of ammo, this is a very addictive sport! Skeet and 5-Stand are 25 bird rounds, Sporting Clays is 50 and 100 birds rounds. At one point 2 of my sons would go with me and together we would shoot 500 to 600 birds in an afternoon (youth shot for half price but the ammo cost the same). My personal record is 7, 50 bird rounds in one day. My most addictive form of Clays shooting has been 410 Bore. I took a friend (who is a much better shot than I am), and after a 12 gauge round, we did 410 and shared my 30" XS Citori. I warned him it was very addictive, but he laughed it off! By his third shot he was yelling at me for doing this to him. In six weeks he and his wife had spent $22,000 for 6, 410 shotguns! I'm just warning you!
Ivan