Yes, in 1994, our wonderful Congress decided to pass what is known as the 1994 Crime Bill. In it, there are provisions that make certain types of features illegal for civilians, and manufacturers cannot manufacture them for civilian use which is why you have that stamp on your Colt.
Basically, the law works out that any rifle with a detachable magazine can only have a total of one of the following features before your gun becomes an evil "assault weapon."
1. Pistol grip
2. Flash hider or barrel threaded to accept a flash hider (this includes suppressors)
3. Bayonet lug
4. folding or telescoping stock
5. ability to accept a grenade launcher
Since the AR-15 has a detachable magazine, it can only have one other feature on that list. Since the pistol grip is required, that's all the features it can have. A flash hider/threaded barrel, bayonet lug, folding stock, and ability to accept a grenade launcher are features that cannot be manufactured into a civilian gun built after September 13, 1994. This is why we Americans have what we call "pre-ban" and "post-ban" rifles. A pre-ban can have all of those features and there is no problem, but you have to prove that it was manufactured before 9/14/94.
Fortunately, this ban has a 10 year limit built into the law that expires on 9/13/04. So, provided nothing replaces the ban, we can have all those evil features again.
The only exception to this rule is if you put a registered DIAS, making the gun a machine gun. Then you can do whatever you want with it that can be done with a machine gun, including putting a sub-16" barrel on it. Unfortunately, you still have to build up a post-ban gun into the config you want, as civilians cannot get a post-ban rifle with pre-ban parts on it from any manufacturer that I know of. So you'd have to get a pre-ban upper with the features you wanted on your machine gun.
Remember the Alamo, and God Bless Texas...