First - the 12 to 1 theory is not mine.

I assume that there are two reasons why Eley or Kynoch wouldn't want the bullets to be any harder than necessary -
expansion in the target is one and the other is cost (tin being more expensive than lead).

The "cup" in the base of the bullet should be for the twisted end of the paper patch.

In my 'ill-spent' youth I made explosive Minie balls. They worked.

I am a fan of Baker and have studied him extensively (including corresponding with his great-great grandson who lives in England).
I am convinced he used explosive bullets but his descriptions don't 'add-up.' He described using a 1/2 pound (3500 grains) bullet
with either "10 drachms of powder" in the bullet or as the propelling charge. In my 8-bore, I used 10 drachms (280 grains) of powder
behind a 1605 grain bullet that produced a velocity of 1040 fps. That charge behind a bullet weighing more than twice as much, in a larger
diameter barrel, would produce an anemic velocity. If Baker meant that the 10 drachms was the explosive charge in the bullet, by
volume, it would displace 2000-grains of lead unless the bullet was substantially lengthened. Seems like a great project for "Myth Busters."