There is no difference between metaphoric and metaphorical, but the latter is more common than the former. Unlike similar word pairs such as historic–historical and economic–economical, metaphoric and metaphorical have not undergone differentiation. They are more like the undifferentiated ironic–ironical, except that in this case the -ic word is the unnecessary variant (whereas ironical is considered an unnecessary variant of ironic).
Related
Metaphoric will continue to appear as long as long as the -ic/-ical suffixes remain tricky (i.e., forever), and a few writers will always favor it for its brevity, but metaphorical prevails for now.
Examples
For example, all of these major publications favor metaphorical over metaphoric:
There is nothing metaphorical about these stomach-churning preparations. [Guardian]
Some guys break their nose hitting the metaphorical wall of their careers. [Toronto Star]
The phrase “shake the monkey off your back” is almost always used in a metaphorical sense. [Sydney Morning Herald]
Chastity belts — even metaphorical ones — hinder rocking. [Los Angeles Times]
Ngram
This Ngram charts the use of metaphoric and metaphorical in English-language books published in the 20th century.
