Originally Posted by Argo44
"fusil de chasse avec poignée de sécurité en stock"

Not bad Raimey...actually I haven't a clue an searching passionlachasse etc. nothing comes up. but here are comments on the French terms:
https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=480959

The gun is a SxS muzzle loader so technically it should be called".
-- Fusil à percussion (percussion gun)
-- Fusil à chargement par la bouche (muzzle loading rifle)
-- Fusil de chasse juxtaposé à chargement par la bouche (SxS muzzle loading shotgun)
-- Fusil de chasse à percussion juxtaposé à chargement par la bouche (SxS muzzleloading hammergun shotgun)

The stock should be:
-- la crosse anglaise (straight stock)
-- poingee (pistol grip)

The safety should be:
-- sécurité; sûreté (safety)
-- levier de sécurité (safety lever)

So I'd hazard the following:
Fusil de chasse à percussion juxtaposé à chargement par la bouche avec levier de sécurité en dessous de la crosse anglaise.

Argo--Haven't looked here in detail in some time. Good catch on Strasbourg/Germany. I spotted that and was about to post, then saw you'd already noticed.

On your translation, I might be inclined to make a minor change in word order: Fusil de chasse juxtapose a percussion . . . Probably works either way.