Cant help you with the v-belts Im afraid, Ive always used timing belts (& pulleys) and never really had any problems (I prefer them to chains as they do allow some slack - the belts arent rock solid - and they are usually lighter). It would be good to know what kind of set-up youre looking at running (i.e. what motor and reduction etc) for the weapon.

Had a look on your site, looks like youve been busy with robot stuff for a long while now. Good luck with it, and please ask us if youve any queries at all.

Ewan