First, determine the diameter of your chamber's throat. Internet search "pound cast chamber throat measurement" to do this. Then find a cast bullet that just fits the throat ( ie 0.0005" smaller than throat diameter).
You not trying to find bullet diameter "a couple of thousands larger than groove diameter". That is too broad, and not the base measurement for selection ( NOT groove diameter). It SOUNDS good, and is in the right direction, but it is not the correct reference point you want to use. Use THROAT DIAMETER as reference point.
Many commercial cast bullet designs available. Look around. Best accuracy will be "near throat diameter" sized cast bullets.
