Birdhunter50,
I now have my reference material at hand, so will try to answer your additional questions you posed:

The breach pressures indicated below are all in British LONG TONS. One Long Ton = 2240 pounds.

.577 3" double rifle data that may interest you:

Bell's Greener rifle, made in 1900. Pictured and written about in DGJ, it was .577 Light Nitro, original factory load was 80 grains cordite, 650 grain bullet. Overall rifle weight 11 lbs, 3 oz.

A British hammer .577 3" had engraved on rifle 75 gr. cordite, 650 gr. bullet, 1850 fps. Rifle overall weight 12.5 lbs. John Taylor said this was load (transitional load) developed to replace black powder.

On flats of Jeffery .577 3 1/4" 177 gr. (black) 650 gr. bullet. Rifle weighed 11 lbs, 5 oz.

Loads for .577 black and light nitro:

Charles Lancaster (London) catalog (and rifle I own): .577 3" 170 gr. black (nitro for black about 68 gr. IMR 4198, 650 gr. bullet.

ICI 1926 catalog:

.577 3" 90 gr. cordite, 650 gr. bullet, 1950 fps.

Nobel 1925 catalog:

.577 3" nitro for black powder, 75 gr. smokeless (not shown as cordite), 650 gr. bullet, 1800 fps.

Kynoch catalog (1902-1903):

.577 3" 560 gr. bullet, 65+ gr. charge (cordite not shown) 1700 fps---6 long tons.

.577 3" 560 gr. bullet, 75+ gr. charge (cordite not shown) 2000 fps----8 long tons.

.577 3" 650 gr. bullet, 90 gr. charge (cordite not shown) 2000 fps----12 long tons.

Black Powder Loads:

.577 3" 560 gr. bullet, 167 gr. black, 1740 fps----10 long tons.

.577 2 3/4" 160 grains black with 610 gr. bullet---1650--1700 fps.
.577 3" 167 gr. black, with same bullet weight 1650--1700 fps.
.577 3 1/4" 177 gr. black with same bullet weight 1650-1700 fps.