No faltering. SKB answered your question.
It's about PRESSURE. Many a Enlish and European guns were made with 2 3/4" chambers but were PROOFED for pressures (Ex: 850 BAR) below the SAMMI maximum pressure.
As I posted in the other thread, even SAMMI mentioned in there earliest documents that shell length is unimportant unless it causes a rise in PRESSURE.
I have two English doubles from the 1870-80's with 3" chambers. The additional length was for additional wadding to help with patterns in the days before the plastic wad. They are both blackpowder proofed and in no way designed for todays modern 3" shell. Why not use a modern 3" shell in these guns? Because they were not designed/built to handle the PRESSURE generated by modern factory 3" shells.
When SAMMI came into existance they set shotshell maximum pressures signifigantly higher than most shells of the day. I think Winchester Super X was one of the first examples of shells made to the new higher pressure limit.
With the regular availability of these 'new' higher pressure shells the manufacturers labeled the boxes so that buyers would be aware and not use them in older guns designed/built for lower PRESSURES.
