Black Powder Shot Shells

I'm reloading 12 ga AA shotshells for trap and the recipe I'm using

gives me a pressure of 9,500psi. I wanted to duplicate black powder

pressures and use some shells in older guns using smokeless powder.

I have an older Cresent single barrel 12 ga, not damascus, but want to

be careful and not blow up the gun.

What pressure does black powder shells have when fired so I can

duplicate the load?