How high was the FPS when recoring that? Could be
related to the inaccuracy in low framecounts.

Also try to use a bigger and simpler shape for the grenade hull.
This might help.

Some small recent games of mine:
Griefer 4k http://damocles11.byethost9.com/griefer4k.html
Kobold 4k http://damocles11.byethost9.com/kobold.html
Cave 4k http://damocles11.byethost9.com/cave4k.html