I am not sure of the BHN of shot pellets. I think they are around 12 BHN. Pure lead is 5 BHN for the most part. You might be casting a harder bullet then you think. Also if your water cooling them at all, your increasing the BHN that way also.

You probably have a 1:48 to 1:70 something twist in that rifle. It might be a 1:48 twist, and you mentioned it is a .58 caliber. I think I would get some pillow tick patch and some .570 swagged lead ball and try shooting patched round ball and see what kind of group you can get. Your rifle might not like the minnies or conicals your casting. You could buy some conicals also and try them like T-7 suggested.

A .58 caliber round ball is a sledge hammer for hunting. The ball weighs 284 grains, put about 70-100 grains of powder behind that and you have a real powerhouse in the woods for taking anything you want to shoot.

Roundball generally out to 100 yards are very accurate. This might be another suggestion for you. I shoot a .58 caliber with a 1:70 twist and with round ball this thing is a fantastic shooter with 100 grains of Goex FFg behind the patch....