Basically, it seems that even when you aim for the head with the Blunderbuss, once you already have a stack, gaining more is very hit or miss.
It seems as though you can often gain and lose a stack with the same shot. With some pellets going for the head, and others for the body then using your Sure Shot stacks. This makes gaining stacks with the Blunderbuss not reliable at all.
It occurs to me that this might be intentional, but I figured I’ll post it anyway.