Really unsure what your question is- BUT-
Firing before bolt is fully closed is known as "firing out of battery" and is VERY bad. The breech of the weapon serves to contain a great deal of force. FOOB means breech is not locked. This can hurt YOU (forget about hurting the rifle!)
When you pull trigger of a semi-auto, it fires, extracts, and loads a second cartridge, faster than most human reflexes. The disconnector prevents the weapon from dropping the hammer again (you still have trigger pulled back, remember?) It MAY fire full auto, but if hammer is riding the bolt closed, more likely to not fire at all (soft strike on primer)
With an EMPTY rifle, draw back bolt, pull trigger, release trigger. Let bolt go forward, pull trigger again. No CLICK- hammer was released when bolt was back. STOP, take rifle to gunsmith.