SHILKAS FOREVER: There are more than 7,000 Russian-built ZSU-23/4 self-propelled anti-aircraft guns in service with 30 countries, making it the most widely used anti-aircraft gun in the world. This is a huge potential market for upgrade kits, and several are available. Belarus, the Netherlands, and Ukraine are offering various packages of new radars, sighting systems, and fire controls. Russia's Promexport is offering an upgrade that includes replacing the four 23mm barrels with two 30mm barrels and installing a new day/night electro-optical sight, this equipment being the same as is used on Russia's new 2S6 Tanguska anti-aircraft tank. State Enterprise Ulyanovsk, which designed the Shilka and built most of them, is offering two upgrades. One replaces the radar and optical systems for improved targeting. The other adds four SA-16 or SA-18 missiles on two separate firing posts on the rear corners of the turret. These Igla missiles, being fire-and-forget, provide the ability to engage targets at longer range without the need to divert the gunner from close-in targets. The missile pods are designed and produced by Kolomna-KBM, which is responsible for all of Russia's shoulder-fired anti-aircraft missiles.--Stephen V Cole