I'm not sure I understand you correctly: is it the shape of the 30mm rounds that tells you they are armour piercing or is it just the color of the tip ?
Is there some special advantage in using armour piercing ammunition as a CIWS ?
I always thought that shrapnel from exploding shells would be good enough to destroy incoming missiles.
It’s a trade-off between hit probability (HP) and kill probability(KP).
Proximity fused rounds increase HP, but each hit will have a much smaller KP compared to a direct hit from an APDS round.
As a general rule, airbursts are more effective against subsonics that have a longer flight time, so greater chance a small amount of damage might be enough to cause it to loose lock or crash into the sea. APDS are more effective against supersonics since the speeds those travel at means you don’t have time for it to meander off course gradually, but instead need to put maximum force on it to drastically change its course in the shortest possible time to prevent getting hit by supersonic flying debris even if you do kill it.
The Chinese are more likely to favour APDS since the monstrous fire rate of their CISW greatly offset the traditional downside of low HP with APDS since they can throw out so many more rounds in the same time compared to other gun CIWS.
