One thing that confuses me a bit is the ROF of modern AAA
 

• Phalanx with its 6000 RPM
• Goalkeeper with 4200 RPM
• Millennium with 1000 RPM (but revolver gun, so faster to spin up)
• Thales RapidFire with 200 RPM
• BAE 40/70 with 300 RPM

Both Millennium and 40mm CTA have Tungsten shrapnel rounds. So is 200 RPM actually enough? I am kinda wondering whether the SPAAG with 40mm CTA should not be a twin turret ...

You are mixing AA and CIWS.
These do not have the same targets in mind.

Phalanx and Goalkeeper also need a direct hit to be effective. Their high ROF is needed to fire the number of rounds to statistically ensure a hit in the shortest possible amount of time. Which is very important concidering the short engagement window.

Millenium allready fires conciderable slower but does not require a direct hit. Whith its larger shell this combines to a longer effective engagement range and needing less rounds to statistically ensure effect.
RapidFire and the 40/70 are classical AA systems. Not CIWS. Their effectiveness against fast and small targets like approaching missiles will be limited. RapidFire to my knowledge is not even advertised with the anti missile role.
A twin Fast Fourty using proximity fused or programmable fused ammo might be effective in the CIWS role but this is quite a large system and the ROF is still at the low end. Which is why there are guided shells for 40 mm and 57 mm guns. Which are supposed to increase hit propability to the point that the low ROF suffices against missiles.
I personally am not convinced that this is a sensible approach though. The cost per shot will go up significantly. Getting much closer to missile based systems like RAM. But the capabilities do not increase in the same way.
IMHO proximity or programmable fused ammo offers the best cost to effect ratio for guns currently. A 35 mm weapon can effectively cover the range until AA missiles become effective. While not being to large and heavy.

graylion said:

I am kinda wondering whether the SPAAG with 40mm CTA should not be a twin turret ...

That depends if you want your SPAAG should be C-RAM and CIWS capable or not. For anit aircraft, helicopter and drone duty a lower ROF if combined with proper ammo will suffice in most cases.
 

schnuersi said:

That depends if you want your SPAAG should be C-RAM and CIWS capable or not. For anit aircraft, helicopter and drone duty a lower ROF if combined with proper ammo will suffice in most cases.

Yeah. But one might need a quad for C-RAM ...

graylion said:

Yeah. But one might need a quad for C-RAM ...

I am not sure.
Typical C-RAM targets fly rather fast but on very predictable trajectories. I think the problem ist mostly with tracking and hitting.
On the other hand C-RAM targets tend to come in numbers. So for rapid engaging of multiple targets a high ROF might be required anyways.

The German Mantis C-RAM system is basically a variant of Millenium or Skyshield. SOP is to use two guns in a fire mission but this is for redundancy should something go wrong. One gun is technically sufficient.

yup, but that's 5 times the ROF of 40mm CTA