The barrel must be ported on top. It keeps the muzzle flip down and recoil low especially on multiple shots in a row.
http://www.herosarms.com/HA1336_Citori_725_Spt_12-32DS/HA1336_Browning_Citori_725_Sporting_Grade-1_OU_12-32MC_DS_Ported_Muzzle.jpg