Here's a picture of the wad I use with my round ball loads, it's called a "Light Brush Wad" from Ballistic Products.
I find it does exactly as Daryl suggests, it acts as the gas seal, has a cushion area and the other end centres the ball down the barrel. Works brilliantly.
Easier to load than a shot charge. Dump the powder, seat a wad, stuff a Liquid Alox lubed ball down on top of it and star crimp as per normal, and go and shoot it.
(Worthwhile marking the cases though, just in case of a mix up!)

The make awesome skeet loads with an ounce of #9's as well.

Regards,
David.