Which RIS system will work on a KJW M4A1 GBBR?

Im not sure if there are any made for that exact gun but most should fit either AEG or gas guns. Some may require a little bit of modification but most should fit pretty easily. Take a look at this one: a href="http://www.airsplat.com/Items/AC-UPE-CA-AO60M.htm" rel="nofollow"http://www.airsplat.com/Items/AC-UPE-CA-…/a