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Technosavant · December 21, 2009, 10:26 AM

Hi Brad.

While Bud is right that Colt is the ultimate authority, this isn't too new for AR shooters.

The stamping on the lower is almost certainly a mis-stamp. Some guys at AR15.com can probably give you the skinny on it, but that's a new one on me. With the AR, though, what is stamped on the lower is essentially irrelevant when it comes to what the gun will actually chamber. There's two sizes of AR lower- the .223/5.56 lower (that will run all kinds of different rounds) and the .308 lower (has a longer magwell because of the extra length). To my knowledge Colt has never made a .308 AR platform rifle.

With an AR, what is stamped on the barrel is gospel. Your barrel is stamped "C MP 5.56 NATO 1/7." The C indicates that it is a Colt barrel, the MP indicates magnetic particle inspection. The "5.56 NATO" is what the chamber is actually cut for, and you can shoot 5.56x45mm or .223 out of it (the size of the two cartridges are the same, but 5.56 runs at slightly higher pressure- you can shoot .223 from a 5.56, but not the other way around; .223 barrels are generally for high accuracy long range varmint style ARs). The 1/7 indicates rate of twist- one turn in seven inches. That's as fast as barrel twists go for 5.56 ARs, meaning your barrel can be expected to stabilize the newer heavy bullet loadings like the Hornady TAP in 75gr.

So your barrel is perfectly normal and actually a desirable one for defensive uses. The lower is indeed mis-stamped, but somebody else is going to have to tell you if you have a rarity or something of extra value- I have no idea.

December 21, 2009, 10:26 AM	#3
Technosavant Senior Member Join Date: May 29, 2007 Location: St. Louis, MO area Posts: 4,040	Hi Brad. While Bud is right that Colt is the ultimate authority, this isn't too new for AR shooters. The stamping on the lower is almost certainly a mis-stamp. Some guys at AR15.com can probably give you the skinny on it, but that's a new one on me. With the AR, though, what is stamped on the lower is essentially irrelevant when it comes to what the gun will actually chamber. There's two sizes of AR lower- the .223/5.56 lower (that will run all kinds of different rounds) and the .308 lower (has a longer magwell because of the extra length). To my knowledge Colt has never made a .308 AR platform rifle. With an AR, what is stamped on the barrel is gospel. Your barrel is stamped "C MP 5.56 NATO 1/7." The C indicates that it is a Colt barrel, the MP indicates magnetic particle inspection. The "5.56 NATO" is what the chamber is actually cut for, and you can shoot 5.56x45mm or .223 out of it (the size of the two cartridges are the same, but 5.56 runs at slightly higher pressure- you can shoot .223 from a 5.56, but not the other way around; .223 barrels are generally for high accuracy long range varmint style ARs). The 1/7 indicates rate of twist- one turn in seven inches. That's as fast as barrel twists go for 5.56 ARs, meaning your barrel can be expected to stabilize the newer heavy bullet loadings like the Hornady TAP in 75gr. So your barrel is perfectly normal and actually a desirable one for defensive uses. The lower is indeed mis-stamped, but somebody else is going to have to tell you if you have a rarity or something of extra value- I have no idea.