UPDATE
Other AEG, my new project
https://www.thingiverse.com/thing:2916390

Warnings
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Project is big and consist of many details. So i will use this thing as central hub and post here links to blocks, like "lower part of gearbox" or "frontset". STL's published here is upper part of gearbox.
Warning! You will need engineering skills, deep understanding of airsoft gun internals and really long time with file to ensure smooth operating of gun. I took for me about 1.5 years to design and create working aeg.
Contents
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Upper gearbox - this part
Lower gearbox - http://www.thingiverse.com/thing:461970
Grip - http://www.thingiverse.com/thing:462057
Mag holder - http://www.thingiverse.com/thing:462840
Hop-up unit and front rail - http://www.thingiverse.com/thing:463221
Frontset - http://www.thingiverse.com/thing:470987
To be published:
Stock
Stock is removable and attaches to a 20-mm rail, so you already can make an aeg without stock, or attach any picatinny stock.
About
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I’ve started this project with inspiration of Scorpion EVO 3 A1, but tried to make TM-compatible parts gun as small, as it could be. Gun is really thin – about 32mm. Overall length is 431mm, barrel – 223mm. Height from bottom of pistol grip to top of upper rail – 173mm.

Gun uses standard TM-compatible:
Gears, short-type motor, 223 mm 6.03 barrel, hop-up rubber, tappet plate (v3), nozzle (ak), cylinder, cylinder head(o-ring and nozzle guide only), piston with piston head, spring, spring guide (v2). All other parts are 3d-printed. Cylinder head self-designed as well as gearbox. Pistol grip holds motor, in bottom of pistol grip installed adjustment screw. Hop-up is adjustable and firmly attached to upper part of gearbox to ensure proper co-axial position and enforce cylinder head. In handgrip placed electronic key and 7.4v Li-Po accumulator. Gun uses airsoft MP5 mags.
Rear end of upper gearbox is quick-detachable for easy spring change. Construction of piston guides allows to change piston without gearbox disassembly. Lower part of gearbox holds gears and antireverse latch. Gun has upper and lower Picatinny rails for attachments.
Right now I use 80 meter per second spring, which outputs 78 meter per second with .25g BB’s.
Initially, I’ve planned using 3d printed parts only for prototyping, but ABS seems to be sturdy enough, at least for cqb gun. Until now I’ve shot about 2000 bb’s and plan to go with this gun on local games for testing.