I got out this weekend and did some more load development with my GPR rifle. I've found the rifle shoots well with 495 round balls, .018 thick patching and a lube mix of distilled water, TapMatic Aqua-Cut, and LessToil detergent. It likes 40 grain charges (center group) and 90 grain charges (top right). This will work for club shooting matches but I need to find a good load using a grease lube that won't dry out in the loading block or during a hunt.
I lubed some patching with lard and shot the lower right group. Obviously, my rifle doesn't like lard.
I've already tried Bore Butter and that didn't shoot worth a darn.
I'm going to try Crisco next. If that won't work I'll try the mink oil from TOW.
I'm looking for something that at least looks historically correct, doesn't cost an arm and a leg, and is easily obtainable. (I don't get many opportunities to hunt bear or harpoon a Sperm Whale.)
I'm open to suggestions.