Well I've had a play with .25cal FX hybrid slugs along with Hades & Predator hollow point pellets and had very little expansion (42ftlb)

These tests, both US & UK are being done at close range, which is utterly pointless

What does it matter if a pellet/slug expands when shot in to a bucket of water from 6" away if I could sneak up to within 6" of a rabbit I wouldn't need to shoot it I could club it to death

Show me the expansion at 50+yds (including going through the fur & skin) when 2/3 of that muzzle energy has been expended & it will be of some relevance.