This brings us to our next issue<g>
It is probably unwise to go shoot an older unfamilar rifle that has not been headspaced and safety checked. I do not know if the french gun you purchased had that done first or not. Also, in a semi auto gun there are many more moving parts and potential disasterous problems that could occur.
I dont know if the MAS 49/56 is semi-auto, bolt, lever or otherwise action. The last post would lead me to believe semi-auto. Slam fires are potentially very dangerous and could occur from any number of worn or out of spec parts.
In semi-auto military rifles the firing pins are usually free floating. This means that when the bolt strips a cartrige from the magazine and chambers it the firing pin will contact the primer lightly. Soft primers or primers seated too high may be forced forward and ignited by the firing pin floating forward. This is a "slam fire". If the the bolt has not completely made it "into battery" when this occurs------> kaboom.
Other things can cause this, worn or rusty/sticky firing pins. Cosmoline goop inadequately cleaned from inside the bolt assembly, etc. etc.
Definately dont put a full magazine into a semi-auto gun and pull the trigger the first time anyways.
Consider having the gun safety checked if you can find a smith who knows anything about french MAS whatevers<g>