I would keep numbers relatively low since you're using 3Fg instead of 2Fg.
"Relatively" means using round-ball charges. They should be plenty, anyway.
A good rule of thumb is that for up to .45 caliber, 3Fg should be used. Higher, 2Fg should be used. However, there are some who are questioning this and find they have good results with the 3Fg in larger calibers.
Now, that said, I recommend a starting load of 50 grains by volume for a .50 caliber. The max load is half the weight of a round ball in that caliber, or 95 grains by volume.
What does your bore look like? You said accuracy has dropped off. How do you clean it? Do you clean it after shooting it every time?
Start at 50. Work in 10 grain increments to 100 grains or so. You have a bit more wiggle room since this is an inline, but you still don't want to tempt fate. To me, 150 grains is an overcharge and a waste of powder. What is your goal using this charge?
Look at it this way: You're shooting saboted .45 caliber bullets.
If you compare the rifle to a cartridge rifle, you've got the rough equivalent of a .45-70. Keep in mind, though, that they also made .45-40 (the .45 Colt pistol load), the .44-40, the .45-70, the .45-90, and the .45-120.
Noplace did they find that anything above 120 grains was a good idea.
Even with full-bore bullets, they seem to stop at 140 grains of BP. (.50-140 Sharps)
We're talking some of the most powerful rifles of the day here, capable of taking anything in North America.
Just back off to 50 or 60 grains by volume and work up until you find something accurate. If you find you need a flatter trajectory, work up again until you find the next accuracy node. I'm sticking with 100 grains by volume being your absolute upper practical limit.
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