Among other weapons, I had an extraordinary rifle that carried a half-pound percussion shell; this instrument of torture to the hunter was not sufficiently heavy for the weight of the projectile: it only weighted twenty pounds, thus with a charge of ten drachms [270 grains] of powder and a HALF-POUND shell, the recoil was so terrific, that I spun around like a weathercock in a hurricane. I really dreaded my own rifle, although I have been accustomed to heavy charges of powder and severe recoils for some years. None of my men could fire it, and it was looked upon as a species of awe, and it was name "Jenna-El-Mootfah" (Child of a Cannon) by the Arabs, which being a far too long of a name for practice, I christened it the "Baby", and the scream of this "Baby" loaded with a half-pound shell was always fatal. It was too severe, and I seldom fired it, but it is a curious fact that I never shot a fire with that rifle without bagging. The entire practice, during several years, was confined to about twenty shots. I was afraid to use it, but now and then as it was absolutely necessary, it was cleaned after months of staying loaded. On such occasions my men had the gratification of firing it, and the explosion was always accompanied by two men falling on their backs (one having propped up the shooter) and the "Baby" flying some yards behind them. This rifle was made by Holland & Holland, of Bond Street, and I could highly recommend it for the Goliath of Gath, but not for the men of A.D. 1866.
The Albert N'Yanza, Great Basin Of The Nile, 1866. By Sir Samuel White Baker. pp. 138
That isn't the reference I remember, just what I could find quickly on the interenet.
The one bore was probably a swivel of some sort. The reference I remember was just to the weight of the projectile not the gun itself.