David, sorry if this is getting off topic a bit, I was doing some online research for some buckshot loads for my GP (as you do

)& came across this. Buckshot My original suggestion of the plastic sheet was based on thinking you were probably using either or both patches & felt wads. I don't know what this guy was using as wads but something a bit heavier for a screen might be called for. Ply or MDF, from memory it was about 12mm MDF that I saw used as a "wad" screen for a chronograph. A bit more looking around on google gave me this Screens Toward the bottom of the page, a very similar setup to what I've seen, this time with ply it would appear, so depending on what you're using for wads you might go with something heavier.
Steve