16 gun.

I killed him with the right-hand barrel, but I lost ground by stopping to fire.
A jungle lay about two hundred yards in front of the herd, and they increased their speed to arrive at this place of refuge.
Giving the little gun, with one barrel still loaded, to Wallace, I took the four-ounce rifle in exchange, as I knew I could not close up with the herd before they reached the jungle, and a long shot would be my last chance.

With this heavy gun (21 lbs.) I had hard work to keep my distance, which was about forty yards from the herd.
Palliser and Wortley were before me, and within twenty yards of the elephants.

They neared the jungle; I therefore ran off to my left as fast as I could go, so as to ensure a side-shot.

I was just in time to command their flank as the herd reached the jungle.