Pyrodex likes more compression than BP. If I remember from when it first came out what, 40 some years ago, it says that right on the bottle. I remember it being soft, like you can crush it between your fingers. I tried it in a muzzleloading rifle, double ML shotgun and in a Marlin 1895. The two muzzleloaders had delayed ignition with it, a problem I never had with real BP in those two firearms before or since. About the only thing with more direct ignition than a shotgun is a revolver or underhammer. The Marlin was fine. I don't remember how accuracy was, it's been so long ago. As I understand it, volume for volume, pressures are comparable.
I thought I cleaned them all good, like I usually do but in a few days they all had rusty bores. I caught it before any damage was done. It sure put a bad taste in my mouth for it. Now how can you NOT get a shotgun barrel clean?