I agree with Brownbear: Rebounding, or bouncing back of hard, but light weight balls becomes a real danger. Always wear eye protection.
I chose not to use wooden projectiles because of this reason. I think using parafin bullets in pistol casings is much safer, and you can get by using only primers. See Bill Jordan's Book, No Second Place Winner, for details on how to make these.
This is not going to be a solution for people wanting to shoot their ML rifles at home, I understand. If you are going to shoot wooden balls, then hang a piece of carpeting or an old blanket to catch them so they don't bounce around. Set your paper targets a foot or so in front of carpet or blanket, so that the balls pass through, and then hit the backstop.
We used a heavy blanket to catch the pellets for our junior rifle team. It also works with BBs in BB gun clubs. You can use this same thing to catch those parafin slugs, but I was practicing speed shooting at a PPC target, so I put a target on a 4 x 8 sheet of plywood, then covered it with mylar (plastic) so that the paper target was never damaged, and the wax would splat against the heavy plastic, and could be scraped or wiped off easily between shoots. Scoring was a snap. An old blanket on the floor at the bottom of my target caught most of the wax, with could be easily folded up in the blanket, and poured into the trash can for disposal. I later added blankets on the sides, to catch the spatter that went sidways. Today, I would simply get another 4 x 8 sheet of 1/4" ply, and cut it in half and stand the two pieces on the sides of the target to deflect the splatter back and down into my blanket. I still think that using an OS shot card over a few grains of powder, followed by a cast wax ball and held in place by another OS card would give sufficient accuracy in a ML rifle. But, you have to seal the gases with a wad of some kind- maybe 2 or 3 OS cards, to get all the velocity out of a small powder charge. I would expect the ball to be squeezed down to an oval slug in the process, but it should be accurate enough at short ranges. I also think that mixing 1 part old motor oil to 3-4 parts of wax will make a more easily maleable wax slug, that will stay together better in the bore, and still shoot fine at short ranges, without having to use more than a few grains of powder.
