MCRP 12-10B.1, page 174

between buildings offer good fields of fire to the enemy. Properly positioned friendly weapons can suppress and eliminate enemy fire. The M203 grenade launcher is effective in clearing the enemy from rooms inside buildings (Figure A-22). Figure A-22. Employment of the M203 Grenade Launcher b. A Marine scaling a wall with a rope should avoid silhouetting himself/herself in windows of uncleared rooms or exposing himself/herself to enemy fires from lower windows. He/She should climb with his/her weapon slung over the firing shoulder so that he/she may quickly bring it to a firing position. He/She should clear the lower room with a hand grenade before going outside the window. Before doing this, the Marine first loosens the safety pin so that he/she needs only one hand to throw the grenade. The objective upper story window should not be entered before a hand grenade has been thrown in. c. The Marine enters the objective window with a low silhouette (Figure A-23). Entry can be head first; however, a preferred method is to hook a leg over the windowsill and enter sideways, straddling the ledge. 16. Rappelling. Rappelling (Figure A-24) is an entry technique that Marines can use to descend from the rooftop of a tall building into a window. (See Army manual TC 21-24 for more information on rappelling.) 17. Entry at Lower Levels. Buildings should be cleared from the top down. However, it may be impossible to enter a building at the top; therefore, entry at the bottom or lower level may be the only course of action. When entering a building at the lower level, Marines avoid entry through windows and doors because both can be easily boobytrapped and are usually covered by enemy fire. Military Operations on Urbanized Terrain A - 19