When shells used fiber wads a lot of guns had slightly shorter chambers with very short forcing cones. This was done to give them a better gas seal. With the change over to plastic wads, which would flex to seal better, it was found a longer forcing cone gave better patterns. Gas was still sealed because the wad skirt would expand to fill any gaps. Many doubles had the forcing cones relieved slightly, which I find a bit better than lengthening the chambers as a whole. But with so many ways to find proper ammo these days it is an unneeded alteration to me.
