I don't think the L115A3 or M107 will make an appearance, we have a lot a 50BMG rifles at the moment in A3 and the AWM (wich is my favorite long range rifle) was present into Arma 2 OA. BIS at my opinion is searching for "futuristic" rifles, with chassis and everything, I expect more a MX338 type rifle like the NEMO 300 or the Noreen Bad news. But they can also put a modular type bolt action just like the MRAD, the MSR, the SRS, the Ballista...
Just remember that BIS now releases modules with their DLC, it's not only content, so we can maybe expect an advanced ballistics implementation wich will be awesome.
I've asked Dwarden for maybe some details but nothing can be revealed at that moment, we should wait next January when the Devs come back.