Why is "ammunition" shortened to "ammo" and not "ammu"?

Solution 1:

I think it's part of a larger pattern in English in which long words truncated at the second syllable often pick up an -o ending instead of retaining the natural vowel sound and spelling that occurs in that second syllable. Here are some examples:

aggro [aggravation/aggression]

ambo [ambidextrous]

ammo [ammunition]

camo [camouflage]

combo [combination]

convo [conversation]

distro [distribution]

invo [invitation]

limo [limousine]

mono [monaural/monophonic]

provo [provisional]

Valpo [Valparaiso University]

When a phenomenon occurs repeatedly, as this one does, it suggests that the tendency to favor -o endings in truncated words over -a, -au, -e, -i, -ou, and -u endings reflects a genuine, though informal preference in the language, at least for the moment. I don't know whether this particular phenomenon has been discussed in a scholarly setting, but I wouldn't be at all surprised if it has.