We all have been shooting longer shell than sometimes the old chamber allowed. If the cone do not allow the shell to open fully the pressure built up will far away superior to the normal pressure of the shooted cartridge. The gun will kick hard, sometime the rib will start to separate from the barrels. Make the cone longer, remove the step in the cone design for the paper shell will lower the pressure but will make the steel thinner.
Shooting the proper lenght and low pressure cartridge as in the old days will be better for the gun on long term. RST cartriges could be find easily in short lenght. They will take care of your old gun. Check with RST.
