Several years ago a friend had a really poor 303 Brit and jacketed bullets were a bit pricy. Naturally, I offered to cast some bullets so he could shoot a bit more reasonably. When I slugged the barrel, I noticed the similarity to a sewer pipe, and it was loose at the breech, tight in the middle and loose at the muzzle. One restriction, do anything I wanted, but do not alter the barrel since he still wanted to be able to shoot the jacketed bullets. So no fire lapping was allowed.
I tried to make some plinker loads, but they would not shoot with any consistancy due to keyholeing. I tried a starting load from the RCBS cast bullet manual, but used the SGC and came up with a load that shoot as well as the jacketed bullets. No custom mold, no special sizer, just a mold that was about right, the SGC and a decent load out of a manual. I believe that the SGC did help seal the gases behind the bullet enough to prevent gas cutting and massive leading. It was a bad barrel, but became a fun shooter but never a really great shooter.
If you have a barrel that leads, it is worth trying the SGC. If you have a reverse choke in a barrel, it is worth trying. Will it add lube to a barrel and produce fliers? I don't know, but if a guy can make paper socks for a bullet and not burn them, it is not too likely that it will melt the wax and coat the bore if only the edge of the disc of wax is in contact with the barrel. But it might be worth trying to see if that actually does or does not happen. Check the information from the link and see if you think it might be helpful.
I also use the SGC when shooting round buck shot in my 6mm TCU and 223 contender barrels. Minute of grass hopper to about 15 yards and a little pistol powder just for fun. If it is not fun, why do it? Dusty