There is so much of this which is purely speculative, but would like a few "Explanations" of certain points. Let's examine a bit what happens inside a shotgun bbl. Upon firing the pressure rises very fast, reaching it's peak inside the chamber & then begins to fall, rapidly at first & then in a lessening curve as the shot progresses down the bbl, increasing the volume the gas has to expand in. Changing the forcing cone angle (lengthening) would give a very "Slight" increase of volume in the area, which would tend to slightly decrease pressure. A "Slight" change in friction could also accomplish the same purpose. Either would affect recoil, to the same extent & in the same direction, it affects velocity, ie increase velocity, increase recoil etc. "If" the same load is fired & achieves the same velocity, but has lowered pressure in the cone area, then it has to off-set in elsewhere. I can see no possible affect lengthening a cone can have on a thin bbl in the forward portion. If you have a bbl with good chamber wall thickness but thin forward & want to "Favor" them, shoot light loads put up with the fastest burning powder which will stay within allowable chamber pressures & forget the cones. The faster & "Less Progressive" the powder used the lower will be bbl pressure while the slower & "Most Progressive" powder used will result in higher bbl pressures. This assuming equal shot wt @ equal velocities. Chamber pressures will be the opposite. One simply needs to study the bbls in question & decide which is the most important low chamber presure or low bbl pressures. If both the only option is "Light" loads overall.
