Very nice rifle. For easy and cheaper plinking loads or even just light game loads I'd go with a cast bullet and a suitable light load of fast burning shotgun/pistol powder. I don't know if you can get French made Vectan powder anywhere in the States but Vectan AS (green and red flake powder) is superbly clean burning for light cast loads. It is equivalent to Red Dot but leaves no residue in the barrel other than a light stain which wipes clean with one pass of a dry wipe.

I used a lot of it for competition trap shooting and never had to clean out the U/O action. Now use it for light 404 cast bullet loads, actual same charge weight (22.0grs) I used in 12G 32gm lead loads gives me 1400fps with a 400gr bullet in the 404. No need for fillers either. I imagine you don't want powder residue of any type falling back into the Webley or similar action type as they wouldn't be the easiest to clean?