Check out Stalker Anomaly, that should fit the bill.
Gaming
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Or if they want to torture themselves, G.A.M.M.A.
Or Stalker Gamma which is a huge modpack on top of anomaly
You're likely thinking about Stalker: Shadow of Chernobyl. I played it and a sequel (Call of Pripyat) recently on semi-modern hardware, but YMMV. I loved Stalker back in the day.
I haven't personally tried it, but I've also been watching Chernobylite on Steam with interest. It seems like it may be intended to be a spiritual successor to Stalker. It's been on sale recently, too.
There was one called F.E.A.R that fits the bill
Great game that is a horror shooter, but I wouldn't call it a MilSim though. MilSims are usually games like Arma or Squad.
Yeah, it has bullet time mechanics.
Which are totally badass ultimately lol