I don't know if the Harvesters are weaker, but they're definately softer than the MMPs. Taking the base of the fired sabots between my thumb and forefinger, I can squeese and flex the Harvester much easier than the MMP.

Maybe the softer material is sealing the bore better and is the reason why I got better groups with the Harvester. At any rate that's just one load. Up the charge 10 grains and the MMPs might be better.

Muzzle loaders are like women. You just never know what they'll do.