What peter said!

The invention of smokeless powder over a century ago made all BP loads obsolete instantly, however the rifles were not obsolete and most would last several more decades, many into this, their second century!

At the time, the trade responded with smokeless loads that would regulate in the old BP doubles, with a number of very significant advantages!

1. No smoke-cloud to obscure the quarry, therefore faster follow-up shot from the second barrel

2. Heavier jacketed bullet than the original BP lead projectile, at the same velocity

3. Lower pressure, meaning less wear and tear on the rifle, and longer life.

4. No corrosive fouling in the barrel all day after an early shot in the hunting field

As an example: my .577 Light Nitro double regulates with a 650gr jacketed soft-point at 1665 fps for between 7 and 8 tons chamber pressure. That's 4000 ftlbs muzzle energy with a bonded-core bullet. Hell of a hunting cartridge!

The original .577 BP cartridge fired a 560 or 570gr lead bullet at 1650 fps and around 10 tons breech pressure. That's 3,450 ftlbs ME. Less energy, and higher pressure! ...with a smoke-cloud!

Pretty simple math!

I consolidated my thoughts on the .577 Light Nitro cartridge a while back, and tossed it up on the web here. Long-time NE.com posters are sick of seeing it, but since you're interested in this cartridge...
(mine is the 2 3/4 inch version, but the same sentiments apply to the 3-inch cartridge).

...and like peter, I strongly recommend Graeme Wright's book, though it looks like we're all waiting for the third edition as the second has apparently sold out. Can you borrow a copy...?

I guess what we're saying is that nitro proofing an old BP DR is generally not recommended, and in the light of the above information, is not necessary!