Looking at George's pitch for that rifle, and the markings on the labels it appears as if this set of barrels was set up for the Eley paper patched bullet.
There were more than one choice for weight - but your chronograph value looks low for the cartridge.
I'd expect to see something in the 1800 fps neighborhood.

I wouldn't try to run a load without a greased felt wad behind the bullet - but that doesn't answer the velocity issue.

Have you tried Swiss 2F or Goex Old Eyensford 1-1/2F or 2F?

Also - many fellows here including myself have gotten excellent results with Nitro for Black loadings and jacketed bullets.
It's important to choose a bullet with a relatively soft core and jacket. This isn't a pachyderm rifle after all, and there are many modern jacketed bullets that are great choices.
I've had great success with the Hornady 350gr Soft Nose Round Nose bullet.

What is the groove diameter of the barrels?