Originally I planned on making a more complete Automag tech page, with information about its parts. However, since there's already a TON of info available for Mags, I decided to concentrate on one specific aspect of their operation...the pneumag.
Pneumag Conversion: Introduction
Internet guys have coined the term pneumag which refers to Automags equipped with a pneumatic trigger valve instead of a direct mechanical trigger. With regular Mags, the trigger pushes on the sear directly, and results in the distinctive trigger pull with which all Mag owners can relate. A pneumag, on the other hand, uses a small trigger valve to actuate a pneumatic piston, which pushes on the sear instead of the trigger. The reasoning for this is to reduce the force needed to actuate the sear, which normally can be very high with a regular mech Mag, but when equipped with pneumag parts the force can be decreased to virtually nothing (just enough to return the trigger after being pulled).
With regards to an E-Mag or X-Mag, the reduced actuation force is still a factor, despite the fact that electronic Mags use a solenoid to move the sear. This means, instead of using the E/X-Mag's whopping pancake solenoid and special 18 volt battery pack, you can use a more conventional custom-installed circuit board running off a 9v battery combined with a small pneumatic solenoid to actuate the sear.
Pneumag conversions are extremely popular, but do take some tinkering. The modifications aren't drop-in so you'll need to permanently modify some parts. There might be some custom installers out there willing to perform the mod for a price, but I don't know off the top of my head. Search around and you may be able to find somebody if you don't wish to mess with the parts on your own (see below links section).
Pneumag Development Files:
Firstly, here are some animations that show the stock parts (first) and one possible arrangement for a pneumag frame (second).
Below are a couple instructional how-to files that circulated a few months/years ago. They're both in .pdf format and will need Acrobat Reader to open.
Below are some pictures that circulated a while back showing the insides and outsides of pneumags. I didn't take any of these pictures nor do I know from where any of them came (if one is yours let me know and I'll post credit). The first row of pictures shows the external profile, to help show possible locations for the LPR and such.
This next row of pictures shows the internal arrangement on a few different mech pneumags. Most of them use the Fabco trigger valve but that's not required.
Not many people have done electronic pneumag mods, likely due to the complexity of the install and integration of the electronics, but I have a couple pictures of some finished ones in case you wish to dare...
The first two pictures are of BigEvil's conversion, which uses a small SMC normally-closed solenoid and an NME board to control it. The third picture (owner unknown) uses the same SMC solenoid but a Morlock board instead. The fourth picture is a conversion I performed using an Ion board with [yet again] the same SMC solenoid. There are other solenoids that can be used, such as the small MAC 33 or 43 series valves, plus some Pneutronics, Numatics, or Parker alternatives, however none of those seem to be retailed individually so you'll have to search around for them (or have them custom made for you by the respective manufacturer).
Automags.org forums - Lots and lots of information about all types of Mags, including pneumags. There are many threads dedicated to the pneumag conversion here, with all sorts of information and experiences.
Airsoldier.com - Retails many components that can be used in either a mech pneumag or an electronic pneumag.
Logic Paintball - Maker of the UEF frame for mags (and other parts)
Ydna's EP Mag - This page has information about the electronic pneumag conversion I performed in 07; this can be used as an example for your own work.