Well one of the basics for shot, is that you don't want any of the gas seeping around the wad and contacting the shot as it exits the muzzle. The idea is that the gas will then cause the shot pattern to be out of round, if the gas impacts the shot as the shot exits the muzzle, and this could really mess up the pattern. So whatever you use to get a good seal is a good idea.
I was taught that the thick fiber wad that we use is not a "cushion" wad. A "cushion" wad is meant to keep the lead shot from deforming from the combination of inertia vs. the rapid acceleration at firing. The back of the shot column gets pressure from the burning powder, and is forced forward in a split second against the front of the shot column, and the shot in between gets squished out of round..., so won't fly straight..., the idea goes. HOWEVER today's shot is a lot harder than the much softer all lead shot of the Muzzle Loading Era, and that apparently (when using black powder), modern chilled lead shot doesn't deform from being shot. (The modern, plastic, shot-cups wrap around the shot and hold it until it reaches the muzzle for this reason, and are actually cushions as smokeless powder in a modern shotshell is a much more dramatic acceleration than BP)
The thick fiber wad, I was taught, is supposed to compress when the load is fired. In so doing it flexes outward, and seals against the barrel walls although it was pretty tight at the beginning. It takes a split second for this to happen, and some of the expanding gas may seep around the wad, but the seal is usually complete before the shot column with the wad reaches the muzzle. The ill effects of any seeping gas have been corrected by the movement of the shot column through the barrel..., by the time the column reaches the muzzle because of the wad seal..., is the theory.
So if the extra nitro card is giving the gun higher velocity..., it's making sure the seal is there.
The other thing I was taught is that black powder shot patterns may be disrupted due to rotation of the shot column in the barrel. This is why it's important that the the wads and shot be rather level when loaded into the barrel, and it might by part of the explanation of why the "Skychief" load works so well. The thick wad over-top of the shot column may be nullifying any random rotation of the shot as it's moving down the barrel, as well as causing a "drafting" area behind the wad for the shot pellets, a short distance as the shot exits the muzzle
LD
Went to a clay shoot yesterday and my SIL shot my 12 gauge double flintlock. He never shot a muzzleloading shotgun before, but hit his first clay. I showed him how I learned to load, but often wondered why we use a nitro card? Just so everyone knows the load procedure we used: 75grains FF, nitro card, cushion wad, with smidge of lube, shot slightly over powder volume, and then overshot card. I have often wondered why is the nitro card in there? Why not just use the cushion wad?
