Honestly, they feel fairly useless overall.
SMG's, AR's, and LMG's are all equally if not more effective at mid-range where the marksman rifles are supposed to shine while still retaining far superior short range power.
Honestly, the only one that seems to have any value is the SVK since it can fairly consistently 2-shot enemies with high powered ammo...though lose most of your mag size.
I've been playing with the AC-42 a lot more recently, especially single shot, and I genuinely don't see a reason to use a marksman rifle over it ever again. It doesn't quite have the same damage at range, but it remains stupidly accurate even when firing quickly. With the bonus of it still being more effective at shorter range engagements.
VCAR I don't get as a weapon in this game in the slightest. I honestly haven't the foggiest clue what niche it fills that isn't filled better by multiple other weapons.