Back in the good old, bad old days, pushing 40 years ago I did a whole bunch of squirrel hunting in a western state. It was not muzzleloaders so I'll say no more about the details. But I did try hard to bark them. Did it a time or two, but it was a very low percentage shot for me, even though I could head shoot them much of the time.
The only sure "barking" shot I found was to shoot THROUGH the limb from the bottom up. The explosion of wood was most impressive and deadly. Maybe I should call it "limbing" squirrels or some such.
In fact it became my preferred shot for squirrels way high in a tree. I've had squirrels take a death grip on a branch with head shots and fail to come down, even though they were deader than dead. But I never lost one from a limb shot. Blew them too high in the air way too fast for their pointy little toes to hold on, I'm sure.
I can say for sure that they didn't have to be laying flat on the limb for it to work.
Next time I get near squirrels with a muzzleloader, I'm looking forward to more speriments with limbing!