Protype done, haven't tested with live ammo...but it seems like it'll work. Pictures are posted
my site.
I'll try to describe the system..
First, the frames. There are two (one for each side). I cut 4 25" long 1x2 and 6 2" long 1x2. Put two of the 25" peices with another peice of 1x2 and 2 peices of cardboard between them (making a 1x2 sandwich). The 2 peices are secured together (and spaced to allow a 1x2 to slide between them with a little extra) with 3 2" peices of 1x2, spaced however ;-). Then make another. These are simply tracks that mount on the inside of a standard 1x2 upright (whatever fits).
The two tracks are held together for the most part by a cardboard target mounted to the front, I also put in a 1x2 crossbar at the bottom for extra stability.
Then, 2 screw hooks are mounted to the inside of the frame (either side) just below (perhaps 1", depends on the balloons) of the -0 center zone.
My original plan for attaching the strings to the uprights was to simply split the top, seems that'd require cutting a notch to wedge them in.. so instead I just screwed a couple of 1" drywall screws to the top of the upright on the rear of course.
Now, the trigger mechanism.. Take a 4"x4" (roughly, no measuring required really) peice of old target cardboard. Fold at 90degree angle. On one part of it, make a 3/4" hole (square, round, doesn't matter) and the other, attach a peice of cord. The idea here is the bent up part of the cardboard rests against the back of the target which keeps the balloon pressed against the target.
Tie a balloon to a length of twine, thread it through the 3/4" hole making sure to get the entire balloon 'nipple' on the side with the string. Take each string (one on either side of trigger), hook through the screw hooks and pull the target up on the uprights..make sure the balloon is centered and wind the twine around the screws at the top...
Voila, bang in COM pops balloon, releasing strings and the target will fall!
The only problems I've come up with so far, I need to adjust the hooks to minimize the possibility of hanging up on the cardboard trigger... and my prototype needs more 'slop space' to account for warped 1x2's.. Oh, and my target frame stands (designed by me for quick and cheap construction), don't support the uprights horizontally (figuring they'll always have a cardboard target keeping things straight).