Jessica (left) and Kalani Windham leave flowers and a candle outside Walmart, near the scene of a mass shooting which left at least 20 people dead in El Paso, Texas. File/Agence France-Presse

Lacey Wallace, The Independent

Americans own nearly half of the world›s guns, with approximately 120 firearms for every 100 US residents. Gun control policies may someday restrict new gun sales. But what impact can they have when Americans already own millions of guns?

Some have pointed to gun buybacks as a potential solution to this problem and in the wake of two distressing shootings in El Paso, Texas and in Dayton, Ohio, the issue is bound to be raised again.

I have spent years studying American attitudes toward guns and gun policies, including smart guns and open carry. I know that gun owners feel strongly about their identities as gun owners, making it difficult to create a strategy for taking guns off the streets.

The sheer number of guns is part of the challenge. The US has the largest civilian-owned stock of guns in the world. At the end of 2017, the Small Arms Survey reported that there were an estimated 393m firearms in the US - and that›s not even counting guns owned by the police and military. That represents 45.8 per cent of the world›s civilian-owned guns.

More than 40 per cent of US adults live in a household with at least one gun. About half of all civilian-owned guns in the US are owned by just 3 per cent of US adults. These gun owners have an average of 17 guns each. Most other gun owners average about three guns at home.

Gun buyback programmes are designed to reduce the number of firearms by purchasing guns from private owners, and typically destroying them.

Following a mass shooting in 1996, Australia banned automatic and semi-automatic rifles and shotguns and instituted a national gun buyback programme. In a year, Australia purchased about 650,000 firearms from private residents, estimated to represent about 20 per cent of the country›s privately owned guns. Research evaluating the effects of the buyback found a 42 per cent decrease in homicide rates and a 57 per cent decrease in suicide rates in the seven years after the legislation passed. But some researchers are still uncertain whether this decrease was due to the buyback, or whether it was simply part of an existing downward trend.

US cities have experimented with buybacks on a much smaller scale, even though the Pew Research Center reports that more than 70 per cent of gun owners say they could never imagine themselves not owning some sort of firearm.

One of the earliest examples occurred in Baltimore, Maryland. In 1974, Baltimore police paid residents $50 per firearm, collecting roughly 13,500 over a two-month period. Rather than reduce crime, homicides and assaults spiked during the buyback. It is unclear why, but two months is a short time period for a clear pattern to emerge and crime rates in cities across the country were increasing through much of the 1970s.

Baltimore is not unique. A 2008 review of the existing research by Matthew Makarios and Travis Pratt in the journal Crime & Delinquency found that gun buyback programmes have generally been ineffective in reducing crime in the US. Challenges include the types of guns purchased, the involvement of law enforcement, and the costs involved.

Gun buyback programs often place no restrictions on the types of guns that can be purchased. Civilians frequently bring in old firearms, guns in disrepair, rifles, or shotguns. Sacramento, California, implemented a gun buyback program in 1993. Nearly a quarter of all guns submitted were not in working order.

The Boston Police Department also attempted a gun buyback programme in 1993 without a restriction for weapon type. Only about half of submitted firearms were handguns. That›s significant because we know from existing crime data that although some mass shooters use more powerful weapons, handguns are the type of firearm most often used in violent crime and in youth violence. If the goal is to reduce crime, getting shotguns or broken firearms off the street will likely have little effect.

Guns obtained through a 1994 to 1996 buyback in Milwaukee also differed from those typically used in suicide and homicide.

The Boston Police Department tried again in 2006. Learning from their past mistakes, the police offered a $200 gift card for each handgun - but no cash or gift card for rifles or shotguns. At the conclusion of the programme, the Boston Police Department reported that more than 85 per cent of submitted firearms were handguns, closely matching the types of guns used in crime.

The number of shootings decreased by 14 per cent in Boston in the year after the buyback and continued to decrease through 2010.

Other jurisdictions followed Boston›s example. In 2015, 13 police departments in Massachusetts instituted a buyback programme with higher amounts paid for types of firearms more frequently used in crime. As a result, they were able to collect more handguns. But three out of five people who sold their guns said they still had one or more guns at home.

Experience shows that some people will attempt to profit from gun buybacks by submitting inexpensive or broken firearms worth less than the cash incentive offered through the buyback.

In Baltimore, one buyback participant claimed she was going to use the buyback money to purchase a larger weapon.

In Oregon, private citizens waited outside the gun buyback locations to purchase firearms and ammunition from owners before they could go inside to submit them to law enforcement.

Gun buybacks are financed by taxpayer dollars and are generally paid for by local agencies rather than through state or federal funding. A local jurisdiction›s budget will limit the amount of firearms it can purchase and destroy, reducing the likelihood that a gun buyback will have an observable impact on local crime rates.

Typically, gun buyback programmes are run by law enforcement. Understandably, criminal offenders may be hesitant to come to the local police station or interact with law enforcement - even if they are promised exemption from prosecution for weapon possession. Boston attempted to address this concern in 2006 by designating sites like churches as drop-off locations. Other jurisdictions have held gun buybacks run by nonprofit groups, but law enforcement officials are frequently on-hand as security, or to help take the guns to be destroyed after the buyback.

So far, gun buybacks in the US have been a community-based, grassroots endeavour with limited impact. Their feasibility on a state or nationwide scale is unclear.

Cost alone may be a prohibiting factor. Assuming a $50 per firearm incentive, reducing the US gun stock by 1 per cent would cost $196.5m. Inevitably, only some of the guns purchased would have been used in future crimes.