The dealer's ad for the Mag-Spark conversion kit states that it comes with a "special 209 primer removal tool" which suggests that
the spent primers may need to be pried off the nipple instead of being blown out by excessive blowback.
My impression of the Mag Spark and British Anvil equivalents is they are a 209 shotgun primer combined with a small booster charge of powder set up to propel the lightest roundball possible.
With no backplate or anything to headspace the back of the primer, charges have to be kept low.
If the flash channel is too small like a standard nipple, with a hot 209 primer inside a pocket (these things are almost as hot as .22 shorts) not enough gasses will escape through and that would likely blow the primer out the back also.
209 primers are meant to be in shotgun shells with the front for the most part unobstructed over the powder charge and held in place from behind during ignition by a properly head spaced cartridge gun. A black powder frame is not set up for that.
The Howell cylinder solves that problem because it is basically a conversion cylinder with a back plate to hold the 209 primer in place.
Maybe Mr Dragoon can be half happy since these cylinders can actually LOOK LIKE HISTORICALLY CORRECT cartridge conversions for us wannabe cartridge gun owners. We can actually enjoy the experience of having something that comes close in looks and feel.
Like the 1873 cap and ball not even close to as over the top as a muzzleloading AR but actually a nice practical item to have and enjoy. It would also ease a lot of the burdens of people in places like Great Britain and NY.
I say fire up the machines and make a whole bunch of them for Colt and Remington as well.
For the 1st batch the question in my opinion is Ruger, Colt, or Remington? Which will sell the most to help fund the production of more options for different cap and ball revolvers.