Tacoma makes an excellent point about gun fit. Gun fit is indeed the most important item regarding shotgunning.
My O/U trap gun has 30" barrels. One of the best improvements I had done to it was having an adjustable comb and adjustable but pad installed.
It provided a vast improvement to my shooting. With my eyes closed, I could then mount the shotgun, then open my eyes and be looking at a perfect figure 8 stack of the beads. Plus no longer was I going home with a swollen cheek bone from being pounded by the comb and a bad flinch to go with it.
A typical registered trap shoot is 100 singles, 100 handicap, and 100 doubles. That is alot of pounding. If the gun does not fit properly fit, you will develope a flinch. And that is why 2 3/4 dr 1 oz loads are used by so many shooters.
Generally 1 1/8 oz #7.5's are used for handicap distances beyond the 24 yds. Amongst 27 yards shooters, there seems to be a split between 2 3/4 and 3 dram loads.
One tip, most beginning trap shooters will leave thier eyes focused on the beads when they call for a target. To be successfull in trap you need to learn to only use the beads as a reference when first mounting the shotgun, and shift your focus beyond the trap house. I have a routine that I use: mount - check beads - shift focus - call - track - shot.
D-squared.