Hank   you might try looking on the American Long rifle assoc website     I have found them to be really helpful, although they are somewhat flintlock oriented they do deal with Percussion arms as well.  A number of ASSRA website members also participate on the ALR site on occasion.

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A second thought:  check the Track of the wolf sight for "Flash Cups"  I spent years in the Rev and F&I War reenactment scene.  we were REQUIRED to have flash guards to protect fellow shooters when firing in close-order ranked groups as "line" troops.  I believe that the Civil War re-enactors do as well.  
  for flintlocks we used a brass deflector that mounted to the frizzen screw. 
I believe the percussion flash guards attach via the percussion cap nipple.  We have a number of site members who are active in the N/S Skirmish groups that probably have first had knowledge much more accurate than none when it comes to the percussion arms.   In any case its a pretty simple fix.

EDIT:    check here     oodles of them  cheap
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the angle of the Percussion nipple will direct any jetting back towards you.  Normally------- the mainspring will keep the hammer down on the spent cap, and the cupped nose of the hammer should keep it contained-------but with heavy loads and  worn parts or deliberately lightened springs in competition flint/perc arms can blow the hammer off the nipple and let some blow-by escape.   Glasses are always a good idea unless you are in a reenactment group the allows or mandates them.


With flinters the gas just jets straight out the touch hole and depending on the lock design (and quality) it should not be a danger on a flinter in good shape.   However the touch holes do erode and a lot of re-enactors made them a little oversize for better ignition with coarser paper cartridge powder as the prime in authentic shooting situations  (the original flint military muskets also had what we would consider "oversized" touch-holes.   they are much more a danger to the guy shooting on your right if you are doing "line drill" unit firing.    thats why the "flash guards" were made mandatory in most flintlock F&I, Rev War, and 1812 military reenactment groups.