The most full information abot 12G Explora I've found in Markevitch book.
He wrote the bullet for Explora been invented by Lesli Taylor in 1899, in 1901 he modified his bullet and sold patent to W.Richards. The design of this bullet looks like lead body with hollow aluminium or brass nose (hope you know it and saw the bullet).
The weight of the bullet can vary from 730 to 750 gr. The weight of Cordite was 2 gramms (metric), that gived velocity Vo=396 meters per sec. Also Markevitch gives service chamber pressure 540 Kg/sq.cm for 12G Explora.
Also there are must several articles about shooting from Explora in English journal Field in 1904-05.
