Shiloh Sharps has the strongest action.
They are made from the very highest quality cast steel from Shiloh's own foundry.
This is a pic of a Shiloh Sharps action that was made by Wolf Droege.
He filled a 45-120 case with smokeless powder, compressed the crap out of it, then pulled the trigger from far far away.
As you can see from the pic the action suffered little if any damage.
The case is still in the chamber, and only the barrel burst.
Maybe in the old original rifles the Sharps action wasn't the strongest, but they sure as Hell are these days.
They will take anything a Ruger #1 will and more.
With that being said these rifles shoot their best with black powder.
About the only way to damage a Shiloh with black powder is by leaving an airspace between the bullet, and the over powder wad.
This can ring the chamber.