Spot on @PathfinderNC!I’ve never heard the National Park Service determining what granulation of powder to use!
I’ve been in Rev War reenacting about 35 years and this is news to me.
The SOLE purpose of a flash guard is to guard your file-mate on the right from getting splatted with burning powder. Period.
Installing one on a Bess or Charleville musket presents NO safety issues.
Frizzen covers are the second requirement.
As for loading, a flash guard presents no obstacle, at least for those with basic motor skills
As for other shooting venues and public range rules, I can’t say.
Although I've only been reenacting for about 25-years, I have never seen the NPS require the use of 2F powder either, which is a good thing. Nor does the BAR or Continental Line require the use of 2F. Some of the powders specifically being sold as Reenactment Powders are a pretty rough mixture of granulations. Since there are no ballistic performance concerns, the only thing that matters is that it goes off and mixed granules work fine.
It is no joke to catch a flash in your left cheek because someone doesn't have a flash guard in place and they are standing too close, especially when some of the granules continue to burn as they stick to your face. Been there, done that, don't recommend it. Before I got a flash guard for my first longrifle (Traditions Pennsylvania Longrifle) I was allowed to participate in the reenactments twice, as long as I was the last one in line with no one else to my right. Hammer Stalls are absolutely required to participate in any reenactment because that is a safety issue.