Chuck;
What you say here is of course entirely correct. I didn't really delve into that portion of it in much depth, merely stating that while the chamber pressure would remain essentially unchanged one would add another layer to the walls.
As I understand the question it was addressing primarily as to what the effect on pressure would be forward of the insert. Here there would be no additional support to the bbl walls, but I cannot help but think the pressure would be lowered. To check this, pressure would need to be recorded at some point in a regular gauge gun of the smaller shell & then recorded at the same distance in the larger bore for comparsion. To be absolutely accurate I believe this would need to be done with a proper Piezo-Electric pressure gun, set up to record at say the 6" or 9" point.
