The numbers on the lower receiver do not affect anything. The lower receiver just holds everything in place and has nothing to do with what ammo you can fire. Some manufactures write the "intended" caliber on the lower, while others leave it blank.

The numbers on the barrel matter. You can safely fire .223 ammo in a rifle chambered for 5.56 but not the other way around.

So it is a very good thing that your rifle is chambered in 5.56mm ammo. Good for you. You can shoot military surplus 5.56mm ammo and commercial .223 ammo safely.

As for Colt...they are great...they are just next to impossible to find and usually cost 3 times as much as the competition when all the specs are even.

There are varring qualities of parts and uppers with every brand name, including Colt. Meaning Colt makes low end stuff, and high-end stuff. It all comes down to the details. Is it chrome-lined? What is the twist rate? Does it have M4 feed ramps? All questions that will help you judge the quality of your rifle.

If all you do is go plinking every so often...just about anything will do. If you want to have the Ferrari of AR-15's...the details come into play!

Hope that helps...just trying to earn my keep.

Oh, and do me a favor and visit my website http://www.tacticalcity.com