Headstamp - SFM

Among various cartridge cases and bullets found in and around my garden (Picardie, France), I have examples from both WW1 & WW2. I also seem to have a non-military case, as it has no date stamp or calibre id., and according to my limited knowledge, is thus civilian. What it does have is a double G symbol, back to back and overlapping (similar to the double C symbol of Chanel perfumes). It looks to be a rifle type case as it is approx 350 mm long, is not necked, and size is perhaps 303 but it may be smaller - there is a lot of corrosion. Anybody out there who can provide an answer to what this might be?

GG = Société Française des Munitions, de Paris.
Notice biographique : Société française des munitions de chasse, de tir et de guerre (SFM). Fondée en 1884 par Victor Gaupillat et Jules Félix Gévelot (anciens Établissements Gévelot & Gaupillat). En 1887, Jules Félix Gévelot rachète les parts de Gaupillat.
GG = Gaupillat et Gévelot.