What about a hard cast lead bullet?

In addition, I guess the thin case neck of a .22 case does not improve the situation.

Heeled bullets phased out by the late 1880's if memory serves correct  - except for the .22LR.

We seem to have drifted off from the starting point of this thread, but while we're at it....

If I was hell bent on using a .22 calibre bullet for subsonic culling, my starting point would be a .22 hornet with monolithics. Because of slow twist 1:16 typical of .22H it would probably be limited to 40 grains max weight. I can recall locally .22H monolithics being available certainly down to 35 grains but can't recall who was making them. That weight range would be the same as a .22LR but unlike .22LR, 22H is easy to reload. A light bullet like that might not have great penetration but the advantage is that those stubby little bullets also won't travel too far if one were to miss, or if it passes through the animal.

GS Custom made a 35gr bullet for the 22H.

For killing larger animals with subsonic bullets I would think that .308 or larger would give enough penetration to ensure a lethal shot. I remember an article in Magnum that mentioned a .32 calibre rifle (specially made) to cull game at night. Only chest shots were used and death was not instantaneous.

GS Custom made a 35gr bullet for the 22H.

For killing larger animals with subsonic bullets I would think that .308 or larger would give enough penetration to ensure a lethal shot. I remember an article in Magnum that mentioned a .32 calibre rifle (specially made) to cull game at night. Only chest shots were used and death was not instantaneous.

I agree that using .308 or larger is first prize but typically if one is doing "discreet" culling, one would want the animal to drop quickly so I would suggest brain shots, not chest. They don't run away after a brain shot, whereas after a chest/lung shot shot they can go far enough that you can struggle to find the carcass.

Having said that, I have observed on more than one occasion, an animal with part of its brain lying on the ground next to it after a night time close range brain shot with a 243/6.5x55/308/30-06 is lying kicking and performing because it doesn't understand that it is dead.

As per my post on GS years ago, when doing subsonic you must break bones, I do not like head shots in general. Double shoulder bone and double lung is optimum, but usually one big bone and into lungs does the trick within a moment.
My rule of thumb when I was shooting subsonic was that you had to break a major bone or it WILL run forever. Skull, spine, shoulder, you must break a major bone.

As per my post on GS years ago, when doing subsonic you must break bones, I do not like head shots in general. Double shoulder bone and double lung is optimum, but usually one big bone and into lungs does the trick within a moment.
My rule of thumb when I was shooting subsonic was that you had to break a major bone or it WILL run forever. Skull, spine, shoulder, you must break a major bone.

That is true, even with a big caliber.