The M4A1 Crystal is claimed to be more accurate then the original M4 carbine. It does not have a scope, like its evolved version (Custom-Crystal), though it is reported that it has more power, and is also more accurate because of the low recoil. It has a blue, crystal like appearence and is favored by many.
Note: this is the M4A1-S Crystal. The permanent version can no longer be obtained in the z8 version of Crossfire.