When I read the title, I said to myself "Depends which gun fits best."
Amen.
During the first few years that I shot competitive sporting clays, I tried 8 different guns in 4 gauges, and ended up shooting my best scores with a 20 gauge Beretta O/U. I did better with that 20 than with any of the three 12 gauge guns I used. I never did a comparison of test patterns, but the results on clays were clear.
In the 20 I used 1 oz Winchester AA target ammo, and once in a while 1 1/8 oz Winchester Double-X field loads (2 3/4") with buffered, copper-plated shot, with excellent results. The only reason I didn't shoot more of the 1 1/8 oz loads was they cost over $12 a box!
I enjoyed a lot of success with that 20 gauge gun before I finally found a 12 gauge with the right combination of weight, balance, and stock fit to equal the 20.
The replies to this thread have used a lot of facts, myths, and speculation in an attempt to prove that 1 oz or 1 1/8 oz loads don't work as well in a 20 gauge as in a 12 gauge. My experience clearly demonstrated to me that, while all the theories may be true, the effect is so small that it is easily overshadowed by other factors.
The bottom line is, forget the theories and the pronouncements of "experts", and shoot whatever works for you.
