This is a design which has the following design criteria:
Smallestest, Lightest, Fastest, Cheapest, Bestest.
Mostly the design is just a test.
I just had a spare half hour today so thought i would edit a wiki page. Check my forum posts 
I did some volumetric calculations and it looks like the entire extruder should weight about 14g for the four filament version (excluding motor and electronics weight)
Hopefully the prototype will start being constructed in three weeks?
Bill of materials
|4||M4 x 25mm bolts|
|25mm||25mm square 3mm wall alloy box section|
|20mm||20mm diameter brass|
|4||M3 x 20 screws|
|6||10R resistors 3W|
|a few ml||fire cement|
|50 mm||2mm heat shrink sleeving|
Lathe drills etc..