More prone to misfire and powder spillage reduce reliability. Typically a generic bolt on is not an improvement whether it's a musket or car. Other than that the NPS calls for FF powder in the pan with the flashguard installed. FF burns slower than FFFF and the pan will redirect sparks up and over a greater distance than FFFF with no flashguard installed increasing the safety distance required rather than decreasing as intended. Couple that with the added difficulty loading, increased possibility of dropped powder, and less reliability you can see why I disapprove.
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@Howard Pippin, I have several rifles and smooth bores with flash guards and other firearms that don't have flash guards.
I do agree that for some, the flash guard does interfere with priming of the pan, especially the King's Musket (Brown Bess, Long Land Pattern), as I use a spare paper cartridge to prime the pan with whatever powder we use to fill our cartridges when firing blanks. There is no difference in ignition performance between firing with any grade of powder from 1F to 4F. There is spillage from using the paper cartridge to prime. I have a small priming horn with a fine tip to prime with 4F. That's not a problem when priming the Bess. As far as redirecting the flash goes, we did have one instance of firing (blanks) in ranks with the first rank kneeling, that the flash ignited the tassel in the miter (hat) of the man in the kneeling rank. The muskets are required to have flash guards.
For the rifles, I use a priming dispenser with valve that clears the flash guard. No problem with priming the rifles. The flash guard does get in the way of picking the touch hole. Often, when I am at the range and am picking the touch hole before priming, I let the flash guard drop. I notice no difference in ignition performance. The flash guard up may be a little faster as the flash guard keeps the heat from the pan close to the touch hole.
Even when I am shooting guns with a flash guard, I will set up a barrier such as an extra target stand to keep the pan flash from spraying the shooter next to me.
Some competitions require the use of flash guards or shields between shooters. In some woods walks where the shooter are shooting in line, the flash guard provides the barrier between the pan and the adjacent shooter and is a good device. Our Woods Walks, such as the Fall Rendezvous at Fort de Chartres require the flash guards and hammer stalls as a group firing does take place and a barrier between the shooters is impractical.
At our club, some members that do not have flash guards will place a target board between benches or have made a barrier to attach to the bench. For
@IanH, I would suggest a barrier be built to set up between his bench and the next shooter. It's a bit of a pain and extra equipment, but with it when pan flashes may affect the shooter in line with the pan flash.