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Old January 8, 2013, 10:23 PM   #3
Join Date: December 13, 2012
Posts: 40
If the barrel markings are C MP followed by 5.56 NATO the "C" indicates manufacturer (Colt) If the markings are C MP C 5.56 NATO the latter "C" indicates chrome plated chamber. If the markings have a "B" or "CB" after the MP then it indicates both chamber and bore are chrome plated. "MP" obviously stands for magnetic particle inspection. As a rule, most if not all Colt match barrels are never chrome plated being it alters bore diameter. The chrome plating was designed for military rifles to reduce erosion between cleanings.
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