A.M. Best has assigned a “bbb” credit rating and “stable” outlook to New Britain-based Acmat Corp., a construction firm that is also theĀ holding company for Acstar Insurance Corp. Back in April, Acstar was given a financial strength rating of “A.” Amcat’s rating reflects Acstar’ strong capital position and its historically strong operating performance, A.M. Best said, although that was partially offset by its limited profile as a monoline writer of commercial surety contracts.