Have one of these little rifles and it's a single shot. Brass trimmings,buttplate,triggerguard,barrel band. Slugged the bore and comes out around .458 or so. Has the flat circular spot on the face of the bolt which I assume if for a cartridge with a base shaped like the mauser "A" base. Tried a 45/70 case in the chamber. Fit the bolt face perfectly but wouldn't fully enter the chamber. So what do you do for shooting these little carbines?.
Frank

the bullet needed will be different if it is a "true" Gras model (1874 model) or a transformed Chassepot (1866-74 model), as the latter chamber was sleeved and can not take a full size bullet. (it should be noted in bold case on the right side of the receiver)

The "best" available oragan donor is the .348 Winchester, although a tad short, it allows for proper sized bullets to be seated and the cartridge chambered with the necessary mechanic play to let the neck expand.

If the receiver says: Mle1874 and M80, you have the easiest and safest version.

the 1880 Modification was to ensure a way out for the hot gases in case of head separation. Not really needed nowadays with modern cases.

The original ammo used a 0.450 bullet paper patched (undersized), 
A full length case loaded with a .458 grease grooved bullet will not chamber. (except if the chamber has been eroded at the throat or modified)

However, using a .348 Winchester case (fire formed), which is 3.5mm shorter, will allow for a classic 45-70 bullet to be used and ensure proper accuracy.

When using a full length case, you have to have a special bullet designed with a smaller base band (like ancient "stop ring" bullets) or shoot an undersized bullet made of pure lead (but there are risks of throat erosion then)

Happy shooting!