I'd read years ago that to increase lethality, the Brits had started putting an aluminum insert into the nose of the jacket to move the center of balance further back: the bullet remained very stable in flight and accurate, but on hitting a 'soft' target tended to go unstable much more quickly. This was the Mark VII ball.
Just looked at the Box o' Truth for the first time in a while and found this post testing just such ammo, including a couple of pictures. I'd never tried cutting a bullet in half to check the inside, here's the aluminum insert design:
And, to make it even more interesting, here's a WWII round with a WOOD insert:
Better than my describing it, go check out his results with this ammo on water jugs. Impressive, to say the least. A few years ago I wound up with several boxes of WWII Brit .303 ammo, I may have to move it up to my 'grab in case of trouble' box.