The reward received depends on the player character's Barter skill and an added in random factor. The following table lists chances of getting every possible reward.
The Lone Wanderer says they will give an "ice cold bottle" of Nuka-Cola in the dialogue box, but only a regular Nuka-Cola is taken from inventory and is required for the quest.
If Zip trades a Nuka-Cola for 5.56mm rounds the dialogue box refers to the ammo as .556 rounds.