There have been a few different versions of the Auto 9 by KSC. These are of course based off the KSC M93r I and II. The first release Auto 9 was called just that, the Auto 9. It featured full and semi auto fire modes. That gun eneded production and was replaced with the Auto 9C in the early 2000's. The original Auto 9C featured semi, full auto, and added the 3 round burst feature. That gun then also went out of production. Last year KSC brought back the Auto 9C but it lacked the 3 round burst feature of the original Auto 9C. The box for both C versions are the same, blue and white. If you want the burst feature in an Auto 9, and who doesn't, grab the original release Auto 9C or the newest version and add the burst kit yourself. Most online airsoft stores should carry the burst kit for about $10-15. They do tend to sell out quick though. If you see one, grab it.
Now you do have a few other options for an Auto 9 (if money is no object). In the first movie they used a real steel Beretta M93r machine pistol. However due to legal and import issues for the second and third movies they used an MGC PFC Auto 9. A PFC (Plug Fire Cartridge) was basically a glorified cap gun. It used a multi piece brass cartridge that a special cap was placed into. This allowed the gun to fire with a bit of smoke, flash, and eject the cartridge just like a real gun. PFC guns were all the rage until airsoft took over the replica gun market in Japan. These replicas, and they made many many different PFC guns, command ridiculous collector premiums. I had quiet a few myself in the late 80's early 90's but sold them all.
There is also the Maruzen Auto 9. This is an airsoft gun as well but does not have any blowback action like the KSC. It has a fixed slide. But it is full size and may be cheaper, if you can find one.
Here are a few videos I grabbed for you.
KSC Auto 9c airsoft- newest version
https://www.youtube.com/watch?v=7EHGpUMyaQQ
Original KSC Auto 9c with burst feature
https://www.youtube.com/watch?v=_jLeO52dw0o
Maruzen Auto 9 airsoft.
https://www.youtube.com/watch?v=tD2jn21r3QQ
MGC PFC Auto 9
https://www.youtube.com/watch?v=l79-2bxeGRw