Pressure data for BPC 12ga shotgun shells?

[tbx-4]

Been looking all over the interwebs but haven't found any pressure data for original black powder 12 gage shotgun shells.
I have two Hopkins and Allen falling block 12 gage shotguns in good working order but I am curious to what pressures these old shotguns worked under and as well as what breach thrust pressure would be. The older of the two shotguns was made between 1888 and 1898 and has the side takedown screw. The newer one was made between 1898 and 1908 with the bottom takedown screw. And no, I haven't tried shooting them.

I suspect chambers to be 2 5/8" but will measure and make sure. I read a post here that breach thrust was in the range of 6500psi. Can anyone confirm this?

https://i.imgur.com/FfX6OuD.jpg
https://i.imgur.com/Qo0sGaH.jpg
https://i.imgur.com/3O4MCSS.jpghttps://i.imgur.com/lwEVFZE.jpg

[waksupi]

I don't know the pressures they were built for, but I don't think you could get into any problems loading with 2FF. Lots of bore there for expansion. You aren't looking for high velocity with a BP shotgun. Less powder, more lead, shoots far, kills dead.

[RED BEAR]

I would definitely stick with bp for them. By the way really nice shotguns.

[MT Chambers]

A lot of BP shotgun shells have been tested to 4300 lup, something hardly possible with the evil smokeless.

[longbow]

This might be of interest:

https://www.shotgunworld.com/bbs/viewtopic.php?f=13&t=259371&start=160

and this:

http://www.tbullock.com/bpsg.html

Longbow

[tbx-4]

A lot of BP shotgun shells have been tested to 4300 lup, something hardly possible with the evil smokeless.

So is 4300 LUP high for BP shells but low for smokeless powder? I'm trying to figure out what you're telling me.

[tbx-4]

Longbow,
Thanks for the links, especially the first link which answered more than one question I had. A lot of interesting data history on shotguns.

The second link I've seen before and found this part interesting and counterintuitive with my limited experience with black powder.
Velocity. Shot velocity is about 1000-1300 feet per second; same as modern loads and the same as it has been for the last two hundred years. There is just no point in driving small shot much faster than the speed of sound because it will just slow right back down again. It's not called the 'sound barrier' for nothing.

Recoil. Recoil is somewhat greater with black powder than with an otherwise identical modern load. It has to do with the greater mass of gas and dirt being ejected from the muzzle. Initial acceleration is sharper due to the large initial surface area of the grains which decrease as they burn. Fiber wads are heavier than plastic too. Get used to it.
Modern smokeless powder is advertised as 'progressive burning' giving 'lower recoil' and 'low flame temperature'; black powder has none of these modern advantages.

Noise. Black powder is a lot louder than smokeless. Get used to it.
It is good for scaring people in the trap squad. The first time I tried it it even startled the guy in the concrete trap house.

[MT Chambers]

I believe that press. figure as low but standard for BP loads, many times trying to produce those low press, levels with smokeless will result in squib loads with inconsistent vel., ignition, unburnt powder, etc. Many years ago Dupont (now IMR) listed some very low pressure smokeless loads with their IMR 7625 powder which I believe has been discontinued.