The way I see it, hardcore gamers are people who have no life beyond gaming. It has nothing to do with the genres they play, all that matters is that they play games all the time, with possible pauses for sleeping/eating.
Most people who claim to be hardcore gamers are either mainstream or, for lack of a better term, "core" (or "true") gamers. Mainstream gamers being ones who only play what's popular and nothing else (even going as far as hating hardcore/core games for completely superficial reasons), while "core" gamers are ones that play games that they find to be of high gameplay quality (amongst a few other things I don't want to waste paragraphs on
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This also brings the need to qualify hardcore games... The way I see it, hardcore games are challenging games. That can mean a lot of things, but to put it simply it's either games with a very high learning curve, massive depth in gameplay, games that are designed to be super time-consuming or really really hard games (as long as it's hard for the right reasons). That's what the term "hardcore game" meant the first time it was used and there's no reason it shouldn't still mean that. You don't need to be a hardcore gamer to play these though
In my mind, there aren't many hardcore gamers out there, and being hardcore isn't exactly healthy or something anyone should brag about.
Most people who claim to be hardcore are mainstream gamers who wrongfully believe that playing Call of Duty makes them hardcore, from what I've seen.