I tested following AA guns:

5.1"/45 SD-2
3.9"/50 "Minizini"
6" B-38 MK-9 Pattern 1938

(Since these seem to be the only useful AA guns)

The 6" B-38 MK-9 Pattern 1938 gun has a very high reload time.. which makes this AA
gun actually not very useful.. The only use I found about this gun is to use it as a
dual-purpose gun in Soviet BB6.. You can shoot some approaching lower tier ships
with these guns in order to save main ammunition... except this it is only good for
scout shooting... But does all worth to have so much weight in the ship ? They add
pretty much weight to the ship.. And with 120 level crew the bb6 almost hits the full
displacement if you install these guns. So they seem to be fantasy guns for BB6..
their only purpose...

5.1"/45 SD-2 This gun has 2.76 sec natural reload time... Even the reload cap hit..
the reload stays pretty bad for an AA gun.. it's range is good but not excellent...
However although the range is good, shell velocity is low... shells fly this good range
like fat bottomed ladies and combined with a below average reload speed these aa
dont pass the class in my opinion.. the damage is disappointing... Again the best use
of these guns is scout shooting (they are better for fighter shooting in comparison to
6'') Forget about fending off the bombers approaching to your ship with these guns.

3.9"/50 "Minizini" Good reload time (1.56 sec) , very very bad damage (you need to
hit a plane often 5-6 times with these in order to shoot them down)... Their reload
speed makes these guns best for bomber killing... however low damage of these
guns renders even this useless. For scout shooting they are not good due to limited
range (SD-2 are better). Shells have low speed, they seem like they don't like to be
fired :) they complain in the sky :).. Using these guns as dual-purpose is fun.. they
are like german dual purpose guns but worse than them.

Compared UK and SN AA

UK is said to have the worst AA, but I think now the things are changed..

Logic of AA is Fighter,Scout and Bomber shooting...
Strategically Bomber and Scout shooting are more important in my opinion...
If you are alone, crippled and need repairs... having AA is very important against
bombers who want to finish you off... The soviet AA fails in this...

I tested UK pompoms against bombers even with a very bad reload... And they seem
to perform excellent against approaching bombers. They completely clean the sky
from bombers...