I admire your............"optimism" for shooting your 1876 that far!! Seriously, the Big Lube bullets are great. I use them in pistol calibers and they are wonderful. However, the big lube groove is a detriment to aerodynamics at long range. As a side note from a man who shoots paper patched bullets in a Shiloh Sharps, the paper patched bullets are making a really strong "comeback" with traditional LR BP shooters as they are as aerodynamic as a modern jacketed spitzer bullet.
Har! No kidding, I can't even see that far most days!
However, our pard, John Boy is making hits at 800 yards with his 1876. There is a thread running elsewhere on this board that discusses PP bullets and the 1876.
I am planning on trying a few PP bullets in my Shiloh just for grins.
May try it in the 1876 if they can be loaded without screwing up the patch.