Not sure where I read this, probably on this forum. As I understand it, in the days before the development of modern plastic one-piece wads with shotcup and an effective gas seal, making the chambers a bit shorter than the length of the fired shell was done deliberately as it was thought that having the end of the hull forced tightly against the inside of the forcing cone helped ballistics by preventing escape of gas around the shot column, since the card or fiber wads did not produce a very good seal.
This is interesting and makes sense to some degree however the UK guns at 2.5 chamber or as someone here suggested most were bored at 2 5/8 only used a 2.5 shell. Were they not concerned about gas seal as well? I don't think their shells were loaded any differently to those in the US when using fibre wads.
I use lubed fibre wads and my thinking is that with the force of the gas (I use BP almost exclusively) they compress enough and along with the lube prevent gas from escaping. I know after firing a round the wad is shredded once it leaves the barrel so its structure must be compromised (compressed) from the force of the gas before leaving the barrel.
