Hi everyone, I believe that the the first cartridge chosen to develop the Martini Henry around (known as the Long Chamber rifle) in 1870 was the .450 x 31/4" cartridge. According to Ian Skennerton (A treatise on the British Martini) it's OAL was 3.75" and the length of the case was 3.22" They dropped it in favour of the Short Chamber because "...there had been complaints regarding the difficulty in loading bent cartridges and of the paper wrapper being caught on the chamber mouth, but the committee stated that - These difficulties have been overcome by the substitution of a shorter cartridge of a bottleneck form on which there is no paper." So you will still be staying with history if you use the .450 x 31/4".
kologha