The high comb makes the gun easier to sight like a rifle rathe than point like a shotgun (more cheek weld). The sloping rib makes the gun shoot high which makes it easier to hit a rising target. If one wanted to shoot trap as an end in itself, the specialized trap gun would be a benefit. If one would view trap as fun and practice for field shooting it would be of less value since that person would be practicing the wrong thing, that is shooting at the target rather than leading it.
I disagree with that. The first and best criticism given to me when I started shooting Trap was to
stop shooting like I was handling a rifle. I was told in no uncertain terms that one points a shotgun as opposed to aiming it like a rifle. When I started pointing, I started hitting.
The entire reason that many, many Trap shooters have their stocks fitted precisely to their physique is so the gun will shoot where they are looking - where the gun is pointed. The point about the rib and comb allowing easier shooting at rising targets is so but those elements also allow for a clearer view of the bird.
One does not shoot
at a target in Trap in the sense of rifle-like shooting. Leading the moving target is basic to hitting it. If one shoots
at the bird, then it is pretty surely a miss.
The basic instruction always includes leading the target and follow through. Take a look at any instructional book or video about Trap and note that leading is fundamental.
In addition to a generally heavier weight and the comb/rib as described, "Trap" guns normally have longer barrels; mine is 34". Why so long? The additional length is and aid to.....you guessed it....maintaining lead by maintaining swing. A longer barrel swings more steadily than a shorter "field" barrel. Short barrels are "lively" - easier to start moving but also easier to stop moving. Stopping the swing destroys the lead and results in a miss. A "lively" light gun is a pleasure in the field, a handicap at Trap.
Pete