In the US, Flight Levels don't start until FL180 - ~18,000 feet AGL (above ground level), using a standard barometric pressure of 29.92 inches of mercury (which is why it varies based on current atmospheric conditions).
Mostly only small turboprops or short-haul passenger jets use flight levels below 24. Commercial air carriers generally cruise between FL250 and FL340. Business jets such as Learjets are capable of flight levels of 450, and some military jets cruise much higher.
Bullet damage != explosive decompression. A small hole in the fuselage will most likely be more than compensated for by the excess capability of the pressurization system, which pressurize (in most cases) to 8000 feet MSL (above mean sea level), not FL100 (which does not exist - flight levels are only for Class A airspace, above 17,999 feet MSL).