I found shilka in game really underpowered. For it cost they have really bad perfomance. Or they have too much cost.
Let's see. All AA guns have poor optics, 0 AP power and 1 HE power. BTW interesting how 20mm and 57mm have same damage.
Flakpanzer M24A1. Two 40mm guns. Accuracy 3. Cost 15. You all know they very good and very efficient even if used on ground targets and easily stun tanks.
ZSU-57-2. Two 57mm guns. Accuracy 2. Cost 20. Not as efficient as M24 but still do good job. Higher caliber only increase cost without increase of damage.
M163 Vulcan. 20mm gun. Accuracy 4/5/8. Cost 25/30/45. Don't know why system with optical fire control got such accuracy. Kills very fast, almost instantly stun anything that can't kill.
M53/59 praga. Two 30mm guns. Accuracy 5/7. Cost 20/30. Again optical fire control and high accuracy without reason. Price/efficiency winner.
Shilka ZSU-23-4. Four 23mm guns. Accuracy 3/4/5 for modifications. Cost 25/30/40. Search radar and radar fire control but only 3 accuracy? With upgrades accuracy don't get enough boost unlike other AA guns. Low effiency against ground targets, 4 shilka can fire soft target for whole minute and it won't be even worried. Remember, all AA guns have 0AP/1HE damage. High price, not better than basic 57-2. Maybe there is a bug, 4 barrels counted only as one? I don't think shilka really cost 25 points for such poor efficiency. 10 points, 15 max.
Here some tests http://youtu.be/FTl2wauvt5k