"And are they really able to defend themselves by buying guns? Lots of people think so but is it true?"
Having spent 30 years as a cop, and witnessed the reactions, thought processes and abilities of trained officers under stress, I can honestly say even some members don't react appropriately in the moment.
Most people out there have the false confidence that, should violence occur such as a shooting, they will be the superhero, take perfect aim and eliminate the threat. Not even close to reality.
In some rare instances, given some time between alert and action, a few will manage, but for most of those situations where no prior warning is given (eg mass shooting), effective response by an untrained, unprepared citizen is a fool's dream.
A couple of the latest examples is the young fellow who called 911 for police assistance, and ended up being shot by the responding officer, and the female officer who, tragically, mistook her firearm for a taser. Even the best and the trained (although who knows how "best" and trained they actually were) make mistakes under stress.