What is NOT a meme, and is useful, but counts as fully automatic, is three or five round burst mode.
Chainguns are a type of autocannon. Heavy, light and general purpose machine guns, like the M2 Browning, M249 SAW, Maxim, Been Gun, MG3, etc... none of these are chain guns. They aren't even all belt fed, which I guess is what you meant to say because I think that's the videogame midwit lingo. They are nonetheless universally accepted as being very valuable in combat.
AIUI, chainguns are any externally electrically-actuated automatic weapons, including both single-barrelled and rotary designs, some of which are autocannons, and some of which are lighter (machine guns).
But, yeah, it makes more sense if GP intended to refer to belt-fed weapons, not actual chainguns.
Most recently, it was Russia dropping 2-round burst on AK-12 after experience with it in Ukraine. M4 is another famous historical example.