The high ribs that one sees on dedicated Trap guns - virtually always single barrels, double guns have more standard ribs - are a convenience for making the gun shoot high. Many Trap shooters want the pattern high so that they can keep the target in clear sight during the entire process of the shot.
Field guns tend to shoot considerably flatter than Trap guns. Generally, when I use any of my field guns for a round of 16 yard Trap singles, I have to cover the bird when I make the shot. With my Trap gun, I can float the bird and follow it until it breaks (and past).
I suspect that a very high rib on a field gun, which is never pre-mounted as it is in Trap, would be cumbersome in the field. In addition, the type of guns commonly used in the field have a different geometry because of their receivers, etc.
Or so it seems to this shooter.
There are certainly more knowledgable shooters than I who can supply more detailed info.
Pete
