One would think that such a juicy story would be well known and reported in contemporary newspapers and later books on Franco-German relations during that period. But the only source for it seems to be an article published in 2007 in Guns Magazine, an American magazine about, well, guns.

Soon after the Lebel reached the French troops in early 1887, a French deserter with the name of Schnabele hopped across the border with a new Lebel and a handful of smokeless ammunition. Arriving in Germany, Schnabele set a price of 20,000 Marks for the purloined booty, but there was little official commitment until Reich Chancellor, Prince Bismarck, himself saw the significance of Schnabele's loot. Communicating the fact Germany was now armed with inferior weapons to the Prussian War Minister, Bismarck got things moving. War Minister von Schellendorf promptly referred the problem to the Rifle Testing Commission at the Spandau arsenal and within a year, the Commission produced both a new rifle and a new smokeless cartridge.

The problem here is that there was indeed a Frenchman called Guillaume Schnæbelé who "hopped across" the border next to the German-annexed Moselle on 20 April 1887, but he did not do that willingly. Schnæbelé, an Alsatian who had chosen to remain French after 1871, was going to visit an Alsatian-German colleague on the German side (it was a trap!) when he was arrested by German officials. He managed to escape briefly to the French side but his pursuers recaptured him and took him to Germany, where he was jailed under the accusation of spying. This was not totally false: Schnæbelé, a police officer (commissaire) working for the French railways, was tasked with monitoring the border. The incident soon turned into a potential casus belli, with warmongers like the General Boulanger calling for war. Cooler heads - notably Bismarck - prevailed, and Schnæbelé was freed on 30 April and returned in France as a hero.

So Schnæbelé was neither a soldier nor a traitor (not from a French point of view anyway) and there is no mention in credible sources about a French soldier defecting to Germany with his newfangled Fusil Modèle 1886 aka "Fusil Lebel".

Sources

• Bodinson, Holt. “Germany’s Gewehr 88 Commission Rifle: It’s Far a Better Rifle than Most Give It Credit For.” Guns Magazine, November 1, 2007. https://www.thefreelibrary.com/Germany%27s+Gewehr+88+Commission+rifle%3a+it%27s+far+a+better+rifle+than...-a0169021346.
• Brose, Eric Dorn. The Kaiser’s Army: The Politics of Military Technology in Germany During the Machine Age, 1870-1918. USA: Oxford University Press, 2001.
• Joly, Bertrand. “La France et la Revanche (1871-1914).” Revue d’Histoire Moderne & Contemporaine 46, no. 2 (1999): 325–47. https://doi.org/10.3406/rhmc.1999.1965.
• Mafart, Jean. “Schnaebelé (affaire).” In Dictionnaire du renseignement. Perrin, 2018. https://www.cairn.info/dictionnaire-du-renseignement--9782262070564-page-704.htm.