Im thinking along the same lines as marksman...
Ive had 2 different batches of 20 cal Zmax differ like that on foxes...first batch of 500 i only had holes blown through skin when hit on the spine...apart from those misplaced shots, every fox i skun out had no pelt damage & was just a big bruised chest cavity filled with minced organs & clotting blood...
The next batch (the one im on now) blows holes on exit 95 % of the time regardless of where they are hit...
Infact, last weekend i knocked a fox at 186 yds in lower neck/just above shoulder blade & there was a neat 20 cal hole on entry, & no exit wound...(actually-- im trying some 32 Nosler Varmeggedon's & cant remember if i used them on this fox...
...so much for conclusive testing eh...
)
Yet I shot only weeks before that one with same loaded batch of ammo, in the neck--it was doing the low down ninja skip across a paddock at 95 yds so i let rip about a foot infront of its head & it blew a huge hole in neck on exit...
Also like you, with the first batch, not one single fox moved so much as a step when hit, but with this newer batch, many move even just a few steps before going down, & some take 2 shots...
So im moving off them, looking for a better alternative, as im sick of the bullets not performing...
So i think its the same thing--a tougher jacket on that batch...
Someone else told me it could be a thicker base too...but i dont know enough about bullet construction to know what the difference would be...
So ive had NO luck on any further fox skins this year--ive had bad luck in every one ive hit with no damage (only 3) have had mange...
(thats since promising some skins to a couple of people for Xmas...
)
Anyway...i digress...
The range will affect the outcome (velocity)...but im surprised a 22 cal 55gn Vmax only leaves a 1.5" hole on a dog...they must be tough ones...
Shame we cant anneal bullets...
...or can we...
Is it worth swapping them over--as a hunter with the experience & expertise you have on shooting dogs , im sure the importer would swap your bullets for a different batch number if you contacted them directly...
The man who knows everything, doesnt really know everything...he's just stopped learning...