Oh it'll work if you're shooting single shot (in full auto it becomes ineffective after the first shot, or if you can pull the trigger really fast it'll also be ineffective). But the CHANCE that it could potentially break is basically the reason why I (and likely other people) are not comfortable with them. If you were rock climbing, would you really want your life to depend on hoping the other persons carabiner isn't some cheap one and snaps? (Not a good analogy but I hope you can at least try to understand what I'm saying).
Also yes, as long as you're within the field limits you can use whatever you want. ie. as long as you can blow less than 1.48 Joules from a straw, there's no reason why you couldn't potentially use a straw and BB's. You could show up to a game with 400 FPS limit with a sniper rifle, shotugn, pistol, or revolver shooting less than 400 FPS and you're good to go. It's only when you start to go above those limits that sniper Minimum Engagement Distances come into effect (normally it'll be like 15-20 feet MED for guns shooting 400 FPS).
Also I'm not sure how it's usually done but usually I would say that you should chrono with .20's just to put everyone on the same standard but you could potentially chrono with the BB's you're going to use and set the chrono to measure energy based on the weight you input into the unit (or if someone's already done the math to figure out what weight will give what energy rating do it that way).
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE