AFAIK they're two cannons, the big one for HE rounds and a small one for AP. If this is not true can someone please correct me. I didn't vote.

Nope.

From Army Guide, describing the GI-30 gun that´s supposed to be mounted in this turret:

"The GI-30 is a unique externally driven electro-mechanical cannon, utilising a drum-cam to cycle the breech to chamber rounds and extract spent cartridges, thus minimising gasses in the confines of a turret and facilitate for controlled firing rate. DLS designed the GI-30 to fire link-less 30mm x 173 ammunition through a dual feeder – a world-first for this kind of weapon.

As a single-shot weapon, it is also unique for its sniper mode of operation. It is fired from a closed breech position, which offers more accurate fire due to no movement taking place immediately before the round is fired.

Another unique feature is that of ‘next round selection’. Having two feeders, with general ammunition on the right-hand and specialist types in the other feeder, the weapon allows the operator to select a specific round, without first firing off or ejecting an unwanted round."

The 40mm CTWS can -also- switch ammunition freely from one round to the other, without ejecting or firing a round of the other type. Both guns have dual feed systems, and have two types of ammo ready at all times. Both weapons systems (of which the latter weapon seems to be modelled) work as shown in the game.

Again, this works as intended and is not a bug.

http://i.imgur.com/au8SiSW.jpg

Silly me, the third one is the camera. :P

InstaGoat - many modern vehicles have such system,even old soviet BMP2 created in 70ths years.This isn't a bug,just change ammo from menu is much habitual to me.

Mass closing ancient tickets with no activity for > 12 months; assume fixed or too trivial.

If this issue is still relevant in current dev build, please re-post.