As I stated before , I load 44 capsules in a 45 Colt with 6 or 4 shot . Just one of those big shots will kill a snake or brake it's neck . Also I do not have a " pinwheel " pattern as my load does not spin , and at 1000 fps it is not underpowered
I am curious, how do you know your load does not spin? Perhaps it does spin, just not enough to cause the "pinwheel" or "donut" patterns??
Seems to me that if you're getting 1,000fps from a pistol then there must be a gas seal between at least some part of the projectile (shot capsule) and the bore.
I can see where it could be possible that between the fact that the shot capsule is plastic, and its contact with the rifling could be very small, no where near the amount of bearing surface that a bullet has, this could create less spin than a bullet gets. It's also possible that there's enough contact to create a gas seal but the rifling is "stripping through" the plastic of the capsule, and so, not imparting the normal amount of spin.
I think its likely that your shot load does get some spin from the rifling, but not enough to cause noticable issues at the ranges your using it.