Remember that 4drams of black is pushing that .9/1.2 oz solid slug into a rifled bore of about .458", not a .729"/.775" smooth bore (equal charges in a smaller bore = higher pressures). Chamber pressure will indeed be higher. As a wild guess I would think in the 20K range. However it needs to be understood, chamber pressure is critical primarily to the chamber walls. They are going to be much thicker than those of a shotgun bbl on that same action. The critical factor for the action itself is not chamber pressure, but "Back Thrust". Several factors play a part in Back Thrust, but a big one is case head area in proportion to the pressure acting upon it. As the head area of a 12ga shotshell is approximately 2.64 times that of the .45-110, then based soley on that we are looking at about 8K in the shotgun being equal to about 21K in the rifle. Other factors of course have to be calculated in & I do not consider myself knowledgable enough to make a solid recommendation. I would think the higher internal pressure of the rifle round would cause the case to take a firmer grip on the chamber walls, thus reducing back thrust at the time of highest pressure, but other factors may offset this. One thing for certain though, simple PSI is not the final answer or no shotgun would be capable of handling a .22LR, they have higher chamber pressures than a shotgun.
