A lot of the research involved in this area depends on the assumptions and definitions that one necessarily uses to produce research.
A perspective that uses culture will often shortchange technology and genetics, a perspective focusing on history will shortchange culture and perhaps ideology, and so on. A socio-biologist's take is naturally quite different than that of a criminologist and that of the psychologist.
There is every reason to suspect that violence is different between intra-society and between societies and that intra-societal violence is different between groups in society and individuals in society.
So do firearms enable mass violence?
In the case of intersocietal warfare, there are far better tools for mass slaughter than small arms. Artillery, tanks, torpedoes, missiles, aircraft, machine guns, explosives of all sorts including nuclear and their delivery systems are better attuned to mass mayhem on a society versus society scale and virtually none of these is available to average Joe on the streets. So whatever insights might apply to criminal use of firearms is absent when dealing with large scale destructive weapons between organized national militaries.
In the case of organized groups fighting within societies, once again, small arms such as rifles and handguns is also not a condition to spur massive outbreaks of violence. Riots, for example, often result in improvised weapons being used rather than firearms. What appears more likely is that societal conditions require breakdowns in authority or authority purposefully withdrawing to allow ethnic/religious/racial hatreds to run amok. The Hutu/Tutsi and other genocides often employed other means of violence than firearms and sometimes firearms were only used for a coup de grace.
I would argue that gang members are more akin to civil war between rival groups in society where their norms of behavior become the gang's rather than society and membership alone in a gang demonstrates the likelihood of criminal violence. Their decision was made when they joined the gang rather than on an ad hoc basis. Thus, gang membership composed of criminal members should be a disqualifier for firearm acquisition.
Finally, violence at the individual level can occur with or without firearms as can familial and domestic violence. While firearms certainly facilitate certain crimes like robbery, they are not necessary to do so as robbery is as old as the hills in human affairs and robbers will use whatever is available to threaten their victims. Instead, I suspect that habitual criminals, living as they do, more or less swim in a criminal society and ironically because they have even less trust in society and cannot resort to the law to settle differences, they carry firearms as much for self defense as for committing future crimes. What matters at this level is the willingness to inflict violent harm on others to get what one wants regardless of morals or laws--e.g. criminals or outlaws as they were called. Literally it is a willingness to live one's life outside of the law and society for perceived rewards in power, money, and status. As GunnyUSMC's seized weapons indicate, criminals want to have status just as others do and thus want whatever is perceived to be a "good" gun. A firearm then often becomes a sign for criminals, just like military officers are given a sidearm, of status above the norm and self defense against other criminal enemies, whether or not they use it. As seen in London, criminals will quite willingly revert to knives, acid, or illegally imported or homemade firearms, to facilitate their criminal activities.
That leaves violence at the individual level which is either from private or public grievances against society. This is the sort of individual that can commit either terrorism or mass shootings depending on how you view their grievances. Firearms for them is a means to a end so that they can revenge themselves on the world on society through their victims but there is much to suggest that any other sort of massively destructive weapon would suffice as well for these folks. E.g. the Columbine mass murderers failed to have their primary weapons explode which were improvised IED's--the firearms were actually fallback. Suicide bombers are a similar example. For a long time, a guy blowing up a school with dynamite due to disgruntlement over taxes was one of the largest mass casualty events in the U.S.
Then there is mental illness that can result in firearm violence but only the sort that either a) a person uses one to commit suicide or b) employs one to fight imagined demons in their head (Colorado and Arizona psychotics among others). This is often overlaid with substance abuse so at some point these people can become literally monsters as their brains literally do not function normally at this point. In most cases, their dangerous nature had already been revealed by prior run-ins with the law/courts and/or psychatrists/social workers before they executed their fantasies of violence but often in these cases, the public safety bureaucracy dropped the ball in various ways either on committal delays, paperwork filings, or reluctance to create a record that would have barred them from legally acquiring firearms. What is unknown is exactly how many other monsters of this type were prevented by law enforcement and the mental health workers from doing something similar.
The question then of "firearm violence" (leaving aside conditions of war) becomes quite a different analysis when one takes the firearm as simply a tool that can be used or not used depending on the nature of the perpetrator of the violence. Different perpetrators of violence will have quite different strategies in obtaining one and their cost benefit analysis will be quite different as well. In some cases, it might make things easier for the perpetrator--e.g. robbers, suicide, or domestic violence but the absence might very well cause the perpetrator to substitute means of mayhem. Regarding terrorism, mass violence, maybe gang violence, or that caused by mentally ill individuals, the methods employed to deny them access to firearms would be different. Terrorists, like those with mental illness or grudges, rarely just "snap". There is usually a long train of behavior that takes place as the person "girds" themself to do something awful--they essentially build up to the event by planning, talking, and even practicing what they plan to do. Terrorists are more likely to have communications with others about doing something awful as an aid to planning. Mentally ill or grudge killers usually demonstrate planning as well as threats. Those with mental illness often are substance abusers as well and have key people in their lives fail to blow the whistle before it is too late.
Most of the small remnant of firearm violence between individuals involve a relatively rare condition where either a) someone (not a career criminal or gang member btw) escalates a personal fight using a firearm with a stranger, uses a firearm inappropriately as alleged self defense against a perceived threat including shooting after the threat legally disappears, or an interpersonal argument (aside from domestic violence), sometimes longstanding, sometimes fueled by substance abuse, that degenerates into a deadly fight. These are relatively rare but often are emblazoned across the media as an example of how people just "snap" or "misuse" firearms. In almost every case, the media reporting is shallow, fails to take into account the victim's behavior prior to the incident, and gives an almost cursory dismissal to the story if the facts prove to be far messier than the just "snapped" narrative.
Then there are legitimate cases of self defense where the tool used is irrelevant whether or not it is a firearm.
Yeah, that simple question about firearm violence, turns out it is rather complicated after all.
