I can clear the clumped up zombies faster with the m60 than with a grenade. Tap firing at head level just wrecks house. The problem is that I start a reload and my buddy is then covering the next batch of zombies at the wall. Rinse and repeat. I'm just looking for some brief pauses of 10-20 seconds in the spawning to do quick repairs or what not. When the zombies batch up they really do some serious damage. Add a demolisher in there and your defenses are just plain gone and you're into backup plan A, B, C, etc.
The alternative is what we've been doing which is just cleaning out a POI that we have a quest in, prep it, get it wrecked, and then just start the quest to fix the POI.