Thought I'd help clarify why the Marine Corps taught us to initiate with a closed-bolt weapon. I saw someone state the fact that if you initiate with an open bolt weapon such as a SAW or 240, you run the risk of the bolt slamming home on an empty chamber, (bingo) leaving a partial team/squad to realize what just happened and only one or half your men initiate opening fire, because as you know, the SAW cycle of operations starts with "feeding" whereas the M27 starts with "firing." Also, now the enemy knows where you have emplaced your heavier "squad indigenous firepower"...so where do you think their focus is now? So yes, idealy, claymores, on-call IDF targets, whether mortars or arty, or even 40mm or some rockets, would be your best bet. And yes, usually a squad/team leader initiates.
FlagdayNCO sort of hit the nail on the head with the replacement of the SAW with the M27 IAR. Contrary to popular belief, the SAW is still very much incorporated with teams and squads in the infantry marine corps and aren't going anywhere. I was lucky enough to carry both, but the M27 gives you the ability to initiate an ambush with fully automatic capabilities and almost none of the risk of a misfire. I can count on one hand how many IA drills I had to run clearing a double-feed/misfire in its dirtiest condition. Not to mention now you have more suppressive fire capabilities in a team and it's the easiest weapon to clean(the short stroke gas piston eliminates most carbon build up in the gas block). If the funding was there, the M27 would replace the M16/M4 altogether, but we all know the USMC does more with less and is the go-to for hand-me-downs
Might hurt some feelings with this one, but the M27 nearly gives DMR capabilities to every team carrying one(without a can on it, obviously). Most versatile weapon in an infantry platoon if ya ask me.