Are you shooting iron sights( open sights??? If so, that is pretty good accuracy, and I don't think it improves too much with practice. You can use a larger diameter ball, you can weigh the balls so that they all weigh the same, you can sort them by size, using a micrometer, or caliper, and you can sift or screen your powder, and then weigh powder charges instead of measuring them by volume, to reduce the groups. Then you can play around with different lubes, as the right lube can reduce group size.
Of course, if you put a scope on that rifle, you can reduce aiming errors, and groups will come down. My brother found that lubing the barrel after seating the PRB produced about 20 fps more velocity, but reduced the SDV in half, down to 10 fps. The smaller the SDV, the more accurate the load will be, all other things being equal.
HOWEVER, not all other things are equal. You can't do much about the weather, or wind speed, or direction. Change the relative humidity, and the powder burns differently, the bore collects more crud, the POI changes, etc. The target shooters keep notebooks of daily information about performance of themselves and their very specialized target rifles, just to try to win matches no matter what the weather may be.
You asked about Hunting accuracy, and I think you have it. If you mess with the rest of this, just remember that you are NOT shooting a target rifle, and it was not designed to be a target rifle. Have fun. Remember, if you are shooting a deer with that rifle at 100 yards, you are Starting OUT with putting a HALF INCH hole in the deer, before the ball can even expand. That is a larger hole than most .30 caliber modern bullet make when THEY exit the off-side of a deer. In most cases, a 50 caliber PRB is going to also pass completely through the chest of a deer on a broadside shot. The exit hole is more likely to be .80 caliber or even larger, by comparison. Deer tend to run a short way, unless the spine is hit, and die within 50 yards of where they were standing at the shot.
Consider also, that the front sight on the rifle you are shooting will generally cover up more target than the size of that group you report, so you should not expect to be able to shoot much better groups than you have achieved. If you are shooting a 3 inch group, or less, that means that you are able to hit within 1 1/2" of where the gun is aimed at that distance. The heart/lung area of a deer gives you about 12 inches to hit with your PRB. That leaves plenty of area for a margin of error, don't you think?
I also practiced and practiced to hit small targets at 100 yards, but the first ML deer I killed was a shot at about 35 yards. My longest shot has been at 40 yards. I can barely see the deer much beyond 50 yards in the brushy area where I hunt, and getting that 40 yard shot was more luck than anything else. The deer stepped into a clear shooting lane, which was only about 3 feet across the entire distance.
