The Chassepot (pronounced /ˈʃæspoʊ/ SHAS-poh), (French pronunciation: [ʃas.po]) officially known as Fusil modèle 1866, was a bolt-action military breechloading rifle.

Many were also manufactured under contract in England (the "Potts et Hunts" Chassepots delivered to the French Navy), in Belgium (Liege), and in Italy at Brescia (by Glisenti).

Gun manufacturers in England and Austria also produced Chassepot rifles to support the French war effort.

The Josef und Franz Werndl & Co. in Steyr, Austria delivered 12,000 Chassepot carbines and 100,000 parts to France in 1871.

In the following year it made its first appearance at the Battle of Mentana on 3 November 1867, where it inflicted severe losses upon Giuseppe Garibaldi's troops.

In 1872 the Empire of Brazil purchased 8631 Chassepots; after being faced with a possible war involving Argentine claims over Paraguay.

Amongst the technical features of interest introduced in 1866 on the Chassepot rifle was the method of obturation of the bolt by a segmented rubber ring which expanded under gas pressure and thus sealed the breech when the shot was fired.

The older Dreyse needle gun and its cartridge had been designed to minimize those problems but to the detriment of its ballistic properties.

To correct this problem the Chassepot was replaced in 1874 by the Gras rifle which used a centerfire drawn brass metallic cartridge.

Nearly all rifles of the older Chassepot model (Mle 1866) remaining in store were eventually converted to take the 11 mm Gras metallic cartridge ammunition (fusil Modèle 1866/74).

[5][17] Large numbers of these captured Chassepot rifles were shortened and converted to 11 mm Mauser metallic cartridge.