The Bud Orr Autococker is a closed bolt gun. That is to say, it fires a lot like a pump gun - with the bolt forward sealing the ball in the barrel. There are three stages of the firing mechanism. The first part of the trigger pull releases the hammer, firing the paintball. The second part is the end of the trigger pull which trips the 4-way valve,
sending gas (CO2/nitrogen/compressed air) to the front of the ram which pushes the rod/block/bolt back. This allows a new paintball to drop into the breech and pulls the hammer back. The third part is the trigger release which returns the 4-way, sends gas to the back of the ram, and closes the bolt. The difference between this and most other paintguns on the market is that the ball is fired from a stationary position in the barrel as opposed to being thrust forward into the barrel and hit with gas simultaneously. Many feel that this leads to a more accurate paintgun, and I'm inclined to agree. It also reduces blow-back (in properly timed Autocockers) to a small fraction of what is seen in most open-bolt designs.
I have recently found an excellent Autococker page with some very nice descriptions of how the Autococker works, and more importantly, some magnificent diagrams of the paintgun's internals. It can be found in Jack's Autococker Infosheet. I highly recommend it.
The Autococker is possibly the ultimate gun for the tinker-freak. If the marker's basic style and design agree with you, given sufficient funds, you can modify the cosmetics and performance to fit your tastes precisely . Because of this, people who are drawn to the Autococker are often those who want to try every modification and upgrade under the sun, and who want to tinker with it themselves to suck every last drop of potential performance out of it. This is probably a good thing since the gun is somewhat more complex and difficult to troubleshoot than most paintguns out there.
Some people report having no trouble with their Autocockers ever. They claim to just clean them off after every day of play and they're set for next weekend. A lot of these people know their gun inside and out, know how to head off problems before they occur, and can fix just about any problem that did (or have a close friend who fits these criteria). The gun can and will perform flawlessly if well maintained and cared for.
I have to admit, though, that the quality of aftermarket and stock Autocockers has improved measureably over the last few years. It is now possible to buy a new 'cocker and do nothing more than rinse it off, clean the bolt and barrel, and run a little oil through it to keep it running perfectly for the long term. It is still wise to get to know your 'gun well enough to fix it yourself, but I'd say that it is much less important than it used to be.
The Autococker is also one of the most personal paintguns made. I've heard one guy refer to the Autococker as a "personal billboard". One Automag looks pretty much like any other, but it's a rare pair of Autocockers that look the same once the owners have had them for a few months.