land_collision_start - does it just not work very well?

I'm currently working on a device which involves slow-moving projectiles which slow down even more when hitting things. One element is that they should certainly not bounce upwards when hitting the ground.

I have collision_start and land_collision_start events which apply a negative impulse proportional to current velocity and mass. It seems to work perfectly well when hitting objects, but on hitting land, half the time the projectiles bounce straight off and in fact continue bouncing for some time.

Is this event just simply not working properly? Any good workarounds?