First, the recently retired manager of the Douglas Long Beach Inorganic Chemistry section and his wife were sitting in first class where the damage occurred. There were two attendants close by, and both were yanked up by the rushing air. He grabbed the legs of one of them (since he had his seat belt on) and held her until the pressure stablilzed or there would have been two fatalities. The engineers at Douglas were part of the industry team that examined the failure. They found a number of small fatigue cracks on other similar Boeing aircraft in the same area. Apparently the design allowed a tiny weakness. All the remaining aircraft in service were retrofitted with an additional sheet of aluminum (a doubler) to handle the stress.
By the way, this failure was quite similar to the five Lockheed Constellations that went down in the early 1960s.
Even a 1/8th inch hole in the skin of a passenger aircraft will overpower the compressor and the aircraft will reach equilibrium with the outside quickly. If the bullet penetrated a simple skin area the main problem would be the whistling that would contribute to driving the passengers to panic while the aircraft dove to lower than 8,000 feet. However, if the bullet happened to hit in a high stress area, I can easily imagine the aluminum starting to tear, and once that happened the tear would continue so that anythng not tied down and close by would be sucked out.