I’m guessing that I know different Marines than you know. Just a hunch...
Doesn’t matter how tough they are - once they run out of ammo, they have short, not very ergonomic, spears and armor that isn’t very effective against blades, bludgeons or arrows.
Unless they made friends (marines are pretty good at that...), it will be over very quickly and very messily - esp. if they ran into Mongols.