Hi,
Just cut the sprue off close and don't worry about it. Put the balls in a bag with beeswax so as they roll around they get a light coating of wax. A 0.648 ball shoots well in my Ferguson, however, I can only get at most 55 grains of powder behind it. I suspect there was some variation in the depth of the powder chambers on TRS's Ferguson breeches. When you push in the ball and then pour in the powder, don't over load it. If you have excess powder that then gets ground into the threads above the chamber, you will foul the action quickly. Use 3F Swiss powder. Also, lubricate the screw plug threads with a mix of 50% beeswax and 50% tallow or Crisco. Melt the lube in a pan and dip the screw plug in it, then install it in the rifle while the lube is still very soft. I am able to get 30-40 shots off without cleaning the breech if I am careful loading and lube the screw plug. The Ferguson was almost certainly designed originally to shoot standard carbine ammunition, which was 0.615 caliber. However, I never could achieve any accuracy whatsoever with that ball size. I suspect the TRS Fergusons are a bit different from the originals. The locks used on Fergusons were a mediocre in design in which the flintcock does not overlap the pan very much. Consequently, you will get best results with long flints. Also, the gun benefits greatly by having a White Lightning vent hole liner installed because the simple drilled vent hole has to go through a lot of metal to reach the powder chamber.
dave