Where are the fences on a shotgun?
The term goes back to the old days of percussion cap muzzle loaders, on which there was sometimes a semi-circular steel fence behind the nipple on which the cap was placed.
The idea was that they would deflect any hot escaping gases, and bits of metal if a cap fragmented, away from the shooter?s face.
Today the term is usually only applied to side-by-sides, to describe the near-hemispherical faces which form the rear of the standing breech.
On quality guns they are often elaborately engraved.