I'm sure you know that it is an expensive device which replaces the regular nipple in a percussion gun to allow the use of #209 shotgun primers instead of #11 caps.
The shotgun primer will produce more heat/flame than a standard #11 cap but usually this is not needed.
Folks who are shooting one of the synthetic black powders that need a hotter flame to get reliable ignition can simply buy Magnum #11 caps or RWS #1075 caps made by Dynamite Nobel.
In addition to spending a lot of money for one of these Mag Spark #209 adapters, after they are installed it can be a real PITA to have to unscrew the top, pry out the spent shotgun primer, insert another primer and then screw on the cap again before the next shot can be fired.
Also, if the gun has a chambered breech with a small connecting flame channel connecting it to the nipple hole, one must first remove the spent shotgun primer, then load the barrel, then continue with the insert new primer and screw on the cap.
This is needed because the flame channel hole between the bore and the nipple threads needs to be open to get the powder charge to blow back to the base of the nipple/adapter when the projectile is loaded or they will have misfires or slow ignition.
I guess I could have just said these things are a waste of money but I think if people can see why I say that, they will have a better understanding.