I bought this Colt off of a friend of mine and I am trying to decide what scope to put on it.

I do have a question though...The lower is only stamped .223. Does this mean that it will only fire .223 and not 5.56 ammo?

It shoots what ever is stamped on the barrel, the lower doesnt always match what the barrel says so look and see if you can find any caliber markings on the barrel it's self to see what you can shoot.

that.

also because that's not a CR6724 lower receiver but a MT6601 one.
the gas block has been replaced as well.

if the barrel is Colt it can handle 556 just fine

Here's the "Skinny" on .223 vs 5.56...

There are a lot of questions about these two cartridges. Many people think they are identical - merely different designations for commercial and military. The truth is that, although somewhat similar, they are not the same and you should know the differences before buying either cartridge.

* The cartridge casings for both calibers have basically the same length and exterior dimensions.
* The 5.56 round, loaded to Military Specification, typically has higher velocity and chamber pressure than the .223 Rem.
* The 5.56 cartridge case may have thicker walls, and a thicker head, for extra strength. This better contains the higher chamber pressure. However, a thicker case reduces powder capacity, which is of concern to the reloader.
* The 5.56mm and .223 Rem chambers are nearly identical. The difference is in the "Leade". Leade is defined as the portion of the barrel directly in front of the chamber where the rifling has been conically removed to allow room for the seated bullet. It is also more commonly known as the throat. Leade in a .223 Rem chamber is usually .085". In a 5.56mm chamber the leade is typically .162", or almost twice as much as in the 223 Rem chamber.
* You can fire .223 Rem cartridges in 5.56mm chambers with this longer leade, but you will generally have a slight loss in accuracy and velocity over firing the .223 round in the chamber with the shorter leade it was designed for.
* Problems may occur when firing the higher-pressure 5.56mm cartridge in a .223 chamber with its much shorter leade. It is generally known that shortening the leade can dramatically increase chamber pressure. In some cases, this higher pressure could result in primer pocket gas leaks, blown cartridge case heads and gun functioning issues.
* The 5.56mm military cartridge fired in a .223 Rem chamber is considered by SAAMI (Small Arm and Ammunition Manufacturers Institute) to be an unsafe ammunition combination

That is all great and correct info. but the question here is about the chambering of this Colt model and it can be accuratley found on the barrel, the lower is not the proper place to look for the chambering of these rifles. Long thread on the colt roll stamp on here somewhere and it was discussed pretty in depth.

I've looked around the barrel and it doesn't say what caliber it is. Could it be stamped underneath the handguard?

Also, how do you know what receiver it is by looking at it? Do I have a bastard Colt?

If that is not already a 'sticky', it should be.

Type in a search for "colt stamp" and didn't find any results...

Well, It looks like the upper and barrel are, in fact, not necessarily Colt. I'm not sure what markings to look for to be sure but unless you paid a premium because it was supposed to be ALL Colt I wouldn't sweat it. It's probably far more accurate than a Colt would be anyway. The barrel may have a small caliber stamp that could be just about anywhere and may not be very well pronounced. It may look like a "nick" or "ding" at first glance.

I bought it off a buddy of mine whom I trust but I knew it was piece meal. Had taken it to a gun smith to verify that it was all original Colt. Don't know how they can tell though...

My biggest concern is the caliber. I don't want to load 5.56 into a .223 dedicated chamber.

+1 It's a MT6601 lower with a switched upper half.

MT6601 HBAR comes from the factory with fixed carry handle, Colt marked chrome moly steel A2 barrel, chrome lined, HBAR contour, A2 FSB, A2 handguard, and either no muzzle attachment or with a muzzle brake.

Have your gunsmith verify if you have a .223 or 5.56 NATO chamber to be safe, if it's a .223 and you prefer to use 5.56 ammo ask the gunsmith if he could ream the chamber to 5.56.

because I have a MT6601 and my lower is exactly like your
just the rollmarks are different
here is the correct lower for your rifle

Thanks for all the info. Sounds like the best course of action will be to take it to a gun smith and have him verify if it can shoot the 5.56. If I knew then what I knew now I would have noticed some of these things right away. This was the first AR I had purchased and was just happy to get my hands on one at the time.

BTW, is there a way to tell if the upper is actually Colt?

Colt put a "C" on their  upper receivers (see pic below), bolt, bolt carrier and extractor.
barrels are usually marked "C MP" followed by other info such as chamber and twist...I dont know how a CAR barrel should be marked but I guess the same as the others



ETA:
if the barrel/upper/bolt carrier group are Colt dont feel bad for not having the right lower receiver...it's just a matter of markings

more info in this thread http://www.ar15.com/forums/topic.html?b=3&f=118&t=425185&page=1 on Colt.

No,,No...IT must be a copy? Because everyone know's Colt's arePerfect.I seen one droped from 30,000 feet, then ran overby a bull dozer. It's still amazing never skip's a beat..........DON'T WORRY THIS IS HARMLESS HUMOR LOL...but Colts Fans don't do not have much humor when talking about their AR's. Well positive  is OK but don't go the other way....lol

Does this mean I can never be made. I feel like DeNiro in Goodfellas. My Colt doesn't come from a pure bloodline. lol

Hmmm, the OP ask's a reasonable question and you act like a dick because why? One shouldn't shoot 5.56 in a .223 chamber, all people are trying to do is to see what the upper and barrel really is. Nothing Cultish about it at all, just safety and reliabilty.

So........get the leade out. Ha Ha LOL, get it?

Lots of that going on around here. Not quite, but almost every other thread some 18 YO funny boy jumps in with nothing to add.

Sorry OP I can't help you figure out your barrel make. Glad you were safe and smart enough to ask.