COD: Modern warfare

Hey guys, i bought the new MW a couple of days ago and i have been running into so many people using the 725 shotgun in multiplayer i just want to know how can i counter them? They just pop out of no where and oneshot me and i think thats not fun nor healthy for the game. Any tips will be appreciated! ^_^

I bought the game for PS4 kinda regret since I've been playing PC for the past 3 years, now if you play competitively the shotguns are disabled which is a plus since they are pure bullshit anyways. I usually just play gamebattles you go against actual good people.