Seen at the beginning of the film when Athos emerges from the water, he wears two shotguns type weapons on his back which fan out into multi-firing crossbows. Later when they are betrayed by Milady, Aramis grabs them to defend against The Duke of Buckingham before succumbing to their drugging.
The expanding effect appears to have been purely computer generated, the weapon is solid wood with metal attachments and guilding down the body. It has been finished to look like fine wood and what looks like detailed metalwork down the crossbow arms are printed adhesive strips. The trigger is static and a piece appears to be missing from the barrel end. Concept art names this as a grappling gun.
