On April 10, 1862, Confederates refused a formal demand to surrender because enemy guns were a mile away, more than twice the effective range of the heavy artillery of the day. When Union troops opened fire, their new rifled cannons fired spinning projectiles that bored through the masonry walls with alarming effect. By noon on the second day, the southeast angle had gapped open, and continued bombardment threatened the main powder magazine. Realizing the hopelessness of the situation, the Confederate commander surrendered. For two months, Union troops had built 11 artillery batteries along the shore of Tybee Island, and rifled cannons there caused massive damage to the fort.