Not sure about your question. I don't know exactly what a "civilian" M2 is-- one made by a commercial manufacturer rather than one made for the military? If so, they would usually be called a commercial gun and they are essentially the same. An older military gun would be a C&R gun. An exampe of a commercial gun might be newly made sideplates riveted to a demilled top and bottom plate and trunnion, and the rest of the gun assembled from surplus GI parts if available, or newly made parts.
There are also semi .50's, and these are vastly different. The sideplates are thicker so the FA parts can't be inserted into the receiver, the internals are thinner, and the trigger mechanism is entirely different.
I hope this helps you and it's what you were looking for. For more information, check on 1919A4.com and see their tutorials on building guns.