UPDATED ON 2-25-16... Added pictures, Field-tested it, added a GATE MOSFET, replaced the wiring and am using an11.1 lipo...
UPDATED ON 12-5-15... Added Internal before and after pictures, flat-hopped it, listed internal pieces, and updated the description, etc...
I was interested to get an AK variant AEG at some point and have done so with this gun. I wanted to keep everything as cheap as possible in case I did not like the feel, gearbox, or size of an AK, Which is why I purchased the CM.028-S Model. After using it for awhile, I have taken a liking to it and will eventually be getting a higher end AK model in the future.
(UPDATES-[AS OF 2-25-16]) As you will have noticed in the pictures, I have given this AK a weathered look as best as I can and have installed a few basic upgrades. As far as the upgrades go, I have attempted to Flat-Hop it, Tuned the Hop-Up, (Thank You Tackett for the Stickied Thread on Fine Tuning a Hop-Up.), Added Deans Plugs, got an11.1 lipo, replaced the wiring, added a GATE MOSFET, Replaced some of the plastic internal pieces that seemed weak (see next paragraph), Added a stronger spring, Corrected the AOE, Re-Shimmed And Re-Greased it. (See the last two pictures for a before and after of the gearbox.)
As far as what I have done inside the gearbox, I will list the parts that I have used:
SHS Full Steel Tooth Piston -http://www.amazon.com/gp/product/B004UC4PZA?psc=1&redirect=true&ref_=oh_aui_detailpage_o08_s00
SHS Piston Head (The stock CYMA piston head was worthless in my opinion as far as air compression goes) -http://www.amazon.com/SHS-Airsoft-Piston-Bearing-Polymer/dp/B00EFAU7T0/ref=pd_sim_200_6?ie=UTF8&dpID=31Dt-3yY64L&dpSrc=sims&preST=_AC_UL160_SR160%2C160_&refRID=0YZMGQK3R1Y2ERCZFR97
LT Air nozzle -http://www.amazon.com/gp/product/B00OF8N5IW?psc=1&redirect=true&ref_=oh_aui_detailpage_o09_s00
Matrix Metal Spring Guide -http://www.evike.com/products/33709/
Matrix Tappet Plate -http://www.evike.com/products/36722/
Matrix Metal Cylinder Head (Again, The stock CYMA cylinder head Bellowed for air loss) -http://www.evike.com/products/35205/
SHS M110 Spring -http://www.amazon.com/gp/product/B00A9ZDTHC?psc=1&redirect=true&ref_=oh_aui_detailpage_o04_s00
GATE MOSFET -http://www.evike.com/products/45436/
Shim Set (I was not expecting the shimming to be perfect obviously, but I have read that the shimming on stock Cyma\'s was iffy/good, but this was one of the Worst shim jobs that I have ever seen on a stock airsoft gun.) -http://www.evike.com/products/31362/
I used Super Lube to Re-Grease the gears. -http://www.amazon.com/Super-Lube-21030-Synthetic-Grease/dp/B000XBH9HI/ref=sr_1_1?ie=UTF8&qid=1450112428&sr=8-1&keywords=super+lube
The Angle Of Engagement was corrected with One Faucet washer and a small amount of Sorbothane using locktite superglue. -http://www.ebay.com/itm/371266030772?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
As far as Outside of the gearbox goes, here is a List of other parts and the battery:
11.1 Lipo -http://www.hobbyking.com/hobbyking/store/uh_viewitem.asp?idproduct=59115
Battery Bag -http://www.evike.com/products/27383/
Hop-Up Bucking (I will be getting a G&G green bucking soon as I\'ve heard that they are the best to Flat-Hop with) -http://www.evike.com/products/42461/
The Nub I used inside the Hop-up was made and cut out of a pink eraser... -http://www.amazon.com/Paper-Mate-Premium-Erasers-70501/dp/B00006IFAQ/ref=sr_1_1?ie=UTF8&qid=1456416587&sr=8-1&keywords=pink+erasers
I currently use King Arms 120rnd Mid-cap Magazines and have had good luck with them -http://www.evike.com/products/27945/
With doing what I have done, I was chronoing near a consistent 350 FPS with .20g bb\'s and am getting an effective range of 175+ft and able to hit targets at 200ft when using .28g bb\'s on the field.
One small plastic piece that was used to adjust the slider on the Hop-Up shattered after adjusting the Hop-Up a few times. However, I made a small metal piece from a chain that works a lot better and feels much more sturdy. The fifth from last picture shows the small metal piece that I made from the chain seated in the sliding part of the Hop-Up. At some point, I might make a quick guide on how I did this in case other people have the same problem and don\'t want to pay $20 for a new metal Hop-Up...
Thank you for reading,