Page 3 of the version 6.0 TO:AUE errata document lists an errata for Field Guns, page 123:
Under “Game Rules”, second paragraph, last sentence
Ultra and Rotary Autocannon Field Guns are immune to jamming or fire control failure effects, and Gauss-based Field Guns are
immune to weapon explosion effects.
Change to:
Field Guns are immune to jamming, fire control failure, and ammunition or weapon explosion effects. All autocannons may use
special ammunition using the appropriate rules, though the entire platoon must use the same ammo type.
Checking my 6th printing PDF, this errata does not appear to have been applied, and should be applied in the next printing.
As a secondary question, is the section stating "the entire platoon must use the same ammo type" intended to apply to a single turn of fire? It would otherwise contradict an earlier line indicating that individual tons of Field Gun ammo may be of various types. If so, the last sentence could be changed to something like "[...] though the entire platoon must use the same ammo type when declaring weapons fire for the turn." to make this clearer.