Hi Bayern,
thank you for helping to date the photo correctly. However, there is a problem, as the reverse of the printed Carte de Visit shows that the studio was awarded a price at the 1889 Paris World Exhibition which lasted till Oct. 1889. The address 21, Rue Chabriére, instead of the older/first address of the studio 15, Rue Chabriére, also points towards the 1890s. The photo was clearly printed from the orignal negative (so no reproduction with internegative of the time).
So: three possible explanations come to mind (maybe there are more).
1) The rifle was used longer in the legion
2) The photo was ordered at least three years after it was taken (because he ran out of copies) and the old negative was still available (which was uncommon after that long a time, but certainly possible.).
3) I know of German photo studios which specialized in photos for the soldiers in the local garison which had uniforms and props ready for their customers. They were not always up to date, of course. So you find soldiers on photos in uniforms and with props that were really past their times.
GreyC