The chamber body itself tapers at a rate of about .005" per inch. Shotgun bbls having less taper than this over the chamber area are few & far between, thus lengthening a chamber is virtually guaranteed to reduce the wall thickness at end of chambder. The cone tapers by .070 ±a thou or so over the length of the cone (to a std bore dia) There are not many bbls which will be thinned by cone lengthening unless it is a very long cone. Also the further down the bbl the lower the pressure. I cannot conceive of any ordinary cone lengthening creating a Danger Spot.
As to pressure, it has for years been standard practise to record max pressure 1" from the breech to ensure that max was always read. Pressure rises very rapidly, but it also falls rapidly. I nighly suspect that giveen proper equipment that would read the pressure curve from max to beyond the cone that change in max pressure from cone length would be essentially undectable. What would be more probable is that the presssure would simply be just a tad slower in falling with the short cone, vs the long one, but not likely anything to get up on your Tricycle about.
As to patterns the biggest aspect here would be of easing the shot from chamber to bore with the least deformation. Modern plastic wads have taken care of very much of this. With old style card & filler wads due to their lack of obturation a long cone often did more harm than good by allowing gas leakage, so its a good bit of a wash here.
The .410 in 3" is of course such an unbalanced & ridiculus load it does indeed "NEED" all the help it can get.
