Main gun

[slim and shady]

well the title says it all, I need a main gun for my tank as armor wars are comming up and we need tanks so what I am looking for is safe cheep ideas for a main gun for the tank. I cant stress enough that SAFTY is the utmost importance although it wont be a direct fire weapon (shot at an arched angle, but noot quite as much as 45*) in the chance it does I dont want to hurt anyone. There are specifics that I am looking for....

1-Has to look like the main gun for a tank
2-cannot be barrel loaded (I dont think I wanna stop the tank have my gunner crawl out to jam a shell down ect in the middle of combat)
3- Has to shoot a fragmentation grenade (water ballon in a sabot)
4-I mentioned SAFTY, but I will mention it again.
5-no PVC chamber designs that are ment to store air pressure within a PVC chamber, that violates rule 4.

So throw me your ideas sketches whatever you can think of. I am open to ideas. I thought of trying to get my EGL47 to work but it doesnt look like a main gun. I dont want to have anything real bulky as I plan on having my main gun fixed between 2 MR2 (Eye versions) markers in a 360* turret.

If you dont have anything positive or helpful to add to this thread please feel free to avoid it and post elswhere