I decided to sit down and make this post in response to the PMs I have been getting over the last 3 months. I figure this might save me some time down the road. Hope this helps...
The cheapest thing you can do is get some .36g bbs and laern to shoot it. I would set the hop-up so that the bb falls in a smooth continuous arc and learn to hit with that at different ranges. If you try to adjust the hop-up so that it has a bit of "over-hop" you will end up tearing the bucking and you will have a very inconsistent hop-up due to the changing fps from shot to shot. You can try .40g or .43g, but you will increase the odds of tearing the factory bucking. I used to try to get max range with out "holding over" the target. This was foolish and the best I could do was about 165' with a stock rifle. The bb will have a more consistent path with the hop-up adjusted just under the point where the bb climbs then falls.
There is a very limited supply of aftermarket parts. KM used to make a TN 6.04 inner barrel that you could buy at www.wgcshop.com
In the glory days, a guy at www.paintball-x.com
was buying bulk orders of upgrade parts from Korea and I was lucky enough to get in on this, but really you can do with out most of the parts and build your own of the rest. www.Evike.com
has a pretty good group of parts from Best Gun, magazine valve kits(comes with upgraded ahmmer spring and firing pins), barrel spacers, nylon hop-up chamber(great if your gun doesn't have a metal chamber), rail adapter(if your gun has the older 3/8" rail rather than a weaver rail). Dees Custom used to make a 6.01 tightbore and you may be able to talk them into cutting you another one. ANGS made a "Light Trigger Pull Kit" that came with several springs, if you can find this the hammer spring will be very helpful, the other springs are really only useful in a pistol since you need spring tension to return the barrel to the forward position after each shot.
After you have played several games and spend a few hours and hundreds of bbs learning how to shoot the gun and how it reacts to different inputs and elements, you canstart thinking about upgradeing it... that is if you are not disgusted with it. Keep in mind that it will never be as consistent as a BAR-10 or other VSR platform. The beauty of this gun is it's power for it's size. It is also in the top 5 for quietest surpressed airsoft guns.
When you are ready to upgrade, you will need to focus on increasing the gas efficiency. This is all trial and error and you HAVE to have a rela chronograph, the coke can method WILL NOT work here. Your goal will be to get a maximum of 550fps with .20g or 436fps with .36g bbs. in 90 degree weather. I would first clip one coil at a time off of the hammer spring at a time, measuring the fps along the way to make sure you do not go too far. I ewould not remove more than 3 coils off of the hammer spring. If you still need to lower the power more, use a file to file a very small amount at a time from the firing pin. Check the fps often so that you do not go too far, remember you can take metal away alot easier than you can put it back.
After you have the power in the correct spot, you will have simultaneously increased the gas efficiency. This will give you a tighter fps spread from shot to shot. Currently the rifle can loose up to 40fps(!) with each shot, after the first few shots. Ideally you want a 10fps difference with each shot, if you hit this mark let me know!
Next up you will want to add a tightbore barrel.
It is very important to remember that the KJW barrel has an 8mm outside diamter while nearly all other barrels are 8.5mm. You can get around this by having a machinist turn down(cut on a lathe) the outside diameter so that the hop-up chamber will fit. You will also need the machinist to cut the 2 needed grooves and the hole for the hop-up patch(bucking). Since you started with a 8.5mm outer barrel, you will either need to have the machinist turn down the end of the barrel to fit into the factory muzzle cap, or have the machinist bore out the muzzle cap to fit the new barrel. Remember that it has to be a loose fit because the inner barrel has to slide forward and backward to chamber each round when the trigger is pulled.
