Just posted up a story of my spring black-bear hunt from last week. The trials of putting together the load for this 130 year old gun have made me a humble man. I used to think I knew most everything there was to know in the world of reloading, but this bpcr has put me back in school, and I am fighting to ward off the dunce cap!
Found an old J&W Tolley, sxs hammergun in 577/500#2. After reading Graeme's book, and conversing on occasion with anyone who would listen, I finally got a load worked out, in the final days before the trip.
My final load, offering consistent sub-2" groups at 50 yds, was as follows:
10 gr of 5744 overtop a 215 primer. Top up with 120 gr of Swiss FFg, cap off with a grease wad, and then crimp in a 360 gr paper-patched cast bullet.
I have yet to chrony this load, but had to drop down in velocity to get the load grouping where I wanted it.
I had originally tried to go with a smokeless load and forego the nasty cleaning issues of blackpowder, but this is the magic the gun was designed around, and it proved up to be the right recipe to make it all come together.
Two bears down with two shots. No problem. Got a little tense when I stalked the last bear within 20 yards, and the powder smoke hung like a fog for waaaaay too long, while the bear was rolling around on the ground with a bullet up the backside. The whole trip was focussed around harvesting an animal with this wonderful old vintage round. Gotta say it was a resounding success!

Some of those old BP doubles kinda take the cockiness out of a fella's swagger, don't they!!! I had a very similar, humbling experience about 20 years ago with a German double rifle in 43 Mauser from the late 1870's or early 1880's. It took me 6 months to figure out that thing. I FINALLY ended up similar to you with a duplex load of 5 grs. of SR-4759 and 67 grs. of Ffg.

It's really, really cool when it all comes together, eh? Congrats on the fine work and the two bears!!!

Using an old BP DR for modern-day hunting....that's something I have yet to pursue.

I got a caribou with a 45-70 Sharps and two bison with a Winchester 76, 45-60, black powder and lead bullets. It is much more satisfying than using modern rifles and smokeless powder. That is real hunting. (No hate mail from the bowmen out there please.)

I just acquired a .577-500 no2 and am reloading extensively for it this summer. The same ballistics as the .500 3" bpe. I've shot black and also smokeless. NO duplex loads yet.
Cal

Congratulations on your persistence, Jimmy! Don't be shy about sharing photos of that rifle. I'll go looking in the hunt reports ...