A M4gery is a name the AR15.com types use to describe an AR15 type carbine that looks like the military-issue M4. The M4 has a 14.5" barrel, collapsible stock, and burst or full-auto capability. A M4 is distinguished by a "notch" in the barrel for a grenade launcher, etc. A typical M4gery is semi-auto, has a 16" barrel or 14.5" with a long muzzle brake or flash hider to make it the legal length, and if it is post-ban, a "fixed-length" collapsible stock. Colt makes a LEO only carbine that closely replicates the military version. There are other differences between a M4 and a M4gery such as pin sizes and such.