Automag body is 6.25" long, 1.125" OD, 1" ID, feed tube is 0.875" OD, 0.75" ID

Front and rear frame screws are threaded 10-32

Frame hole spacing is the same as used on the Autococker and Sheridan pumps - 4.125"

Doc Nickel had mentioned that the firing chamber of the Mag has a volume of approximately 1.1 in^3 at a pressure of 375-425 psi.
 

O-ring that AGD sticks onto powerfeed bodies is a dash 212 buna-n

Pump kit:
The Automag pump kit hasn't been produced since 1997, according to AGD.  However, interest in the pump kit seems to have been picking up recently, with pump kits fetching over $100.  The idea behind the pump kit is a simple one - the standard blue bolt bumper is replaced with a wave spring and black bumper.  When the trigger is pulled, the bolt flies forward, fires the ball and then the bolt spring pushes the bolt back into place.  However, the wave spring keeps the bolt from returning fully (causing what is known as 'bolt stick').  A pump rod lying in a groove in the rail is pushed backward to return the bolt fully to the rear.

Here are the two pages showing the parts and assembly instructions for the pump kit (taken from AGD's site): pump.pdf

The black bumper is 1/16" thick, 3/4" OD and approximately 0.465" ID.

The wave spring is about 0.93" OD, 0.737" ID with a free height of approximately 0.2" and a compressed (solid) height of approximately 0.04".  The spring is made of three turns of 0.094" wide, 0.012" thick strip.  The closest match from Smalley appears to be the C100-M1, with specs here: http://www.smalley.com/SpringProperties.asp?PartNbr=C100-M1

Note that you must have a body with an enlarged sear slot to be able to use the pump kit. According to the above .pdf document, all bodies made after June 1994 have the enlarged sear slot. However, recently made guns appear to have a narrow sear slot - AGD must have returned to the old style slot after discontinuing the pump kit. [Anyone with further information, please contact me] This enlarged sear slot also has a small bevel by one corner where the pump rod passes under.

Kolea Hara provided this photo which shows the difference between a standard body and one with the enlarged slot for the pump rod:

Thanks go to Connieandsimon@sprint.ca and the Toronto Recreational Paintball Club for these two pictures of the pump kit:

Here's a scan from an issue of APG that shows how the kit gets installed:

Many thanks to Keith for these dimensions of the body slot and rail slot. He says "I was given the files by someone else. I cant say for sure they are correct so they are use at your own risk."

He writes:
The curves aren't exact, I guesstimated on the thicknesses (based on other frames), and it's missing the center cutout, but the profile is pretty accurate. It's perfect if you need to add for looks or check for alignment issues.