I have had 4 .54 caliber barrels , aGRRW. an ED Rayl, a Douglas, and a GMT barrel. Like most barrels they have a lower velocity load and a higher velocity load. If you are shooting chunk gun range, 6o yds. I would experiment with 2f loads between 55 and 65 grains. You will probably find a good load in that range. I believe your barrel is 1 in 66 twist. Your upper velocity load is going to be some where between 85 and 110 grains. Those upper velocity loads will let you know that you are shooting., specially from a rest.
Be good to know what that upper accuracy load is for a windy day.
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Your patch to ball combo will have a lot to do with your barrels performance. That's something else you'll have to experiment with.
Hope this helps