I have a fine load for my Ruger No. 1 in .45/70. It also has the 22" barrel.

My bullet is the Lyman 457122HP, which from my mould casts 342 grains from pure, soft lead. The powder charge is 65 grains of Swiss FFg Sporting Rifle black powder. Primers are Federal 210's in Remington nickel-plated cases (these are better for black powder because theydon't corrode and turn green like brass cases do. And they are easier to scrub out clean after shooting with BP).I use a .45 caliber greased felt Wonder Wad of the type made for cap & ball revolvers on top of the powder charge. This is the ONLY lubricant I use in this load, and it does NOT LEAD the bore. MV is 1350 FPS, and it is very accurate!

I have found that by using 44 grains of IMR 4064 smokeless powder and TWO Wonder Wads on top of that powder with these pure lead bullets produces the exact same MV as the black powder load, but of course is not nearly as dirty as the BP load. NO LEADING with the smokeless load either. The lube in those Wonder Wads must be great stuff! Both the BP and smokeless loads shoot well in my rifle. That old Lyman-Gould 457122 hollowpoint bullet was highly regarded as a game bullet 130 years ago, for everything from deer to grizzly bears!

You could probably use the same, or a similar, load with your 300-grain bullet. I had to cut my powder charge to 65 grains with the Swiss powder in order to have enough space in the case for the Wonder Wad and still be able to seat the bullet and crimp the case. I don't know if you'd need to cut your charge as well-probably depends on the make and granulation of the powder you have. You might want to try the IMR 4064 load first to see if it gives the kind of performance you want, before you dirty up your rifle with black powder!