srose:
The 177 refers to 177 grains of Black powder, and the 650gr bullet is the heavier option for the family of .577 Express cartridges.

The standard .577 Express on the 3-inch case was loaded with 165 grains BP and a 540 or 560gr lead bullet.

You would be able to make a wonderful 'nitro for black' load up for this rifle. Woodleigh makes a superb 650gr Weldcore jacketed bullet, and a powder like Varget would allow you to match the original ballistics at perhaps 8 tons pressure rather than the original 10 tons. Easier on the gun.

The .577 NforB load is very underrated IMHO. It is a good killer with the bonded core bullet, and is nowhere near as vicious to shoot as the full nitro load. Five stars!