OH I see what you are going for.
Yes if everyone is toting an M4 and you have a M14, G3, Fn Fal then everyone knows you have a 308 and you are the one that is the long shooter and that, the Saw, Brass, and the bolt action make anyone carrying it a target.
There have been some talks about using the AR-10 as the marksman rifle. Same AR profile but in 308 instead.
Ammo is ammo. And the M249 shoots ball, tracer and AP (armor Peircing) and all could be a benefit for a marksman.
AP to get through bunker wall or building wall, FMJ is your standard stuff, and the Tracer could be used to illuminate a target for the rest of the squad to shoot at. (Army officers carry a Full 30 round mag of tracers for this same reason)
I was never a fan of the tracer, but in ROTC they taught us to load three rounds then a tracer and load the rest of the mag. That way when a tracer poped out you knew you had three rounds left in the gun.
As far as parts, any thing that is issued should have parts in field. M1A is still made and parts are available.