On May 17, the First Missouri Brigade was part of a rear guard holding the crossing of the Big Black River. However, a Union charge broke the Confederate line and routed the defenders in the Battle of Big Black River Bridge. The regiment then entered the defensive works at Vicksburg, which were then besieged by Union forces. On May 18, Cockrell's brigade was engaged in a small action near Mint Spring Bayou. This forward position was determined to be dangerous due to a large ravine between it and the main Confederate line, so it was abandoned on the 19th. The 3rd Missouri Infantry was used as a reserve when Union forces attacked on May 19. On May 22, the men of the regiment manned the Confederate line at a point known as the Stockade Redan, where they, as well as other elements of the First Missouri Brigade, fought off Union attacks against the position. When the attack failed, a number of Union soldiers were trapped in a ditch in front of the Confederate position. After the Union soldiers refused calls to surrender, men of the 3rd Missouri Infantry secured a number of artillery shells, lit the fuses, and then threw the explosive shells into the Union position as improvised hand grenades.