I had a jacket-core separation in a little more modern context. Let me explain…
I was bringing a pre-war 275 Rigby to RSA circa 2019. I felt obligated to bring “275 Rigby“ brass vs “7x57” brass so as to avoid potential hassles at entry. Moreover, I choose to use the factory Hornady 275 Rigby ammunition, as the accuracy was good, rather than shoot it all and develop reloads with a 140 grain swift a-frame or similar (which I was using in the US in 7x57 marked brass to good effect).
Well, its performance on an impala was lackluster. Lethal, but lacking. It did not exit, which provided a chance to see what actually happened, a complete jacket core separation.
The cartridge is fully capable of taking larger plains game well, but not reliably with that particular bullet. I thought it may be bonded - it was clearly not. Fortunately, I had other rifles for other quarry.
While lethal, I consider it a failure, as on larger game it would have been problematic. An a-frame, partition or any number of other bullets would be far more effective.