[yulzari]

Just seen this thread. There are any number of Gras converted to shotguns and with cut back stocks for sale here in France at @ 165$US. Not accurate stocks for a Carabine de Calvalrie but one of these would get you up and running by swapping out the shotgun action for yours. I use a 20 gauge example as I can't afford a proper Gras.

 

[yulzari]

JPS. Full length stocks are exceedingly rare and more expensive., Both because civilian buyers wanted a civilian shotgun look and because others wanting to restock rifles used them in the past.