New bit of info to think about on foregrips as well. Got this from a friend who is a NG armorer. He has been talking with some of the guys from our local SF group who have rotated back home from Afganistan.
He said that they have had a few guys put sufficient downward pressure on the Knight foregrip that they actually broke the locking lugs while the M4 was being fired.
I'm not at all familiar with the Knight system, but I guess that there are 2 kinds, one that mounts like regular handguards, and one that screws on like a float tube. This problem was with the first one.