The M203 is seen mounted on the Commando assault rifle in nearly every mission it is available in. It has a very similar reload animation and the same aiming system as the Modern Warfare games. It was seen in the E3 demo of the game. The level that had the grenade launcher was "Payback". The grenade launcher can also be found mounted to several other assault rifles in single-player, such as the M16, FN FAL, etc. (but not the AK47 or Galil which utilise the GP-25).
Treyarch has taken many steps to balance the M203; it costs 3000 CoD Points, which is more than most attachments and cannot be used with the perk Warlord, like all other under-barrel attachements. Its grenades are not replenished by the Scavenger perk and there are no explosive damage increasing perks like Danger Close, Fireworks, or Sonic Boom. However, the perk Flak Jacket returns. It also cannot be used for the first 5 seconds of the start of a round, to prevent players from shooting grenades across the map at the enemy spawn point. This applies for all grenade launchers. This grenade launcher is smaller than it appeared in Call of Duty: Modern Warfare 2.
When attached to most weapons, the weapon gains some visual differences, normally a heat-shield. When the weapon it is mounted on has camouflage applied, the very end of the M203 will have a lighter tint of the same camouflage. This has been confirmed on the Xbox 360 and PS3 version of the game.
The M203 is only available on the Pack-a-Punched version of the M16, the Skullcrusher. It has 9 grenades at highest. It applies somewhat high damage until approximately round 23, where it loses its ability to kill with one hit. One interesting note is that it does not have a 'safe distance', meaning that it will explode at any distance. Therefore, PhD Flopper is highly recommended, as even if the player would survive the explosion itself, one will still be stunned for two seconds, and during that time the player will be very vulnerable to zombies' attacks