It is customary and considerate and a requirement on most black powder ranges and clubs to notify the shooter to your left (if shooting a left handed flintlock) or vice versa to audibly say "Flint" before shooting. Some close quarter re enactments use flash deflectors on muskets having an outside frizzen screw. Youo will have to do your research to find out if they are period correct. Loose or poorly fitted touch hole liners, worn drum threads on convertibles, poor bolster to barrel fit of the lock can be cause of identified flying objects The latter is a source of your priming powder falling inside the lock mortise, accumulating and one day giving you a heck of a surprise. One "range officer" had a cap fragment shield installed on his percussion rifle but did not have enough threads on the nipple engaged and it came flying out, missing his eye by fractions of an inch. Loose flints in the jaw have been blown toward the lock side as well. Lots of safety details to keep informed and aware of. There is an old movie about making gun flints that shows early NMLRA members shooting flint rifles in close formation and you see a lot of guys flinching when hit by touch hole expectorants. Period correct eyeglasses are not a good substitute for shooting glasses or goggles. Be aware of the condition of not only your firearms but those next to you.