What is over-boring and lengthened forcing cones? Do they really make a difference in recoil?
The definitions for “over-boring” and “lengthened forcing cones” of a shotgun are straightforward. Their impact on felt recoil, barrel pressure and pattern integrity is more controversial and deserving of a longer explanation. When a shotgun is “over-bored”, the diameter of the barrels are increased by “reaming” or removing a slight amount of steel from the inner walls of the barrels. Over-boring reduces friction between the wad and the barrel as the wad travels from the chamber through the bore and out the muzzle. This reduction in friction is generally thought to reduce felt recoil and deformation of the shot pellets which results in improved pattern integrity and an increase center-pattern density. This school of thought is more common in the US than in Europe, where over-boring is frowned upon, thought to reduce internal barrel pressure and thus velocity. Because of the divergent philosophies on opposite sides of the Atlantic, some manufacturers even offer two different bore diameters for the same gun: one for the US market (larger bore size) and one for the European market (tighter bore and theoretically higher barrel pressure). There is somewhat less controversy about the utility of lengthened forcing cones. There are actually two forcing cones in each barrel: one from the chamber to the bore and the other from the bore to the choke. The term “forcing cones,” however, is most commonly associated with the tapered area of the shotgun barrel between the chamber and the bore. Let’s say the average internal diameter of the chamber of a 12 gauge shotgun is 810/1000” and the average bore diameter is 735/1000” when measured using a micrometer. When expelled from the casing, the wad must travel through the tapered forcing cone and into the smaller diameter bore. For some shotguns, this 75/1000 transition from the chamber to the barrel can be short and abrupt or, in the case of other shotguns, long and gradual. It is generally thought that, with all other variables being equal (weight of the gun and velocity / payload of the shell), longer forcing cones make for lower felt recoil and less pattern distortion. Jim Eyster of Heritage Gunsmiths, and his father Kenny before him, have been “Eyster-izing” or optimizing barrels and chokes for decades. Jim says that, “when fiber wads were common, lengthened forcing cones were not as important because these wads were more pliable than the modern plastic wad. Most of the damage to the shot column occurs in the forcing cones, either at ignition or right in the forcing cone. With modern plastic wads, longer forcing cones induce better center-of-pattern densities.”
If you have your barrels over-bored, forcing cones lengthened or both, be prepared to accept some consequential changes in the “feel” of your gun. By removing metal and weight from the barrels, the gun may feel a bit more “whippy” or lighter toward the front. While you can compensate by adding weights in the foreend, the weight will be redistributed and the gun will swing a bit differently. Additionally, your factory chokes will no longer fit and you will need buy custom chokes to achieve the appropriate amount of constriction for the new bore diameter. If you don’t overbore, but just lengthen the forcing cones, you won’t need custom chokes but you will still feel less weight between the hands and a bit more toward the front and back ends of the gun. Kreighoff International started lengthening their forcing cones years ago. Gun manufacturers like Beretta are now offering and actively marketing lengthened forcing cones (692 and DT-11) to cater to the growing awareness and demand in the American market.