FINALLY ready, Tippmann Arms has released a full upper receiver assembly for their M4 airsoft platform. This allows you to build out another upper receiver, so you can have both CQB and Field/DMR uppers. Coupled with the Tippmann outer barrels, you can configure your gun in any way you like, just like any AEG or GBBR.
This is a fully functioning upper receiver that does not come with the barrel, or sights. Imaged credit:Ken Farris.