Argo, please let me correct some mistakes and add quite a few more.
best regards,
WC-

Acier - steel
Acier damas (also "damas") - damascus steel
Acier tremp - quenched steel
Acier revenu - tempered steel
Anson et Deeley, Anson- box lock
Arme de poing - hand gun
Arme dpaule - long gun
Armurier - gun shop owner
Armurerie - gun shop, not a maker, historically a body armor vendor
Arquebusier - Gun maker
Artisan - artisan
Avantage (a droite, a gauche) - cast-off, cast-on
Bande - rib
Prolongement de la bande - extension of the rib (to the receiver)
Balle - bullet or slug
Bascule - receiver/action
Bille acier - steel shot
Bois - wood
Bois de noyer - walnut
Noyer blond - blond walnut
Boitier - housing
Bouche - muzzle
Bouchon de percuteur - Firing pin bushing
Bourre - wad
Brisure, a - top break, center break fire arm ???
fusil a Broche- pinfire gun
Bronzage - barrel blacking
Busc - comb (for a stock)
Cale - wedge (in a double rifle)
Canon - barrel
Canons juxtaposs - double barrel; SxS
Canon ray - rifled barrel
Canons superposs - over/under
Canon lisse - smoothbore
Carabine - rifle, carbine
Cartouches plombs - shot shell
Cartouches balle - slug shell
Chambre - chamber
Chien - hammer
Chiens extrieurs - external hammers
Choke - Choke
Demi choke - Modified choke
Cl - key
Cl douverture - top lever key
Cl sur le dessus - top lever key
Coffret - case (to put the gun in)
Corne - horn
Crochet - barrel hook
Crochet du canon - same
Crochet encastr - dovetailed lump
Crosse - stock
Crosse anglais - English stock
Crosse joue - stock with cheek piece
Crosse demi-pistolet - round knob grip stock
Crosse pistolet - pistol grip stock
Crossier - stock maker
Culasse - gun bolt
Damasquin - damascened
Demi-coquelle - half cock (for hammer guns) (???)
Demi-bloc - chopper lump
Dtente - trigger
Devant - forearm
Double detente - double tigger
double verrou de fermeture - double locking bolt
Etui - brass (for a cartridge)
bne - ebony
jecteurs automatiques - automatic ejectors
Extracteur - extractors
preuves - proof
Express (also double "express") - double rifle
Banc dpreuves - proof house
Double-epreuves - double proofed
tranglement - choke
(arme a )Feu central- centerfire (gun)
Fermeture en "T" - Jones underlever
Forage - bore (barrel)
Fusil - firearm (normally a shotgun except but also military rifle)
Fusil de chasse - shotgun
Fusil a percussion - muzzle loading, percussion rifle
Fusil a piston - percussion gun
Gachette - sear
Gachette interruptrice - safety sear
Graveur - Engraver
Gravure - engraving
Grenadire - sling swivel
Goupille - round pin
Goupille de charnire - hinge pin
Guidon - front sight
Hausse - back sight
Illisible - illegible
Jaspage - case coloring
Juxtapos - side-by-side
Levier - Lever
Levier de scurit (ou scurit) - safety lever
Longuesse - forestock
Longuesse a poussoir - pushrod (forestock with pushrod release)
Massif - water table..holds the barrel locks I don't agree with that
Moul(e) - molded
Noix - Hammer axis
Or - gold
Or fin - fine gold
Oreille - Sideclip
Percussion centrale - center percussion, center fire (hammer gun)
Percuteur - Firing pin
Piece d'armurerie - piece of firearms manufacture ???
Piston (Fusil a-) - percussion gun
Planchette - back sight folding leaf
Plaque de couche - butt plate
Platine - side lock
Platine Holland - front lock as made by H&H or others
Platine arrires - back locks
Platine avant - front locks
Platine encastre - embedded locks
Platine dmontable - hand removable sidelock
plomb - lead shot
Poinon - stamp
Poinons rglementaires - legal stamps
Poinons d'preuve - proof marks
(Crosse) Ponce huile - sanded with oil finish (stock)
Pontet - trigger guard
Pontet volute - trigger guard with volute in front
Porte - range (as in shooting distance)
Poudre sans fume - smokeless powder
Poudre Pyroxyle - smokeless powder (French)
Poudre Vive - smokeless Powder ( Belgian)
Poussoir - push rod (forestock)
Quadrillage - Checkering
Quadrilleur - Checkerer
Rallonge de crosse - stock extension
Ray chargement par la bouche - muzzle loader rifle
Ressort - spring
Ressort boudin - Coil spring
Sret - safety
Stphanois - from Saint-Etienne region
Tirette - action rod
Tonnerre - action ball
Type anglais - English style
Verrou - lock
Verrou de fermeture - closing lock
Verrou vertical - rising bite
Verrouillage - locking system
Vis - Screw