Try using a filler of corn meal, farina, etc- whatever you have in the house-- don't go out and buy something just for your gun that you don't already eat!--- to act as a gas seal. The use of any gas seal will push all the energy into your PRB so that it upsets quickly in the barrel.
Then try this with the .350 ball, and 30 grains of FFFg powder. You choose the patch size, but I would use the .018 or .020" patch, well lubed. If the barrel is sealed properly with your patch, you should NOT have to use as much as 40 grains of powder to get one- hole accuracy out at 50 yards.
And, you should not have to use the .360" balls. The .350s will upset well enough, if they are cast from pure lead, and using the smaller diameter ball will also make it easier to load the gun faster for second shots at those squirrels.
As always, never assume the exact diameter of any barrel: measure it with calipers to know exactly what the bore diameter is, and then what the groove diameter is. The depth of the grooves is used in selecting the right thickness of the patch material. use a micrometer or caliper to measure the actual diameter of your lead balls, too. You might be surprised at what you are actually using, instead of what you might think you are using. :hmm: