roundball62
I'm sure your Rice .62 caliber barrel has a bore size of .620. With that slow twist I'm also sure that Rice would have used rifling grooves that are at least .010 deep.
My reason for mentioning this is because if I take your .610 diameter plus two .005 patch thicknesses I get an answer of .620.
Now, if the patched ball is .620 in diameter and the bore is .620 in diameter, what's filling up the rifling grooves?
I'm betting that if you retrieved your patches they were burned all to hell. That would explain the 3 inch diameter group. Your gun really should do better than that if you are a good shooter.
The bottom line when it comes to patches and ball sizes is the sum of the ball size plus twice the patch thickness should always equal .005 or more in diameter than the groove size.
If it doesn't, mediocre accuracy will be the result.
Now, this is just my opinion but folks have to get rid of some of the modern ways of thinking about killing power when they are shooting a roundball out of a muzzleloader.
Velocity is
not the answer.
Accuracy
is the answer.
A high velocity with a poorly placed shot will not kill the animal.
A low velocity with a well places shot, with that big old piece of lead that starts out larger than a modern bullet will expand to, will do massive destruction to the game.
Most muzzleloader shooters have also noted that usually the big round ball will either stop on the far side of the animal or just barely poke its way thru.
What that means is, unlike a modern guns bullet, 100 percent of the roundballs energy is spent inside the critter destroying vital organs and nervous system parts.
Once this is realized and one has developed the most accurate load their gun will shoot they should have no fears of loosing the game animal they have shot.
I suggest that you use a .610 diameter ball with a .018 thick cotton pillow ticking patch.
That equals a pre loaded ball/patch diameter of .646. That ball/patch in a Rice .62 cal with .010 deep grooves (.640 groove diameter) will provide .006 interference (.003 per side) and should seal off the powder gasses and improve your accuracy greatly.
Yes, it will be hard to get started. Use a "Short starter" to get the patched ball into the muzzle and pushed down about 3 inches.
After doing that, a regular ramrod will have no difficulty in pushing the patched ball the rest of the way down to the powder load.