Shotgun, Forcing Cone.
The forcing cones in a shotgun barrel force the shot charge or column down in diameter to enter the bore and then to enter the parallel section of the choke. The one we will concern ourselves with here is the chamber to bore forcing cone the other is contained within the choke or choke tube.
We will start with identifying the forcing cones:In the diagram above:
(2) Chamber Forcing Cone
(4) Choke constriction (forcing cone)
(5) Parallel or lead
Chamber to bore forcing cone:
There has been a lot written on the subject of chamber to bore forcing cones, some good, some bad, the most experienced and authoritive goes to the late Ralph T. Walker this appears to have been his life work "shotguns" his book Gun Digest Book of Shotgun Gunsmithing By Ralph T. Walker. I highly recommend this book to all who want the maximum from their shotguns. This is where I started my research as a gunsmith seeking out the ultimate in shot patterns. It is here where the issue of forcing cones came into light for me. Chamber forcing cones vary in length from 1/2" to 2-1/2" with many saying that this length or that length is perfect.
The 1-1/2" forcing cone:
This is what I found after many hours at the lathe, chronograph and patterning boards, 1-1/2" length will give you the best performance but there is a catch to this and one that I did not see in any writings anywhere that really addressed this one issue, it is the amount of finish that you put on that 1-1/2" forcing cone. This is the one thing that makes or breaks lengthening a forcing cone, if the forcing cone is cut with a reamer of high quality the shot pattern may actually decrease if not honed some to remove the reamer ridges, if you proceed to hone the ridges then a pattern increase will start to come forward, the more you smooth out this area the more the pattern increases.
The end result to this is simple cut the forcing cone length to 1-1/2" hone then polish the forcing cone, then when it is glass smooth, burnish it, this will seal the metal and a substantial gain in pattern density and evenness will be yours.
Note: As an experiment we had sent out four barrels to four gunsmiths to have the forcing cone lengthened and we got back four barrels with forcing cones lengthened with LITTLE or NO degree of finish work done on the forcing cones all had reamer ridges left in them so keep this in mind.
Shotgun Bore Effects - Chamber To Bore Forcing Cones:
(1) Can a longer forcing cone reduce recoil? No!
Simply put, the law of physics says for every action there is a opposite reaction. In a shotgun you detonate powder which creates expanding gases that push to escape, in other words if you are pushing an object that weighs 1 ounce at a given velocity of say 1,200 feet per second in one direction the push factor in the opposite direction is exactly the same.
(2) Can a longer forcing cone reduce felt recoil? Yes!
Felt recoil can be adjusted to give different effects by altering time frames to reach a given high point in recoil and by doing so you get more of a push effect than a punch effect at your shoulder.
Once a powder charge is ignited it starts to push, in a barrel with a short forcing cone, the shot column that has to move is met with resistance from a steep angled forcing cone trying to squeeze the shot column down to fit in the bore, therefore more pressure is needed to start moving it, with a long forcing cone the shot column will instantly start moving freely into the bore requiring less force to move it because it is met with less resistance, as you can visualize here we just altered the time frame for the felt recoil from a punch to a push in both cases the shot column will accelerate till all powder is burned giving us a desired amount of power from the expanding gases.
(3) Can a longer forcing cone increase pattern density? Yes!
The reason that longer forcing cones give increased pattern density is that less pellets are being crushed at the back of the shot column. The reason for this is simple the pellets in the back of the shot column have 2 forces working against them, the expanding gases are trying to push them forward and the weight of all the pellets in the shot column are acting as a wall preventing them from moving forward, in a short forcing cone the push to start moving all pellets forward is a sudden burst of pressure and the longer forcing cone allows the pellets to enter the bore in a slower push with less pressure thus not crushing the pellets and keeping more of the pellets in the pattern do to a lack of fliers with flat spots.
(4) Can a longer forcing cone increase velocity? Yes!
Actually, by increasing the forcing cone length you are removing metal from the bore and adding volume and this will change the expansion ratio for the powder being burned and add more velocity, the increase in volume is small so the overall velocity gain is very small.