It's interesting to see how opinions of this have changed in the last five years when there was about a 50/50 split on the advisability if lengthening cones.
I did a test about five years ago at the Southern Side by Side and a couple of other shoots. I had a Fox A grade with cut 26" barrels. I had one forcing cone lengthened and left the other as it was. I allowed those interesting in participating to shoot four shots two from each barrel and found no statistical evidence at all that there was a change in recoil.
On the card the shooters filled out, I did ask if they thought lengthened cones reduced recoil and those who were of that opinior almost always noted much more recoil on one barrel or another.
I didn't let them peek but several tried.
