Children’s toys no longer: a porcine model study of the lethality of modern air rifles

Air rifles have been long thought of as beginner’s or children’s guns in New Zealand and around the world with no significant lethal potential.^1,2 However, modern air rifles can reach muzzle velocities analogous to small-bore rifles such as a .22 calibre long rifle (.22LR). Recent incidents of potentially lethal thoracic injuries with homicidal intent in the authors’ practice sparked the need for an assessment of the lethality of these weapons that are available with no licensing requirements in New Zealand.¹

Determining the range at which these weapons can cause a lethal injury is important because this has significant effect on what harm can be done to the public with them and differentiates them from other non-restricted weapons. If these weapons are reliably lethal at more distant ranges they can be potentially used in a wider range of criminal activities, as well as the more expected accidental injuries.

Air guns can be classified based on their means of providing propulsion into compressed gas, spring loaded and pneumatic mechanisms. Compressed gas guns typically use cartridges of compressed carbon dioxide and are usually less powerful than spring-loaded or pre-charged pneumatic (PCP) weapons. Spring-loaded guns rely on the manual coiling of a spring, which usually requires a break barrel mechanism for higher powered weapons, while lower powered weapons such as BB guns and airsoft weapons also use a weaker spring-based system that can be manual or battery powered. The reservoir in PCP rifles can be pressurised by an external air source and many are able to be fired in a semi-automatic fashion.² In New Zealand, legislation was enacted in 2010 following the death of a police officer that made ownership of PCP rifles require a firearms licence.^3,4

We designed a porcine model study to assess the severity of injuries caused at intermediate ranges into a torso. Porcine models have been used in ballistics research extensively and in air rifles specifically. Porcine models are an effective approximation of human anatomy and can be used to determine the likely injuries that can be inflicted by these weapons.⁵ We aimed to produce qualitative data on what injuries were possible in the first instance by testing a complete porcine model. To further assess reliable lethality and to control for anatomical differences in the model, as well as misplaced shots, we designed a second model to quantitively assess penetration through a chest wall into a 10% ballistics gel medium, which has likewise been assessed as an effective model of human tissue.⁶ We hypothesise that contemporary spring-loaded air guns can reliably cause lethal injuries at an intermediate range.

Methods

Two porcine cadavers were obtained that had been used previously for basic biological research under The University of Auckland Animal Ethics Committee, approval 25159. They were euthanised with an intravenous overdose of sodium pentobarbitone while under anaesthesia. They were then transported and secured on an outdoor shooting range. The cadavers were measured at 46 and 48 kilograms respectively prior to starting the experiment. A Gamo Swarm Fusion 10X Gen3 Air rifle that was chambered in .22 calibre was utilised for testing (Figure 1). This is a spring-loaded rifle with a reported muzzle velocity of 975ft/s and a muzzle energy of up to 40J. Testing was performed with H & N Hornet pellets, which are 16.2grain (1.05g) with a brass pointed tip (Figure 2).

View Figure 1–5, Table 1–2.

Five pellets were fired at the anterolateral chest at 5 metres into the first cadaver, and five pellets were fired at 10 metres into a second cadaver. The pigs were shot in a variety of locations of the thorax to simulate realistic shooting conditions. Necroscopy was then performed, and the injuries assessed and recorded. Specifically, whether the projectile breached the chest wall or internal organs. In addition, the likelihood of a lethal injury based on an assessment of the injury pattern was recorded.

Following this, a segment of the chest wall was harvested that had not been injured in the initial shooting and was secured in place behind 10% ballistics gel. This construct was then shot five times at 5 and 10 metres respectively. The penetration through the chest wall and then through the ballistics gel was measured and recorded.

Results

Shots taken from 5 metres caused two likely lethal injuries, two potentially lethal injuries and one non-life-threatening injury. At 10 metres there was one likely lethal injury, one potentially lethal injury and three non-life-threatening injuries (Table 1) (Figures 3, 4). The chest wall was breached in 4/5 shots and 3/5 shots at 5 and 10 metres respectively.

On the ballistics gel component, the chest wall was measured at 45mm, which was added to the penetration through the gel to give a total penetrance. The chest wall was penetrated 5/5 times at 5 metres and 4/5 times at 10 metres (Table 2, Figure 5). The fifth shot at 10 metres fractured a rib, which deformed the ballistics gel component, but the projectile did not enter the gel. The range of total penetrance was 55–140mm and 45–125mm at 5 and 10 metres respectively.

