Dan: They never did come up with a good designed deflector to protect the forearms, and occasionally, your forehead, from flying fragments of a cap. The skirt around the hammer is pretty good for keeping everything flying away from your face, but if you put a brass cup under the cap to deflect fragments upward, they occasionally come back at your face!
I have been hit in the cheek several times with cap fragments. The occasional burning powder is nothing to having to carefully pull a fragment out of your face, so you don't leave a bigger scar than necessary. I shoot a DB percussion shotgun, so I have fragments coming back at me, as well as heading towards my forearm. Your advise to wear a long sleeve shirt when shooting these is sound advise to new shooters.
BTW, I have an idea for an underhammer, and sidehammer percussion ignition system to keep the fragments and burning powder out of your face, or forearm, but its still to be tested. I just can't get the same idea to work, in principle, with a top action percussion system. Gravity is working against me.