There are things you can do to help keep the consistency of the PRB out of a smoothbore after multiple shots. But, understand that without spinning, the Lead RB is basically going to act like a "Knuckleball " in Fast pitch( Major League) baseball. After 50 yds, or so, as velocity drops, even the heavy .58-.75 caliber balls are going to be affected by air and humidity, and will veer off the line of fire, opening the group.
Try running a Lubed cleaning patch down with the PRB when you load the PRB down onto the powder charge in your barrel. The lube helps keep fouling softer, and makes it easier for the Patch around the ball to push what little residue will exist down behind it into the powder, where it will be burned with the next shot, and expelled out the barrel.
My chronograph shows that lubing the barrel this way will reduce the SDV by half, and increase the velocity from 20-50 fps- a small amount to be sure, but a indication that adding more lube to the barrel is not going to hurt your MV. If you don't use a bore brush between shots-- and who even thinks to carry a bore brush into the field when hunting??!!!--- the residue that can build up in the bore will begin to affect the velocities of consecutive shots, and that increases the SDV, and contributes to larger groups.
I have tried this with Young Country 101 Lube, a predecessor to Bore Butter, Wonderlube, and Natural Lube 1000, but basically the same stuff. I have not yet tried this using a wet lube either based on an oil like ballistol, or a water based lube. I shy away from wet lubes for hunting, because I don't know when or if I am going to fire my gun, and don't want to take the chance of fouling my main charge with anything wet. But, that does not mean that these lubes won't produce the same results as the thicker lubes do.
And, using either an OP wad, or a filler between the powder and PRB, particularly in the larger caliber barrels) has given me lower SDVs in my testing. I increased the velocity of my 19 ga. fowler by more than 200 fps, by using a Walter's Fiber OP wad in the gun, and got a much more complete burning of my 2Fg powder charge behind that wad, also. I tried some Farina as a filler for a couple of shots, and the balls hit in the same POI as those fired in front of the fiber wads. I did not have my chronograph set up the day I used the filler, so I need to go back and fire some shots for a group over the chronograph, and then compare them to a fresh group fired using the OP wad.
Finally, most shooters are going to get ONE shot on a deer, and that is it. A 3 shot group that consistently shoots a very small group of holes at a given range is all you need for hunting.
I always smile when I hear shooters talking about 5 and 10 shot groups for MLers. Yeah, they are needed if you are target shooting, as these long strings are the way to separate the winners from the darn good shots!
But not for hunting.
When I am testing loads, I have no problem with a 5-shot group, but I also have that bore brush at the loading bench at the range, and don't hesitate to use it to clean the barrel thoroughly between shots. My purpose is to remove all the variables humanly possible so that I find out what a given combination of loaded components will do out of that gun on that day. I don't expect my off-hand, or field position shooting,( may involve rests) groups to be anywhere near as small as I might get on the target range. :hmm:
