teeny 350,
This issue is reviewed in detail on pages,260-262 0f Gough Thomas[G.T.Garwood] Gun Book.We read that experiments conducted in conjunction with Eley showed that for a given shell charge intended for use in a gun chambered and proof tested as a 2 1/2,increasing the case length from 2 1/2 to 2 3/4 did not result any significant difference in breech pressure.
At one time Eley sold a shell called the Maximum.This shell contained 1 3/16 ozs; of shot and was safe to use in guns proof tested at 2 1/2 chambers. Perhaps lapogus could comment on its current availability?
As suggested by others use only shells designed for use in 2 1/2 chambered guns[ie; do not exceed allowable breech pressure using home loads] stay safe!

Roy Hebbes