If you ever tear the hop-up patch, you can trim a little off the front and the back of a Tanaka M700 patch to get it to the correct lenght to fit(use the factory patch for dimensions). If you decide to keep the power over 600+fps, you will need to use the G&G red patch for the Tanaka M700 to get the rifle to "hop" .36g or heavier bbs with out tearing the patch. http://www.airsoftextreme.com/store/index.php?main_page=product_info&products_id=1319
You can also make or buy a remote air source adapter to increase the consistencly of the gun. Here is one for sale on Evike.com that will work ans save you some hassle: http://www.airsoftpost.com/adaptor-marui-compatible-series-magazine-p-28100.html
Making one is hard. I used a 5mm allen bolt, drilled a hole down the center, then drilled and threaded a hole into a 1/8" NPT female cap/plug, then use JB Weld to seal adn glue the bolt into the cap.
Mock mock mock mock suppressor adapters will have to be made or modified from an existing adapter. However, the latest KJW Mk1s come with a new muzzle cap that comes with the adapter hidden under a thread protector cap. If you opt to make your own supressor adapter, buy one and take it to a machinist to turn down the outside diamter so that it just slips into the muzzle, then drill and tap a hole through the outer barrel and into the muzzle adapter. You can either add a set(grub) screw for a flush finish, or use a allen bolt to double as a front sight. Pictured is one made from a King Arms VSR-10 adapter.
The 23" barrel kit is nice if you want a longer gun. It comes with a longer outer and inner barrel, but the inner barrle is just as bad as the stock barrel. The outer barrel is great for covering a custom tightbore, but the muzzle cap has differnt(coarse instead of fine) threads so if you have one with the new supressor mount muzzle caps, it probably won't fit.
If you opt to increase the hammer spring tension to increase the power, it will wear the trigger lickages down more quickly. The trigger linkages are all cast pot metal and rubbed enough of the camming surface off of a set so that it wouldn't work on every trigger pull. You can also put a bb under the hammer spring to increase hammer tension even further for more power. All this extra tension will increase the trigger pull enough to make your finger hurt after shooting/testing for awhile. To fix this, I cut off the trigger guard and made a trigger extension out of part of the trigger guard. I bolted it on and used JB Kwik Weld to contour it a little better and help hold it on.
If you leave the mags full of gas in the heat, they will expand/swell enough to make it difficult to get them out of the gun. I usually left just a bit of gas in the mags to keep the seals good, just not full. If one swells, you can fix it by emptying the gas, placing it in a vise with the large flats sides against each jaw, then very gently squeeze it back. Be sure to check it in the grip as you go so that you do not compress it to far. BE CAREFUL, this can cause small fractures in the mag and eventually make it leak beyond repair. A better option is to make a magpul for it. I drilled through the base plate, under the bb chanel and tapped it for a eye bolt, then added a key ring. I also went a step further to counter sink the hole so that the eye bolt sinks in tigher, but this is not required. To make it a bit easier to remove, I made a heel on the mag release lever. I drilled into the existing lever and added a small screw, I then molded some JB Kwik Weld around the screw to form the heel. This makes it easier to press the lever with your thumb so that you can pull out the mag with the same hand.
I made my own full lenght barrel spacer after I switched over to a 8.5mm outside diamter tightbore. I found a brass sleeve at a hardware that was about .010" larger than the barrel so that it would still slide smoothly inside. I then found several differnt brass and steel parts to completly fill the inside of the outer barrel. This was pretty easy, but I did have to run a 1/2" drill down the center of all the larger brass peices to get the other pieces to slide inside. I also used aluminum ducting tape to make the brass pieces fit nice and snug inside the outer barrel.
For stock options, you can modify pretty much any stock so that the grip will fit through it, then make some new mount points. If you do this, do not bother trying to mount the grip. Just mount the outer barrel to the stock and let the grip hang down unsupported. If you try to mount both, you have to get it EXACTLY perfect or the mounting of the grip will cause a bind between the upper and lower receivers causing the trigger parts or the inner barrel to bind and not allowing a second shot. This is why there are no grip screw holes in the first gun, and why they are missing from the second pictured rifle. Both of these stocks are intended for the Ruger 10/22, note the resin filled magwell area on the first stock. I chose these stocks because they had a similar grip angle to the KJW Mk1.
Now, all that said, feel free to PM me if you have any specific questions that I did not answer here or if you would like more information about a specific modification.