According to Etymonline, ammo has been used as a shortened form of ammunition since 1917. Why does the shortened version end in o instead of u? The only reason I can think of is that it matches other shortened forms like info and photo, but information and photograph actually have an o in them, so I'm not sure that's it. Does anyone have any insight into this?
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:
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.