If you have one of the older WE M4 gas blowback rifles with the older ''closed bolt'' system with the brass tube running though the middle of the mechanism then you probably became envious of the newer ''open bolt'' M4 system that came out later.
That said, do not worry; you can purchase this kit which contains everything needed to convert your closed bolt WE M4 into an open bolt version.
Please note that this is just the internal parts conversion kit and does not come with a WE M4. You need a complete working WE M4 to combine with this kit to make an open bolt version WE M4 gas blowback rifle.
This kit comes with the bolt carrier group (including nozzle), trigger mechanism group, inner barrel, hop-up chamber, dust cover and various other little pieces to complete the functional set.