🔴 How Much Hollow Point Expansion Does How Much More Damage?

Your points are valid but you are leaving out penetration.  We need a big enough hole but we also need a deep enough hole to get to vital tissue.  Admittedly this is only an issue at the lower end but it can be an issue.  The first squirrel I shot at this house was with a 1377 with about 5 fpe.  The pellet stopped after breaking the near side front shoulder.  So the squirrel died slowly due to insufficient penetration.  A bigger hole would have had the same effect.  Once we get to the vitals, how big the hole is matters.  But we have to get there.

When we start adding the variable of expansion, we again need more velocity in our projectiles.  We are going to penetrate less if our projectile expands.  I tested the 22 pellets I had on hand in wet phone books to see how much penetration I got.  There are some that think wet newsprint gives results similar to muscle tissue.  My little 1377 only went about 1.5 inches using a gamo redfire that did not expand.  The only 22 pellet that expanded was a metal mag weighing about 18 grains.  It penetrated about the same as 14.3 grain Crosman domed and hollow point pellets.  The deepest penetrator was a 21.3 grain Baracuda.  I later shot a dead squirrel hanging by it's tail to see how a couple of these pellets penetrated the body cavity.  Both the 14.66 grain H&N and the 21.3 grain Barracuda, both domed, went through the squirrel but the skirt caught in the skin on the far side.  Definitely far enough to hit something vital but a shot all the way through would bleed more. 

In powder burners there is a lot of debate about penetration versus expansion.  Some people use highly fragile varmit bullets on deer.  A hit in the lungs will put the deer down quickly but if you miss and hit the shoulder you only injure the deer.  So most use controlled expansion bullets to get some expansion to increase the permanent cavity.  With the higher velocity the temporary cavity may also destroy tissue and expansion can help with that.

I believe the criteria is to get sufficient penetration first and foremost.  In the less powerful airguns this means you do not want expansion of the projectile.  But if you have sufficient power to have both, bigger deep holes will kill faster.  But shallow wide holes work worse than narrow deep ones.

With air guns, I've always valued accuracy first.  I never really gotten the best accuracy out of any of the hollow point designs.