I've used modern plastic shotshell wads without them melting at all. I used a base wad of balled up newspaper that was tamped down and compressed well. I've also experimented using triple size cotton balls under and over the shot load, and also successfully used newspaper as an over shot wad.
Maybe the tighter seal of the newspaper wad kept the plastic from melting while loading with Pyrodex.
The loads certainly patterned better using plastic shot wads than without them from 2 shotguns, a 12 gauge and a 28 gauge.
I used charges of 90 - 100 of both powder and shot. And I also added even more shot to fill in the pattern. Even the very weight light 28 gauge caused no problem handling the heavy payload with the help of a slip on recoil pad.
While I opted to use modern 2 3/4" plastic wads, CVA also made these softer plastic 12 gauge BP wads below which I can't vouch for their resistance to heat and flame. Notice that their capacity is 1.5 ounces of shot though.
I just stuffed the wads in and then loaded the shot into the barrel. If a few balls of shot don't fall into the plastic shot cup, it doesn't matter. The 3 inch plastic wads would better accomodate even larger shot loads as long as the over powder wad fills in any spare room in the shot cup since some modern wads have smaller shot cups & less shot capacity. Any excess petal length could also be trimmed if necessary. That's why I recommend using wadded newspaper because it holds tight and forms well when tightly compressed. Since I last shot, I bought some 12 ga. BP cushion wads to use over the powder, but I'll still probably use a base wad of newspaper to better seal the gases and flame since that worked so well to protect the plastic from melting. There weren't any noticiable plastic deposits left in the barrel either.
BP shotguns usually shoot much more open patterns than modern shotguns do, even if they have screw in choke tubes. That's why larger shot loads and plastic shot wads are useful in obtaining effectively better shot patterns.
