Gentlemen,
Are we not missing the known unknown?

Pamtnman in his post on 10th February says the “original bullet has the copper peg in the tip”.

If, as I take it the bullet is for a British black powder express rifle, the “peg” is likely to be the closed top end of a copper tube, extending perhaps half way down the bullet and filled with air (or perhaps fulminate?) to promote sudden and violent expansion on impact.

In case it is fulminate I would suggest not trying to dismantle it.

The hollow is likely to be at least the volume of a 40 grain .22 bullet (perhaps narrower but longer) and would easily account for the difference.