Michael, the best information, IMO, to figure out what I think you are trying to determine, is the pressure data for commonly sold shells that were in service during the time the guns were made and used. Much data exists on what those pressures were. Many readily available loadings of the day were in the same range as our modern shells (up in the 10,000 psi range).
A comparison using pressures necessary to cause a gun to fail when new would be useless unless you knew what it would take to cause that same gun, after 100 years of use, to fail. Bell comes the closest, in modern times, to doing that of anybody we know of. But, it's only 100% valid for that particular gun, because of the way condition varies so much with vintage guns. It may, however, be somewhat meaningful for us.
Some have graciously posted charts and figures that show the pressures of vintage loadings, and you could probably find them with some searching, or they may read this and repost them.
When we call a loading a "lower pressure" load it is usually the result of a personal comparison of the pressure of the load(s) in question with the pressures of the loads of the day. I base that call on the individual gun in question ....... it's condition, and what it was designed to shoot originally. I've loaded and used shells that range anywhere from 5500 - 10,500 psi in my vintage guns, though not all of them were "lower pressure" loads. Remember, higher pressure does not hurt wood on a gun. It only places increased stresses on the action, the lockup and the the barrels. Recoil resulting from a heavy payload, and higher velocities, is what is a detriment to old wood in poor condition.
So, what it comes down to is whatever an individual considers "low pressure". If the gun was designed to shoot 10,000 psi commercially loaded shells I personally consider anything from 8500 psi down as being "low pressure". Others would not consider 8500 psi "low", but it certainly is lower.
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