It actually is a knife

Sorry to break the news.. I thought it was punch at first also.
Play in 3rd person and look closely. Your guy pulls out his knife very quickly and you can clearly see the knife.
In 1st person, however, it does look like a punch and doesn't make the sound effect. I suppose that's because everything is sped up and it changes the sound effect;when holding a care package marker - you run faster, knife faster, and jump farther.
If anyone wants to have a 'punch' free for all private match on PS3, add me. My PSN is BEAST_SantaClawZ
Still, quick knife nearly as effective as tactical knife. Commando, lightweight, and marathon would own with this.
Note this works with all 'airdrop' killstreaks, so if you want all 3 killstreaks to enable you to punch use care package, sentry gun, and emergency airdrop

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+REP if this post helped.