The Metal Gear box of your gun is the starting point you want to focus on and you are looking for 3 primary performance points for upgrades; 1.Durability 2. Accuracy 3. Fire Volume Airsoft Durability Upgrades -To upgrade for durability you must first decide what FPS, Foot Per Second and ROF, Rate Of Fire you would like your gun to shoot and then replace all stock airsoft gun parts that are not capable of operating in your desired range. Many airsoft gun manufacturing companies use a base grade of plastic airsoft gun internal parts in stock models to keep manufacturing and retail costs low. Plastic spring guides and plastic bushings are quite common in many models and should be upgraded to metal parts as soon as possible. Other considerations are the piston, cylinder head and cylinder as these are plastic composite in most stock airsoft guns. These items will not wear out as fast as the plastic bushings and spring guides but if you plan on a high speed or high FPS upgrade you should consider upgrading these parts to metal for durability purposes as well.
Your airsoft gun battery size, type and voltage must also be considered with durability. Some airsoft gun configurations will only allow for a certain size or style of battery. Your gun battery voltage can speed up or slow down your ROF. Be sure your chosen battery will fit in your gun and power it for at least 4 hours of moderate use.
Airsoft Gun Accuracy Upgrades - To upgrade your gun for accuracy you will look at the hop up unit parts and the inside diameter of the stock barrel your gun came with. Most stock barrels are 6.04 diameter size and work great with most upgrades but there are tighter bore airsoft barrels available. Keep in mind the tighter the bore of your barrel the higher the quality your bb's will need to be. If you have upgraded the gearbox beyond 350 FPS you will want to try out heavier bb's. The .23 or .25 gram seem to work nicely in most guns that are upgraded to higher velocities. For more information and pictures about hop up repairs and upgrades visit; http://www.airsoftgunrepair.com/airsofthopuprepair.html
Airsoft Gun Speed Upgrades: To upgrade your gun for speed or a higher ROF you can increase the battery voltage for a quick speed upgrade or you can install high speed gears for a more durable increased ROF. There are high speed airsoft gun motors available but I recommend a battery upgrade to 9.6 volts, as most stock airsoft guns use an 8.4 volt battery. This will increase your ROF significantly with minimum cost as most stock airsoft gun motors can run efficiently on up to a 12 volt battery, which brings me to my next point. The new High discharge Li-Po style 11.1 battery systems. These batteries bring a whole new level of fast bb fire into the airsoft game but most stock airsoft gun parts are not engineered to handle these extreme RPM's. I recommend high speed modular type gears be installed for this kind of high speed upgrade for durability. 
I use modular gears, metal tooth piston and metal cylinder head (See Photo) in all my guns using the Li-Po style batter. These guns feature a M100 spring and metal spring guide with bearing along with hard steel bushings. Despite my heavy trigger finger and extreme ROF My last yearly PM showed hardly any internal parts wear since last years PM. That equals allot of welts shared since 2008!
I use modular gears, metal tooth piston and metal cylinder head (See Photo) in all my guns using the Li-Po style batter. These guns feature a M100 spring and metal spring guide with bearing along with hard steel bushings. Despite my heavy trigger finger and extreme ROF My last yearly PM showed hardly any internal parts wear since last years PM. That equals allot of welts shared since 2008!Armsdealer...OUT