Discussion

Air rifles have had significant proliferation in New Zealand in recent years. According to the New Zealand Customs Service, from 2012–2016 over 130,000 airguns were imported compared to 97,855 powdered rifles and 39,991 shotguns.¹ While this number includes low-energy air rifles such as BB guns, it sheds light on the magnitude of how many are coming into the country. Non-powdered weapons also make up a significant portion of weapon sales in the USA, with 2–2.5 million units purchased annually.⁷ According to the Accident Compensation Corporation (ACC) there was a mean of 235 active claims for air rifle injuries annually in New Zealand over the last 10 years.

A porcine model has previously been used to assess thoracoabdominal wounds from air rifles in 1995.⁵ The authors compared a common air rifle and air pistol fired into the thoracic wall, showing a breach of the wall in 8/18 shots between 0 and 5 feet from the target and causing a variety of significant injuries. The rifle used in this experiment used a smaller calibre (.177 vs .22) and slower velocity projectile (700ft/s vs 975ft/s), as well as the model being shot at a much closer range (1.5 vs 10 metres) compared to our study. Our study, however, builds on this with data on contemporary rifles at longer range that appear significantly more lethal.

Lethality or “stopping power” of a projectile is a multifactorial concept based on factors such as mass, composition, velocity and behaviour of the projectile.⁸ With increasing velocity and mass of contemporary projectiles we can expect increased lethality, which was demonstrated in our study.

Our research was inspired by observed cases in our professional practice that were notable for being attempted homicides using air guns. This is rarely reported in the literature, with unintentional discharges being the most common cause of severe injury.^2,9,10 As these weapons become more refined and other firearms become more restricted, it would be reasonable to assume that wounding with criminal intent may also become more common.

The most common air rifle pellet comparison is to a .22LR cartridge, which is commonly a 36–40gr projectile and fires at approximately 1,000–1,300ft/s with standard ammunition.^11,12 Air rifle pellets in .22 range from 16–32gr, which in high-end model spring air rifles can also be fired at 1,000–1,300ft/s.¹³ While .22LR is superior in terms of stopping power, modern air rifles are not dissimilar.

Previous lethal and potentially lethal injuries from air rifles have been reported in numerous case studies from around the world, including developing and developed countries.^14–16 Approximately one fatality is reported annually in the UK, and 33 were reported over a study of a 5-year period in the 1990s in the USA.^10,17 These injuries are across multiple anatomical regions, including great vessels, cardiac, pulmonary and cranial regions.^10,14,14,18 Children appear to be at particularly high risk of serious injuries from air weapons, which is likely a combination of easier access as well as less durable bony structures to penetrate.^7,19

Our air rifle was able to reliably penetrate the chest wall at 5 and 10 metres. Once the chest wall is breached, further life-threatening injury becomes possible and relies only on a shot placed over a vital organ in these regions. In that sense, this weapon is very likely to cause potentially lethal injuries at 5–10 metres with a thoracic shot.

The range of fire that homicides occur within can be estimated with powdered weapons based on the powder burn and residue on pathological samples to point blank, close (<1m) and distant (>1m) ranges.⁸ With the addition of crime scene data more specific ranges can be inferred, but this remains an inexact science. A study of gunshot homicides in Denmark over 14 years gave a percentage of combined point blank and close-range homicides at 31%, with the majority of injuries either indeterminate (41%) or at a range of >1m (28%).²⁰ We suspect that high-powered air rifles could be used in a significant portion of homicides, even with a conservative lethal range of 10 metres.

Legalisation around air rifles is variable around the world, with some countries considering them the same as firearms regardless of power, such as Australia or Singapore.²¹ Other countries have a variable muzzle energy limitation at which the weapons are considered firearms, such as the UK, which restricts above 16J.¹⁷ Canadian restrictions are based on a muzzle velocity of >500ft/s, with weapons below that velocity not requiring licensing.²² The USA does not consider air guns a firearm on a federal level, but has variable restrictions placed on them depending on the state. Twenty-eight states in the USA have regulations on air rifles, ranging from outright bans on sales in certain city limits to minimum age of purchase requirements and velocity restrictions.² If further restrictions for these weapons were to be considered, further data on lower calibre (.177) and lower powered weapons might be required, which is beyond the scope of this study.

The medical voice in trauma and harm prevention has sometimes been lost in recent times when it comes to major programmes, like road traffic safety, as we move to a more punitive approach where the focus is on the legal strife you may enter if you do not comply with rules. Our study adds some medical validity to the opinion that these higher velocity air weapons should be considered more like a small-bore rifle in terms of the lethality. Thus, we believe it would be reasonable to restrict them in a similar way to this level of weapon as appropriate within different countries’ legal framework. This would not affect the members of the public that use these guns for legitimate recreational purposes but would potentially have the effect of keeping them out of the hands of unsuitable people, such as those who would fail background checks for gun ownership and would potentially use these weapons for unsavoury purposes.