I have built a tank platform(photo attached) and I want it to shoot a projectile on top. Crossbow,nerfgun, catapult…? I want all ideas even if it doesn’t shoot something.
In the photo I can move the cortex and other stuff of the top
It depends on what you want on top. If you want a blaster that big, you’ll need a bigger tank. Projectile type is up to you, and size is dependent on that. I don’t quite understand what you’re asking specifically.
I think the OP was asking for recommendations on projectile. Dimensions of the base would help, but I would recommend a flamethrower some sort of squishy ball, shot by a flywheel. You can pretty easily find designs, just google “vex nothing but net flywheel”.
Water gun.
Video camera. Robot’s eye view touring a competition.
Motion tracking software for autonomous firing. That will be sick!
At worlds in either Clean Sweep or Round Up, someone made a color sensing nerf gun that was supposed to shoot people wearing red shirts. The carpet was red.
Lol rip carpet. I don’t think they thought it all out 
A classic ** REALLY! ** moment.
You could build your own basic ping pong ball shooter pretty easily with VEX materials.
One of my teams mounted a nerf shooter on the spare base that they had made. They built a little cradle to hold it. Trigger pulls was done by tandoff mounted on a gear that rotated and pushed the trigger back.
Make the mount on top of a turret base. Turrets are fun.