This Mako slug was shot into clay at 27 yards from an R7 that pushes 8.44 pellets at about 7.5fpe.
Expansion in clay looks good but really does not have much to do with expansion in critters. Even the ballistic gel which is far less dense and more flesh like shows better expansion from projectiles than in actual animals for the most part.
The real question is this: A .177 slug out of a springer at 27 yards..... Did it hit where you were aiming? I'm looking at the circumference of the slug that expanded for any evidence of engagement from the lands, and I really don't see any.
It looks like that is their only slug. Is that true ? I would like to see a .178.