FWIW...Reading my Speer #13 manual the other day regarding shot capsules. P. 593; "You will notice that the loads shown for Speer shot capsules are relatively light..."(?)
It goes on to say; "A little experimentation will point the way to the most effective charge weight for good patterning in your revolver. Often,dropping the charge weight slightly improves the shot coverage."

Amazing the information that is provided in Reloading Manuals, isn't it?

For years, all my shot loads have been according to the recommendations on the shot capsule box, and no ill effects, even though the load shown...5.5 gr. Unique...actually a little more than a .38 Spl. +P, and was my IPSC load for years under 150-158 gr boolits. Hmmmm...that should kick the ~106 gr of shot + the capsule weight out with excellent velocity... The manual sez 1,111 fps/6" barrel. So now, I am going to try slightly milder loads...5.0 gr., then 4.5, and maybe even 4.0gr. Unique with the capsules. The 4.0. load is sufficient to push a 158 gr. projectile out of my 4" barrel and should fracture the capsule. But I use 357 Magnum brass, not .38 Spl., so who knows what the velocity will end up at?