I had many issues shooting at max loads of powder,,,back off your powder charge and see if that brings back the grouping,,ive had my bullets shooting all over then once I backed off the powder it started grouping well..I only hunt w 80 gr of 777 behind a great plains conical in .54 cal,,,if yours is only a .50 you may need to back off even more than that...at longer range you will need to figure out your bullet drop and you can dial it in.I hit milkjugs at 250 yrds w mine,,and I get about 28" of drop at that range.let me know if that made a difference for you?I don't think its the barrel unless extremely pitted.also I use a felt under my conical and that I think also helps too,,,and that's prim style that is required out here in Oregon.