Also does the firing pin not reach the primer until the bolt is fully closed? I tried to figure what keeps the AR from firing on an open bolt?
The firing pin does not reach the primer until the bolt has locked into the barrel extension. When you disassemble the bolt carrier group to clean it, look at how the bolt interacts with the rest of the BCG. As the bolt locks into the locking lugs, it is pushed backward and makes a 1/8 twist. This shortens the over all length of the firing pin channel and allows the firing pin to reach the primer. This mechanism prevents the AR15 pattern rifles from firing without the bolt being locked closed.
This video from Brownell's might help: http://www.brownells.com/.aspx/lid=11417/learn/