also i read this from http://gtwy.hl2arena.com/
01:10:14] [Hitman8] Grid Near-term Milestones
[01:10:14] [Hitman8] The following milestones are steps along the way to a full September beta release. This information is proprietary and confidential and is not to be shared outside Valve and...
get the feeling they have set breaking points, and the amount of breakage allowed.
I also think projectiles with a higher kinetic energy rating will cause greater breakage
Guessing ofcourse :)