Are you certain that your rifle has a heavy barrel? Have you checked under the handguards to verify that it does not have the tapered down section of a government profile barrel designed to accomodate mounting an M203 grenade launcher? If it does indeed have a true heavy barrel on it, some parts swapping has been done somewhere in it's past.
What you have is an R6550 Government Model lower receiver, if not complete rifle. This model has a serial number range that begins with the GS prefix. It is basically a Green Label clone of the M16A2. It is my belief that fewer than 10,000 R6550s were produced.
In short, in my opinion you have one of the more, if not most, desireable pre-ban Colts.