This is an extension for the external barrel of an M4 type Rifle. It extends the barrel about 5 inches which can allow you to turn a CQB airsoft gun into an M4 length airsoft gun. I used this with a JG Brand HK 416 to allow me to put a longer barrel into the gun. Basically all you need to do is remove the flash hider and then this extension screws right on. You can then screw the flash hider back on at the end of the extension. With this barrel extension I was able to go from a 275mm inner barrel to a 363mm TIGHT BORE barrel with room to spare. You probably want to use loctite or something to lock the extension into place, but it does not come with the extension. This upgrade, combined with the 363mm Tight Bore has increased the accuracy of the gun by a lot. This extension was designed to work with Echo 1 and JG M4 style guns, but would probably work with any brand as long as it has the 14mm counter clockwise thread.
-Can increase the length of your AEG if you want a longer inner barrel
-Metal, nice weight but not too heavy
-Easy to install
-Very nice finish, dosen't look like it will chip
-Can give you the ability to switch between a CQB version and a full length version using the same gun
-It's not really needed since the barrel fits pretty tight, but it does not come with any locking adhesive which would be nice to make sure the extension would hold on tight
-There was some red dust in on the inside of the threads on the extension. I do not think it was rust because it cleaned off easily with a paper towel and some silicone spray