There are a couple of maps that are pretty infested, but most have multiple/covered routes and you start with access to smoke and gas grenades. Snipers also tend to tunnel-vision. Most maps devolve into one big furball, and most snipers are just going to sit scoped in on that. Approach on a curved route that stays in their peripherals or out of sight entirely and you can usually get the drop on them. If it's a big nest of people in tight quarters, Assault is great. Once I got the OP shotgun I just started running full balls-out: approach like that, gas their position, mask up, rush in and shoot + stab until I go down. With the confusion and broken close-range damage you can usually get two or three before you go down, sometimes even pick off a couple and keep running.
Otherwise, grab an unscoped bolt action and the sniper shield, range properly, and aim for the scope twinkles. A lot of them have shit aim anyways, so they probably won't hit you even if they see you missing shots.
As a tip, you can turn up/down the ironsights zoom level on your weapon options for each weapon in each loadout. I keep a max ironsights zoom Recon loadout specifically for counter-sniping. It's also helpful to turn pistol/shotgun zoom down to 1.0 instead of 1.25, IMO. Some of the MGs also give you the option to chose between ironsights and AA sights.