I have a Colt Light Match ,year 2000, and on the receiver it is marked .223
There are no markings on the barrel.
My gun dealer said all Colts are OK with the 5.56 ammo, even though 5.56 is not shown.
Do you agree?
It does not matter what is stamped on the receiver that counts. It is what is stamped on the BARREL that counts!
If your barrel is stamped 5.56, then you can use 5.56 ammo.
Please study the following link if you have not done so already:
The chambers for .223 and 5.56 weapons are not the same either. Though the AR15 design provides an extremely strong action, high pressure signs on the brass and primers, extraction failures and cycling problems may be seen when firing hot 5.56 ammo in .223-chambered rifles. Military M16s and AR15s from Colt, Bushmaster, FN, DPMS, and some others, have the M16-spec chamber and should have no trouble firing hot 5.56 ammunition.
Military M16s have slightly more headspace and have a longer throat area, compared to the SAAMI .223 chamber spec, which was originally designed for bolt-action rifles. Commercial SAAMI-specification .223 chambers have a much shorter throat or leade and less freebore than the military chamber. Shooting 5.56 Mil-Spec ammo in a SAAMI-specification chamber can increase pressure dramatically, up to an additional 15,000 psi or more.
The military chamber is often referred to as a "5.56 NATO" chamber, as that is what is usually stamped on military barrels. Some commercial AR manufacturers use the tighter ".223" (i.e., SAAMI-spec and often labeled ".223" or ".223 Remington") chamber, which provides for increased accuracy but, in self-loading rifles, less cycling reliability, especially with hot-loaded military ammo. A few AR manufacturers use an in-between chamber spec, such as the Wylde chamber. You can generally tell what sort of chamber you are dealing with by the markings, if any, on the barrel, but always check with the manufacturer to be sure.
Typical Colt Mil-Spec-type markings: C MP 5.56 NATO 1/7
Unless your rifle says.
I'd stick with just .223.
Was the rifle built by Colt?
If so call them and give them the Serial # they should be able to answer your question.
Yes. I have Colts marked .223 on the receiver but they are 5.56.