Maybe we ought to make this thread, or one like, it a sticky. One of the most asked and often misunderstood question, is what load to shoot in my gun? A gun in proof, which has been inspected by a knowledgeable gunsmith, should be safe shooting a “service pressure” load of 740 bars which is 10,732.79 psi. For additional margin of safety you can always go lower, as I do. A Black Powder proofed gun gets loads comparable to what was original to it, and those are 4,500-5,500psi. Or if you can load them, real black powder loads which always draws a lot of attention.
Drew has posted a lot of very detailed information on this subject, along with pressures for period loads for the 1890-1900 timeframe. Look at his website to see real numbers. Look at the CIP link I posted, for real numbers. I posted the formula to calculate bars to psi. Go to Hodgdon or Alliant to find real pressure numbers for any reloads. Any factory CIP approved load will be at or below Service pressure. But the bottom line is normal service pressure is 10,732.79 psi. That goes for 2 1/2”, 2 3/4” or 3” if proofed at 850 bars. I also think 2” are now proofed at the same but check it on the CIP to be certain.
We know shooting 2 3/4” shells in 2 1/2” chambers does increase pressure some. Most reports have bee 1,000psi or less. You decide what to do about that. I cut hulls down to 2 1/2” but if the pressure was 5-7,000, in a 10,000 gun, that is over caution. I still do it.