Synonyms for Bullet-headed (same or very similar meaning)
WordNet sense 1 (obstinate and stupid):
bullheaded, pigheadedWordNet sense 2 (tenaciously unwilling or marked by tenacious unwillingness to yield):
obstinate, stubborn, unregenerateWordNet sense 3 (having a short broad head with a cephalic index of over 80):
brachycephalic, brachycranial, brachycranicFrom the ODE community, based on WordNetadd/edit