I bought a Knights Armament 20" rail that doesn't seem to fit my Colt Match Target.
The Delta Ring doesn't fit over the the top rail.
Is it possible these don't fit Colts?

There are bazillions of KAC rails installed on gov't issue Colts in mil service. However, most of those are gov't profile barrels. Do you have a RAS & is the barrel on your Colt a hbar? Some hbar diameters are not compatible with the RAS. A RAS can fit a hbar but it depends on the particular diameter of the hbar you are trying to use it with. They are not all the same but I'm not familiar with Colt civ models other than the gov't profile barreled LE6920.

Edit: The ras has a swing lock mount on the top rear of the RAS underneath were it meets the barrel nut. The hbar issue I was talking about doesn't allow enough clearance room for the mount to slide under the barrel nut depending on the particular diameter.

A RAS will fit a Colt 20" HBAR barrel.  Your installing it wrong.

Are you sure you've released the clamp on the upper rail?  

There should be an allen head screw on the top rail, in the back, you need to remove it entirely, and let the clamp drop down.  

I've found the easiest way to install any KAC RAS is to angle the weapon at about forty five degrees, muzzle down, the clamp will naturally hang down.  Make sure to first insert the leaf spring on the front underneath the handguard retaining cap.  Depress the delta ring, and allow the clamp to fall around the gas tube, and it should engage the barrel nut.  

A nice smack on the rail to seat the rail, replace the allen screw, and you should be good to go.

HTH,
~Augee

WHY ARE YOU SHOUTING AT US?

The barrel reads "C MP 5.56 NATO 1/7 HBAR."
i'll try again with the screw out.

clasky,
Shouting is when you use all capitol letters.
Increasing the font is so people can read short threads easier.