How to shoot big weapons without crouching

How do i shoot big weapons that need crouching (like sniper cannon) without actually crouching? I know this is possible because i saw someone else doing it during pvp (he was shooting me with sniper cannons while he was in the air). But i just cant get it working.

I don't think that is possible. He might have crouched in the air but I haven't actually played in a while so I forget if that's possible. It could've looked like he was in the air unless he was clearly in the air. If both are not possible it could be he was hacking but you never know. As of now I think you have to crouch.