Finished the detail on the 40mm powder chamber. The 2 deg per side 4 deg total taper is for the obturator.
I believe I understand your intent is to give up the 'Beef Tallow impregnated Asbestos Pad' which lies in back of the Obturator Spindle's rear shoulder in DeBange's Breech design. If that is true, then will the 2 degree taper you show in your drawing mate precisely with a 2 degree taper machined on the surface of your breech block, starting at the point of tangency where the rear edge of the mushroom cap meets the forward edge of the breech block's 2 degree taper? Will these two surfaces provide the gas seal or will there be some other accommodation?
Tracy and Mike
A DeBange breech used in 1893 on the British 6" B.L., Mark III. From Wiki Commons, draftsman unknown, this drawing clearly shows the major parts of the Debange system breech, the interrupted screw-locking block, the floating mushroom cap obturator spindle and the asbestos pad under the spindle's mushroom head. Upon firing this pad, lubricated by the beef tallow component, squeezes outward to seal the breech, preventing hot gasses from escaping.