Bell's tests on long shells in short chambers mostly showed some slight reduction in pressure (a few hundred psi) when cones were lengthened but chambers still short. Longer cones on modern guns are quite common, the usual rationale being "improved patterns". But as Jim asks above, what does "improved" mean?
The advice to shoot long shells in short chambers goes back to the paper hull/felt wad days. Shooters actually got tighter patterns when they did that, because with the case mouth extending slightly past the end of the chamber, the shot charge was given some protection where it would normally have first contacted the barrel walls. But modern plastic wads provide an even greater advantage when it comes to protecting the shot, so that old advice really doesn't pertain any more (unless you're shooting fiber wads).
Usually, short forcing cones aren't a problem with longer shells. That's evidenced by the fact that the Brits and Europeans have been shooting long hulls (loaded to appropriate pressures) in short-chambered guns, going way back to before WWII. Both Burrard and Thomas commented on this practice, and noted that as long as the pressure was appropriate, hull length in and of itself was not a danger. However, I have seen a few anecdotal reports of very old (all 19th century, I think) Brit guns with very short and sharply angled cones having problems with those long Brit shells--as in excessive recoil and blown ends on the hulls. Charles Fergus, for one, reported such results in an article in Shooting Sportsman. He shot the long shells in two Brit guns, one pre-1900 and the other from the 1930's, both with 2 1/2" chambers and original cones. He got the results I described in the older gun, but no problems in the newer one. Modern loads in 2 1/2" hulls, however, worked fine in the older gun. If you get the results Fergus got with his older gun, it's certainly a good idea to stop, try true 2 1/2" shells, and see whether maybe you own one of the (relatively rare, I think) vintage guns in which you ought to avoid longer hulls.
