All i know us there is No way in hell i'm going near the side that has the trebuchet if i'm not on it 

go ahead call me a pansy see if i care

I really like your Crossbow Idea, it can give the impression of  handling a real siege weapon, but less lethal. It seems to me that the pouch would be the biggest obstical. The ballons outerwall would be scraping against the wood while lanching, unless the pouch rides the channel. Wouldn't that cause it to burst before actual flight?

Interesting....

Unsapien

I'm considering what types of tactics and etiquette might be appropriate for slinging in groups. I can't find much on slinging formations.

I was considering starting a new topic on this very subject, but I haven't found the right words. I think it would be a worthy topic, but am testing it here first.

Obviously I am interested in this because it would add to my WB slinging war, but in a larger sense it would be interesting to see what was practical for a slinging corps.

-Unsapien

I Agree, I was actually pondering the same thing!
That said you woud have to determine how wide the bars need to be, and what gadge.

That also offers another problem, mounting the bars onto the cross-bow.

Hmmm

-Unsapien

then WD-4o to remove all remaining resistance .
maybe not for a waterballoon war though...

Sorry it's taken awhile to get post.
Here are three things that can be done to prevent the water balloon from popping. Much sanding and then applying paint and then airbrush a coat of clear automobile polyurethane. Gives it almost a glass like surface. More elaborate than needed and not worthwhile in my opinion.

Another thing that can be done is to use a larger double strung pouch that will wrap around the top and bottom of the wb as well as the back. It will then cradle the water balloon, exactly like sling shots do. This style pouch, I believe, should be used no matter what, unless it catches on the frame. Which I don't think is possible unless you poorly make the frame.

And then a very different approach is to design the frame like a prodd crossbow and use the pouch mentioned above. This would then be very similar to sling shots and worthwhile.

The cost/benefit ratio is not entirely rational if you were to buy a model crossbow like mine to use only in wb fights.  It's more of a novelty item, I suppose.
I have been thinking of a more practical design to launch water balloons very far distances that would be very useful in a wb fight and simply cool looking. I’ll get back to you guys once I have some initial plans drawn up, whenever that will be.