I don’t know about a 50 caliber GM LRH, but here are couple 100 yard targets shot with a 45 caliber GM LRH. The first target shot with PRB with a slightly oversized felt wad over 60 grains of 3F Swiss. This ‘let’s see what happens’ group was 2-3/4” according to my notes. Only time I have tried PRB in this particular barrel. Second target (1-1/16” group) was shot with a 400 grain paper patched bullet over a felt wad and 80 grains of 3F Swiss, a load I shoot a lot out of this barrel. Bore was ‘cleaned’ between each shot with two wet and one dry patch as I was looking for clean/cold barrel groups. Both targets shot on the same day.
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Don’t know what you consider ‘shoot decent’ to mean, nor do I know how your particular barrel will shoot. The 50 caliber GM barrels I have are in round ball twists and really handle PRBs very well.