Quite possibly a Mau Mau gun but rather fancy. As I know little about guns and ammunition I am unsure how this gun would function. Many Mau Mau guns were very simple often having a door bolt as the firing mechanism, just ramming the bolt into the bullet. Very similar guns (Zip guns) are made by Solomon Island rascals local name for gangsters, criminals and so on.
https://www.iwm.org.uk/collections/item/object/30036466
https://www.rememuseum.org.uk/blog/c...mau-mau-pistol