How legal are tension springs?

[andre_ribeiro97]

We were thinking that tension springs would put more force out per inch of movement than surgical or latex tubing. After searching the rules multiple times and checking the list of legal and illegal parts, tension springs are not mentioned. In order for our initial shooter idea to work in the least amount of space, however, we need to use them. Anybody know if they're legal, or if theres another springy material that would work just as well?

Thank you on behalf of Team 2191 Flux Core.