.256 Win is based on the .357 Mag case.

Got no clues on the reason for different web thickness. Although two things strike me. Both thick webs are rifle cartridges and both have a smaller bottle neck. Don't know if there is any significance.

The weakness of the Martini Cadet or more correctly the BSA Francotte Martini:

1. weak extraction
2. small barrel tenon
3. difficult load geometry-entry angle

The common misconception is that the striker hole in a Rimfire cadet breech block is off center. It is not. As many as I have looked at I have never seen a Rimfire BSA with an off center striker or striker hole. The proper way is to simply adjusted the block height to lower the point of impact. No other changes are needed. Striker diameter is already correct. You may also need to adjust the load position. No MIG, No TIG, no facing off, No reheatreating.

The .thin side ..310 cadets do need their strikers modified and breech blocks bushed…the striker diameter being to large for smokeless powder use.

You will find the adjustment instructions buried in the LoC for Armorors of the Martini Henry. You can find them reprinted in several locations including in Ian Skennerton’ SAIS 15 on the Martini Henry.

Remember the Francotte is a Martini Henry with all the internal parts moved to a subframe, nothing more.

The thick side actions are generally rimfires. I don’t think I have seen a thick side centerfire. Some do enlarge the barrel tenon hole on the thick side, but I don’t feel the need.