When a tank or an APC is zeroed to less than 5 meters, in some cases they will shoot up in to the sky or even backwards. Some of them will "just" shoot about 30 degrees off the barrel line.
For example the Rhino will shoot almost literally 180 degrees back. Also some tanks like the Slammer will shoot into the sky even though the barrel is parallel to the ground.
The trick works on every APC and tank.