The Chruchill rib is a raised rib. They were marketed in combination with his short 25" barrels on his shotguns. He made the arugement that the raised rib gave a visual illusion that the barrels were longer than they were (though he always said in his book not to look at the barrels when shooting...). Other ribs can be flat or convex, and can have different finishes (file cut etc).



Peter Blakeley's book "Successful Shotguning" has some nice pictures and explanations about this.

most people don't do instinctive shooting unless they are shown,when someone is used to shooting a long gun,they then inturn shoot there shotgun the same way.than there are trap shooters,the high rib,wether it is flat along the barrels or just high at the rear use the rib as a full plane sight.for thoes who do not want there stock altered,the rib can be made to change the point of impact.this works good for the shooters that do skeet,trap and sporting clay's.the stocks can be factory while the ribs do the adjusting for the different poi.beads do the job of getting the eye to pick-up the sight plane,if you try to line up the barrel,sight or not with the bird,your going to miss,birds and clays move to fast for alot of people,they shoot with out thinking and hit,some people just can't get down on the target quick enough,that is why some barrels are also bent,to taylor the shooter to the target instead of re-learning how to shoot.we have been building and installing ribs for 60 years,on all types of firearms,some people like them and some don't,it's just more choice for more people.