If it's stamped 5.56x45 on the barrel, then yes. It should also be stamped with the twist rate on the barrel, too, and will probably be 1 in 7.
The majority of Colt rifles are 5.56x45mm chambered, some of the target versions are .223 only.
Still need a bit more information on it. I can't give you a value, but what price are you looking at? Hopefully a good bit under $1000 (because you can buy a new, up to date Colt 6920, flattop, with a 16" barrel for that price rather than a rifle with fixed A2 sights).