Note the pressure difference is from two different points. At 3 5/16" from breech the 11,500psi 1" 16ga pressure would have fallen quite a bit already. Also note the 11,500 is "Nominal Maximum" for the average pressure of 16ga ammunition, that specified for the Gauge-mates was actual measured. The 28ga shell will have a lighter powder charge & shot weight than a noiminal 16ga load & will produce as high or higher chamber pressure (even in the Gauge-Mate) than will a 16ga shell. It will of course have the extra wall thickness of the insert to help contain it. Of course after the nominal 28ga load enters the full 16ga bore it gains 45% bore capacity over a 28ga bore. Pressures should fall considerably. I suspect though if you loaded a 3/4oz load in a 16ga hull with a suitable charge of fast burning powder to acheive the same ballistics, you would be surprized just how close the pressure of the two would be at the 3 5/16" point. We simply have to get past the idea the whole of the bbls are pressurized to that breif max pressure recorded at the 1" point.
