Dave - given that information, the .610's, if having sprues, will stick at some point, where the sprue runs sideways against the inside of the tube. If they have no spures, they aren't round - or - aren't actually .610" in diameter.
In that tight a bore, I'd be using a .590" with a descent patch - maybe even a .585" ball and a .022" denim patch.
It's amazing how much this helps - rotating the thumb in a circular motion with 320 grit emery or wet/dry paper. Rotate the barrel every now and then to make certain the 'grinding' is perfectly even around the muzzle. This 'crown' allows the ball and patch to swage together into a perfect fit that wipes (cleans) the bore down each time it's loaded and so there is only 1 shot's fouling (the last one) in the bore at any time but it's only there until the next one is loaded.