Tony M Feb 29, 2016:

@ Asker I think that's the wrong kind of 'volt' — here, it is a 'bolt' as in a cross-bow (arbelette); i.e. the projectile, as Florence says.

Cécile Gaultier (asker) Feb 29, 2016:

Thank you very much, I originally thought of "culasse libre" but I am not sure I am completely hitting the point. Thanks anyways for your help!

Tony M Feb 29, 2016:

@ Asker I would imagine (but it is only a vague hunch!) this is referring to a 'bolt' that is not retained in the 'gun' in any way, so it is 'free' to 'fly' as it were. You may have to research some manufacturers' literature for a more accurate explanation. Then by comparing with FR manufacturers' information, you may strike lucky!