The M249 exists in most games in two versions:

Either the M249 Para or the M249 SAW. The Para is konfigured for paratroopers mainly, this means, it has a shorter barrel, which makes it more easy to use in CQB environment.

The SAW (means: Squad Automatic Weapon) is used mainly for supressive fire, the barrel is longer. This makes the SAW more heavy than the Para, but it also gives a better acuuracy when firing in full auto mode.