chain fire with just powder loads would be a big pfut and over in a second.

Caps you would have trial and error to find best fit, and a good push with a bit of wooden dowel to seat them well. A problem with m/l guns is that often the cap bursts and bits go everywhere, not good next to your horse or without eye protection. One plus is without the back pressure from a ball its less likely to blow the cap off, nipples with smaller holes would help as well. My bad as theres little powder larger holes would be best.