These terms are chiefly found in modern Latin texts. English speakers are unlikely to recognize them. Neither the Chicago Manual of Style (14th ed.), the American Heritage Dictionary (3rd ed.), nor P. Kenneth Seidelmann's Explanatory Supplement to the Astronomical Almanac (1992, University Science Books) mention AC, ACN, or Ante Christum Natum.
These terms were not used in medieval and Renaissance Latin texts. Bede the Venerable, who was the first writer to identify a year as before Christ, used the Latin ante incarnationis dominicae tempus (before the time of the Incarnation of the Lord) in his Historia ecclesiastica gentis Anglorum (I.2) in 731. Most comparable early Latin terms referred to Christ's Incarnation or conception, not his birth nine months later.