GB, kamml,
Actually 20 gauges are used for trap a lot at my local range. Why? Because there are a lot of kids and small women shooting them. My wife is a case in point. She stands 5' 1" and weighs 103 lbs soaking wet. A 12 gauge, even with a cut down stock is just WAY too large/cumbersome for her. Shortest arms and smallest hands I ever saw in an adult female. Plus, she doesn't compete. It's just for fun and warming up for Pheasant season.
The difference in scores between a 12 and a 20 is small. For me, it amounts to about 1-1/2 birds per round. And, I'm talking about 7/8 oz. loads of 8-1/2 shot. Anything more than that tends to beat her up a bit, so I just shoot the same stuff as she does to simplify things.
I also shoot about 8 rounds of trap a year with my Browning 20 gauge for practice for Pheasant season. The reason is simple. The gun I compete with is a typical Trap gun. Shoots about 14" high at the 27 yard line. My 20 gauge is a typical field gun. Shoots dead on. I have to shoot a little trap to get back in the practice of using a field gun. I could raise the comb some and make the little Browning shoot like a trap gun but when a bird flushes, I tend to revert to shooting anything I am carrying like a field gun. The trap practice just reinforces that for me.
My gun is an IC/Full (typical for upland hunting) and I use the full choke for trap. The IC will break them but just in a few large chunks and I shoot fairly fast. My IC patterns more like Skeet 1. My wife's is Modified/Full (she's more deliberate than me) and she uses the Modified for Trap.
Incidentally, her gun will actually smoke (not break) the clays from 16 yards using the Modified choke and 7/8 oz of 8-1/2s. Our local State champion even asked to shoot it once as he couldn't believe it himself. I think it changed his mind about some of the misconceptions he had about a 20 gauge.
Ka6otm