M4 and M16 airsoft parts alone could fill up a small airsoft warehouse, now add to that the companies that make non standard guns (parts will not fit other airsoft guns) and you have many more options and parts to catalog.
Then there are the upgrade parts or "High Performance Airsoft Gun Parts" designed to increase your durability, FPS and accuracy. High torque gears, High speed gears, and hardened steel gears, don't forget the bore up cylinder sets and silent cylinder sets, spring guides with thrust bearings and tight bore barrels which brings up the importance of properly maintained hop up parts. Some guns feature a stock plastic style hop up and some have a full metal unit, but what is absolutely critical is that all parts are properly assembled and the bucking and nub are in good working order. Hop up unit rebuilding kits are available to purchase now as well.
Now if you look at both internal and External airsoft parts available the already large list turns into an encyclopedia size inventory sheet. That's why we tell our customers to call us if you are shopping for parts at our online airsoft gun parts store and do not find the part you need listed for sale. We probably have the airsoft part you are looking for...just not online yet. Many have asked about the ICS split mechbox parts and G&G L85 and other specialty parts. Yes we carry hard to find airsoft parts, and easy to find airsoft parts as well as upgrade and high performance parts. So the next time you need repair parts for your AEG swing by the Airsoft Battle Zone parts store or visit us at the airsoft gun repair website and see if "we have what you need" to get you back in the airsoft game quickly.