I am NO Abrams expert but I have learned a lot during my Abrams build and will be including this installation on my tank.
It is the "Counter Sniper / Anti Material Mount"
manufactured by Gunmasters Defense System LLC.
It can be used as pictured with the 12.7mm M2HB or a 7.62mm M240B. It is as Alwyn said a "remotely operated" it can be operated by either the tank commander or gunner form the safety of the turret armor. Using the main gun aiming systems the 12.7mm is accurate out to 2000m.
Photo provided for educational purposes only under the Fair Use Act.