Picked up a S12 when they were still $450. and want to shoot clays with it. The barrel is only 19" long, so trap loads spread about 10' @ 30yds. What is the tightest pattern @ 30yds I could ever hope to achieve using extra full choke?
I wouldn't think you would get much better than a 20" spread at 30 yards. The general rule is 1" of spread for every yard. With an extra full choke the 20" spread or slightly less may be able to be achieved.
Why don't you just pattern it and find out. A lot of trap ranges has targets you can use to pattern your shotgun. Then you can see where it patterns and how many bb's you are consistently getting at a certain range.
barrel length of 19 inches will still be able to pattern. Have seen (as Chucky said) peoples patterns on 20 inch all the way to 28 inch pattern similarly at 20 and 30 yards using the same choke