kDan: I went back to re-read all your original comments on this project, trying to understand what you are doing. The biggest concern is that the depth of grooves in a ML rifle barrel are much deeper than that found on a cartridge gun, even the .45-70. IF Bobby Hoyt is going to make a liner or new barrel for your GPR, he will have to make the grooves only about .004" deep at the most, if you are going to use cast .45-70 bullets in it.
Stick with the shorter, smaller, lighter, 300 grain bullet. You will have to cast it from pure lead rather than an alloy, in order for it to expand on firing and grab the rifling. Tell Bobby what bullet you are going to use, so he can rifle the barrel, or liner with the correct ROT for that bullet.
You definitely are going to need to seal the barrel with over powder cards, like you use in the .45-70 cartridge. You will want to use a couple of the oversize, .460 diameter felt wads. Oxyoke made the prelubed kind and those work well in both the .44 Ruger Old Army Revolver, and in .45 cal. ML rifles to get down into the grooves and seal the bore.
In order to get the best accuracy, you will need to drive the bullet down the barrel consistently so that it is properly aligned on the same axis. Most cast bullets are made .001 " smaller in diameter than the bore, or land diameter. In a cartridge gun, with shallow grooves, this allows the bullet to upset a little on firing, and fill the grooves, making its own powder seal.
If I were trying to do what you are doing, I would want the bullet to be sized to the land diameter, even if I have to tap it down the barrel with a rubber or wooden mallet. I would consider using a false muzzle or coned muzzled to center the bullet, like the new T/C barrels are set up, so that the bullet is loaded straight, and centered every time.
If you have a barrel made with the more typical ML rifle depth of rifling, I think you are going to have to use a heavy card wad over the powder to seal off the gases. Gas cutting will be fatal to accuracy with that bullet. The deep rifling of a ML barrel will allow gas cutting unless something of Groove diameter is used to seal them off. Normally, a cloth patch around a soft round ballis used to do this job. YOu are going to use overpowder cards to do the job that the cloth patch used to do. BTW. the reason I suggested you consider using the small .452 cal. 250 grain bullets, is that you could paper patch them and let the paper help seal those grooves in the barrel. Yyour 45-70 equivalent barrel willl be a specialty barrel indeed. It not only will have a much faster rate of twist than you want for round ball shooting, but it also will have the shallow rifling.
Take a look at some of the reproduction .58 cal rifles like the " Zuave " guns. They also come with the shallow rifling, because they are mainly set up to shoot what the original guns shot, and that is a soft, cast, hollow based conical bullet, that had to seal the barrel with its skirt on the base of the bullet. The rifling was very shallow compared to what you see on Hawken and plains style rifles.
Finally, if you get a load that produces 1500 FPS velocity, it will be more than enough for any deer hunting, or Elk hunting you will ever do. I think 1800 fps would be extremely hard on the barrel, breechplug, and stock of your GPR, not to mention your shoulder. Even at that velocity, you are going to go through nipples at an alarming rate, whether they are platinum lined, or not.
You might want to visit with some of the Slug Gun shooters at Friendship and talk to them about the problems they face with the heavy powder charges and slugs they shoot. There are only about 80-100 Slug Gun Shooters across the lower 48, and they are a friendly bunch. They have been there, and done that, as they say, and they could help you avoid some needless expense, and damage. Even in a .45 cal. rifle shooting round ball, if you put a lot of powder behind the ball, you can burn out the nipples pretty fast.
I recently saw it happen, when a standard nipple was used in a gun that was loaded to the max. Shots were fired over a chronograph, as the owner was working up his accuracy load at 100 yds. All of a sudden the velocity dropped more than 100 fps, and then became very eratic, with a much wider SDV than he was getting originally. ( a SDV of 12 fsp!) He checked the powder, the balls, by measuring them and weighing them, and even the bore of the gun. When all those things check out okay, he looked at the nipple, and it had been burned out with less than one hundred shots. ( probably less than 50 shots!, as he was not counting.)
Good luck on the project.
