1st thing to do is to measure the groove diameter and get a mould that casts a ball .005" to .010" smaller than the bore.
2nd is to use pillow or mattress ticking or 10 to 12 oz. denim in the .020" to .025" thckness. If the rifling is very shallow, a ball .010" smaller than the bore will be just fine. I definitley would not use a smaller yet ball, as they will not be as accurate. The larger the ball you CAN use, the more accurate it will be.
3rd is to radus the crown of the muzzles. ALL production and most custom rifles have muzzles that are too sharp and are difficult to load.
use 320 emery between the end of you thumb and the muzzle's crown and rotate, turning the barrels every now and then. If the muzzle's crown has angles already cut, then about 5 minutes per bore is all that's needed.
You can also use a tapered wooden plug, or tapered gringing stone for an eletric drill. Wrap the embery around the stone of wooden plug and rotate that in the muzzles. Finish with your thumb as that adds a smooth radius and leaves no sharp angles that grab patches and cut or push up ridges on the ball when loading. The small angles swage the ball & patch together into the rifing. I use a .684" ball in my .69, with a .025" patch and have no difficulty loading.

With a very tight patch on a long jag, try 'flitzing' the bore. This will not hurt the rifling, but will polish the bore nicely. Flitz polishing compound is easily available most everywhere. It is more aggressive than JB, and quicker to use.