CapeGunner
The thing to watch for when using Pufflon filler is if you don’t compress it sufichantley it can shift and co mingle with your charge. I would be more inclined to use a foam or Dacron filler just be absolutely sure you fill the case completely with some compression.

K80
Ken, the reason RL 15, with a filler, is one of the best loads for any of the big bore doubles is it most closely duplicates the burning rate and pressure curve of Cordite. This helps with getting many of these guns to regulate.
This also puts the pressure peak with in the same area of the barrels. When you use a slow burning powder it moves the peak pressure curve further down the barrels where they are thinner. This combined with older not so great steel can be dangerous and expensive.
Just a general statement it is not the use of fillers that is dangerous but the improper use that is.
Bill