A 1:70 twist will generally require powder charges in the 70-120 grain range for best accuracy. What is the depth of the rifling? If the grooves measure 0.016" deep, as do most modern muzzleloading barrels such as Colerain & Rice, then a patch measuring at least 0.022"-0.028" thick, as measured compressed under the barrel of a micrometer, and 0.575" diameter pure lead ball would be a good place to start. With a properly relieved crown, this tight combination will not tear, will probably need a short starter, and once 8"-10" down the bore, load easily all the way to the breech. The most important thing is to completely fill the grooves with patch material. If you do so, then there will be no need for cleaning between shots, as the patch material from each subsequent patched ball will push any fouling down on top of the new powder charge, in essence cleaning the bore for you.
And, by filling the grooves with cotton duck patch material, a complete gas seal is made, insuring no gas blowby, and max velocity.
The key is the crown of the rifle. It should be smooth and RADIUSED, not angled as from the manufacturer. A radius crown may be achieved by taking increasingly finer grits of wet/dry paper, and using one's thumb to radius the factory 60° angled crown. Plug the barrel with a wad of cotton to prevent the grit & steel particles from dropping down the bore. Use your thumb to press against the crown, and rotate the barrel back-and--forth 10-20 times. Then rotate the barrel 90°, and repeat. Keep turning the barrel 90° until you have completed 360°. Keep this up until you have radiused the crown approximately 1"/16 deep. You will know you are close when you can push a greased, patched ball approximately three quarters of the way into the bore with light thumb pressure.
