Been shooting flint for a few decades now & never fired one with a flash guard (truthfully, I didn't know what they were until just a few years ago).
A brand-new RO at my range sought to establish himself as being the ultimate know-it-all for all firearms old & new and approached me one afternoon while I was shooting a newly aquired 35 year-old Cabelas (Investarms) .54 cal. 'Sport' Hawken rifle (it has checkering on the stok, which makes it sporty, I'm guessing).
Anyway, RO-kid approached me and told me I couldn't shoot my rifle there anymore, since it was obvious I didn't have a flash-guard installed.
I told him I didn't need no stinking flash guard & he got kinda' pi$$y. He said I could burn someone standing next to me with hot gases.
I had to point out to him that I was firing a caplock & there is no burn hazard to bystanders.
He withdrew & afterward avoided me as much as possible, although I made it a point to greet him when I arrived & asked if he had any sound wisdom of the day to impart.