A 12 gauge will pattern differently than the same weight of charge in a 20 gauge. This was true in the days previous to shotcups and hard shot. It's not true today. Pattern 10 examples of 7/8oz 20ga. loads and compare them to 10 examples of 7/8oz 12ga. loads, shot from the same choke, and you'll have a hard time seeing a consistent difference.
This has been extensively tested, and trap shooters would have changed a long time ago, if there weren't an advantage with the 12 gauge. The advantage in the 12ga. has always been its ability to handle 1 1/8oz trap loads, which were and are the standard (maximum) allowable trap load. As an aside, trap shooters have no commercially-available 1oz or 1 1/8oz trap loads in 20ga. There are no BT-99's, 682's, XT's, etc, made in 20ga., either. I'm fairly certain this plays a part in the lack of 20ga. shooters on the trap ranges.
There are any number of 20 gauge loads which handle 7/8 and 1 oz. loads. Yet, none of the top shooters use 20 gauge guns.
Again, non-availability of suitable trap loads or guns, in 20ga, plays a major part in this.
The difference in bore is huge, and this has a real effect on the pattern needed to break trap targets between 16 and 27 yards.
Again, today's ammunition technology has pretty much evened the field, and a 20 will pattern 7/8oz loads as well as a 12....skeet scores attest to this.
You can have a lot of fun using a field or skeet type gun on the trap range. Just don't expect to win a lot of shoots against guys who shoot dedicated trap guns. What Dennis meant to say is "Just don't expect to win a lot of shoots against guys who are better shots than you." :wink:
I looked at the top 100 trap shooters equipment, and they all use the 12 gauge. No available 20ga. trap loads, no trap guns available in 20ga, have more to do with this than perceived inferior 20ga. patterns. Believe me.
There are also any number of recoil reducers which assist the trap shooter, either spring-loaded, hydraulic, or mercury types. They are sold by the thousands, and they do the job.Right you are.