If you have Honest Hearts, you can use the Survivalist's Rifle. It uses 12.7mm rather than 5.56mm. In the same DLC, the Grunt perk gives a 25% damage bonus to .45s, 9mm, assault and marksmans carbines, grenades and service rifles. Additionally, there are two mods: upgrading springs (faster rate of fire) and one that improves the condition 50%.
Trigger Discipline is a great trait. Of course a high Guns and Survival stats.
Survivalists rifle and having raul as a companion will slow down the breaking of your weapons espescially when he has full maintenance. If your using the reg service rifle, then fully mod it. if you have raul, idk if it does but i assume it does since plenty of effects seem to stack, then is reg miantenance perk which gives 50% better condition plus the 50% better condition from forged reciever mod should in theory give you 100% better condition. idk never had raul when i had my service rifle, always had a better gun by then like trail carbine or Marks carbine or whatever