That is a substantial skirt on those slugs. Unless you fill the base ( hollow base) with a lube, and then use a OP wad to protect that skirt, I doubt that you are getting effective obturation, even using those heavy powder charges. Using an OP Wad would allow you to seal the gases, and reduce the powder charge needed to get the same level of velocity you now are getting. Using grease in the base will allow hydraulic pressure to work for you and expand the skirt, so you get an evenly upset bullet, ( concentric with the bore) and good spin to the bullet from the rifling. Without the grease in the base, the skirt can expand unevenly, and tilt the bullet in the barrel, sending it out of the barrel with a yaw already started. That is not good for accuracy. The slug is likely to fishtail as it spins, with that yaw, too, shedding velocity quickly, and going God Knows where.