MCRP 12-10B.1, page 185

Figure A-26. Hand Grenade Thrown Through Window WARNING After throwing the grenade, the Marine should immediately announce, “FRAG OUT,” to indicate that a grenade has been thrown. He/She then takes cover because the grenade may bounce back or be thrown back, or the enemy may fire at him/her. g. If Marines must enter the building by using the stairs, they first look for boobytraps. Then they throw a grenade through the stairwell door, let it detonate, and move quickly inside. They can use the staircase for cover. h. The best way to enter a building is to breach the exterior wall. Again, a grenade should be thrown through the hole while using all available cover, such as the lower corner of the building (Figure A-27). i. Another way to enter a room is to blast mouseholes with demolitions. In moving from room to room through mouseholes, Marines should use grenades to clear the room as they do in moving through open doorways. j. Although buildings are best cleared from the top down, this is not always possible. While clearing the bottom floor of a building, Marines may encounter stairs that must also be cleared. Once again, grenades play an important role. Before climbing the stairs from the bottom floor, Marines should first inspect for boobytraps then toss a grenade to the head of the stairs, if appropriate (Figure A-28). The use of a grenade in these situations is based on stair/building construction and building layout. Throwing grenades up staircases is extremely A - 24 MCWP 3-35 .3