Here is my Kynoch 3 inch .577 FBSP with 750 Grn projectile I have another with solid a projectile as well but looks basically the same.
The rifles came in single barrel and doubles.
The .577 Nitro Express 3-inch [76 mm] is a conversion of the .577 Black Powder Express 3-inch, it fires a 750-grain projectile at over 2,050 feet per second. This cartridge was to become the most popular of the three and a standard round for African elephant hunters in the early 20th century.
MkVII 303 for scale.