Just to baseline, I don't advocate everyone go and lengthen the cones in their guns.
I do have a small number of guns I've modified cones on, all of which are .410, 3" chambered guns, with one exception, a 12 ga BSS which I modified with a 1 1/2" long cone in one barrel and later, a 4" long cone in the other. The 12 ga was modified back in the early 90s during the height of the fad, mainly for my own testing curiosity. My first .410 was modified in an effort fix the butchery done by some moron who ran a common chucking reamer and who knows what else into a 2 1/2" chamber to lengthen it to 3". It was so hacked up inside that the longer cone was the only thing I could come up with to fix it...and it was worth fixing. (See below). I had a reamer maker friend grind a "taper pin reamer" so it would match the chamber dimensions while cutting the new cone at the standard taper pin angle of 1/4" per foot, which gave about a 3" cone in the .410. After that, a fellow Fourtenner saw my patterns and asked me to modify his CSMC 21 .410 and later a M42. I believe I only have modified 2 or three of my .410s and the one below was bought by a friend. So, the discovery of the improvement to the .410 was incidental.
I would not be an advocate of modifying a treasured vintage gun without other rationale than seeking pattern improvement, since it is small potatoes with the exception of the 410.
