On board the USS Monadnock in action during several attacks on Fort Fisher, 24 and 25 December 1864; and 13, 14, and 15 January 1865. With his ship anchored well inshore to insure perfect range against the severe fire of rebel guns, Dunn continued his duties when the vessel was at anchor, as her propellers were kept in motion to make her turrets bear, and the shooting away of her chain might have caused her to ground. Disdainful of shelter despite severe weather conditions, he inspired his shipmates and contributed to the success of his vessel in reducing the enemy guns to silence.