The French adopted the Gras as their first mass-issued metallic cartridge rifle in 1874, replacing the needlefire 1866 Chassepot. Quite a lot of Gras rifles were manufactured, and they became a second-line rifle when the 1886 Lebel was introduced with brand-new smokeless powder and its smallbore 8mm projectile. When it became clear that the quick and decisive war against Germany was truly turning into the Great War, France began looking for ways to increase the number of modern Lebel rifles it could supply to the front.
One option that was used was to take Gras rifles from inventory and rebarrel them for the 8mm Lebel cartridge (which was based on the Gras casehead anyway). These could be issued to troops who didn't really need a top-of-the-line rifle (like artillery crews, train and prison guards, etc). Then the Lebel rifles from those troops could be redirected to the front.
The rebarreling process was done by a number of contractors, using Lebel barrels already in mass production. The 11mm barrel from the Gras would be removed, and only the front 6 inches (150mm) or so kept. A Lebel barrel and rear sight would be mounted on the Gras receiver, and that front 6 inches of Gras barrel bored out to fit tightly over the muzzle of the new 8mm barrel. This allowed the original stock and nosecap to be used (the 8mm barrel being substantially smaller in diameter, and not fitting the stock and hardware by itself). It also allowed the original Gras bayonet to be fitted without modification, since the bayonet lug was also on that retained section of barrel.
These guns are of dubious safety to shoot, since the retain the single locking lug of the Gras, designed for only black powder pressures. However, this was deemed safe enough for the small amount of actual shooting they were expected to do.