My old guns when I put them up to shoot, I really don't see anything except the target. I shoot with both eyes and to tell the truth I don't pay attention to the barrels.
Now on a few new (old guns) ones, I have not shot yet, I will take them and "aim" them looking down the barrels and just seeing the bead at the end and see how they shoot. Most have between 13 3/4"-14 1/4" LOP, drop between 2 3/4"-3" at the heel. As far as castoff I have no idea what that means.
In your question you had asked about how much rib you should see, I think that the more you see of the rib the higher you will shoot. They say that guns with two beads that you are supposed to "stack" them on top of one another for rising shots.
I used to do a reasonable amount of skeet shooting, was good for mid 40's out of 50, more fun than anything, just practice.
Taking a gun and putting it up to your shoulder in your house or at a gun shop, we do look down the barrels, but in hunting, I don't see the barrels at all, almost like the International way of shooting.
