hansol - plastic wads beneath the slug will probably generate higher pressure than the wad atached to the slug as I suggested, merely because the plastic wads create a better gas seals than card and fibers.

I assume you missed the past pages of thread dealing with shooting and developing loads for smoothbore doubles. Yoogi and other double smoothie shooters from Australia have worked on these types of loads using round balls. Jeff Tanner's moulds are very reasonable and he will make any size you want. If you want a heavier ball, go with a .725" as I eventually did. It is useable in bores from .725" to .740" & is accurate - as noted below.

If one uses the hemispherical cavity, cup-shaped base-cup from a trap wad like the WAA12 to hold the ball in the middle of the bore, accuracy is about guaranteed with some load development. Merely cut the gas check at the fingers and throw the tops away. I use spent wads picked up on the trap range - All that is needed, is just the base wad, seated cup up, so the hemispherical cavity crables the ball and any size ball can be used from about 14 bore up to 12 bore. If the ball does not touch the walls of the barrel, it will not leave lead streaks on the inner surface. The cup centres the ball in the middle of the bore and delivers it perfectly straight from the muzzle, no bouncing as described by some writers for the old 'punkin-ball' loads of the 40's and 50's.

You use other wads, like fiber wads to take up the air space between the smokeless powder and the ball w/cup, or as some, use a plastic wad (smokeless powder only), cutting off part of the fingers to leave just enough to centre the ball. If used separately the plastic base wad part or gas check, is used to centre the ball in the barrel. With smokeless loads, I've even gone to using one cup down on the powder, a filler wad or wads, then a base cup, cup-up underneath the ball with pefect results with smokeless powders - and in black powder loads, a hard card or two on the powder, then the ball with the cup beneath it. You have to protect any plastic from black powder flames as BP produces more heat than smokeless powder and will melt plastic if in contact with it. The melted plastic will coat the bore and is sometimes difficult to remove.

All loading with smokeless powders should be started with low pressure shot loads of the same weight.

Attaching a wad to the copper slug will not generate any more pressure than having the same wad beneath it, but not attached.

Switching to round balls would be a positive move now that you've gained some experience with your lead-filled copper slug loads with both black powder and smokeless.

I used folded crimps on all my round ball loads in plastic cases. In some instances of using plastci cases with reduced capacity and with 7 dram black powder loads, the ball was not completely covered by the plastic case when crimped, leaving the ball visible in the centre of the crimp. This is not a bad thing. In other hulls of of even less capacity, the plastic only folded into a coned, 45 degree angle against the ball's sides and nose. None of them ever opened to release the ball until they were fired, and these 'pointy' cases with RB told me instantly they were BP loads. If loading in a pump gun or other repeater, then more perfect crimps are necessary of course, but not so in a double or single shot.