c mp chrome bore
From The Black Rifle II Christopher R. Bartocci Collector Grad Publications 2004 pp 285:
Quote:
The chrome plated chamber was adopted on May 26, 1967, and the late-style M16A1 barrel with chrome plated bore and chamber was in production by 1971 or 1972. In lieu of the earlier "CC" or "CB" prefixes, Colt began to mark their barrels (ahead of an initial proprietary "C" for Colt) and the "MP" (Magnafluxed and Proofed), with an additional "C" to indicate the that the barrel had a chrome chamber only, or "CHROME BORE" or "B" to indicate chrome bore and chamber.
under the rear site is C M no markings