MCRP 12-10B.1, page 190

c. Once the door is open, Shooter Number Two immediately throws a hand grenade into the room. Shooter Number One maintains cover for Shooter Number Two as he/she prepares to throw the grenade into the room. Shooter Number Two then returns to the original stacked position behind Shooter Number One (Figure A-32). If Marines have no grenades, the procedures remain the same except for throwing the grenade. Note: When the door is already open or nonexistent, the stacked position is always used. WARNING Because fragments from M67 fragmentation grenades may injure Marines outside the room, they should not be used. Marines should use MK3A2 offensive hand grenades instead. Also, the technique of cooking-off hand grenades can be deadly unless properly performed. Figure A-32. Throwing a Grenade Into a Room (1) Throwing Grenades. Two techniques are available for using grenades in room clearing. The preferred technique for Marines is to throw a concussion hand grenade into the room so hard that it skips and bounces, making it difficult for the enemy to pick up and throw back. The skip/bounce technique should be used by Marines during training and combat. The least preferred technique is to cook-off a concussion hand grenade by removing the grenade’s safety pin, releasing the safety lever, counting off two seconds by thousands (one thousand and one, one thousand and two), and then throwing the grenade into the room. Cooking-off the grenade will be used only as appropriate during combat. (2) Nonverbal and Verbal Alerts. To alert all that a grenade will be thrown, a visual showing of the grenade is made to assault element members, and a visual acknowledgment from them is received. A nonverbal alert may ensure that the enemy is surprised when the grenade is thrown. If the situation demands, a voice alert can be used, but the element of surprise may be lost. When or if a voice alert is used, the voice alert is “FRAG OUT;” when an enemy grenade has been identified, friendly forces shout, “GRENADE.” This allows Marines to distinguish between warnings for outgoing and incoming grenades. Military Operations on Urbanized Terrain A - 29