The simpler solution would be to make an enclosed arrow guide on top. Look how Jörg has done this with some of his rubber launchers.
These "flipper" designs have individual arms which are individually powered, so even slight differences in construction lead to the arms not moving in perfect unison. Thats just a quirk the designer has to overcome by additional means. somehow there has to be a connection/synchronisation of the arms.
Yours looks very similar to robert van Houses design. http://www.freepatentsonline.com/4169456.pdf
. Look what he has done to synchronize the arms. some additional connecting bars which meet at the stock and are riding/guided in a groove there would do the trick.