Thank you Larry and Pete.

I'm postulating that the "poudre" specified is 81 grains (3 Dram) black powder. 50 grains of "E.C." is 3 1/2 Dram Eq.

Why would a 70mm chamber 16g (image on the original ShotgunWorld thread linked) be recommended for only a 24.5 gram - 7/8 oz. load of "plomb"?
Ballistite was the only Dense Smokeless in which 24 grains = 3 dram equivalent. Mullerite was marketed by Louis Muller & Cie S.A. des Explosifs de Clermont, Liege; No. 2 was a Bulk powder and 33 grains of Mullerite No. 2 = 3 Dram Eq.

Yes, proving with "E.C." was not uncommon, but I've never seen E.C. No. 3 marked.

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