The punched-out tissue complex (skin-bone ”imprimatum“) in shots from captive-bolt guns: does it act as a secondary projectile?

Powder-activated captive-bolt livestock stunners are still of great forensic importance, especially in the central European countries [43, 45]. Such devices are used to cause immediate unconsciousness in animals to be slaughtered. They are composed of a cylindrical metal tube containing an inlying steel rod, the so-called bolt, which is propelled by the powder gases of a blank cartridge [12]. The strength of the ammunition is indicated by color markings on the base of the cartridges. The bolt has a diameter of about 10 mm, a circular cross-section, and a conically grooved end acting as a sharp-edged punching tool when driven out from the gun’s muzzle with an initial velocity of about 50 m/s or less [9, 36]. The penetration depth depends on the bolt’s length and usually does not exceed 10 cm. After firing, bolts mostly return into the barrel by a rubber bushing and/or by a withdrawal spring although some models lack a return mechanism and must be drawn back by hand [42]. Several makes of captive-bolt stunners have two or four openings for the combustion gases. They are typically located in the muzzle plane beside the central hole for the bolt and determine characteristic smoke soiling patterns in contact and close range shots [6, 25, 31, 36, 40, 44, 45].

At the entrance site of the bolt, its grooved end produces a sharp-edged punch lesion of the skin and a circular hole of the underlying (flat) bone. The hole in the outer table of the cranial vault roughly corresponds to the diameter of the bolt whereas the inner table is broadened in a crater-like manner. The skin-bone complex (possibly accompanied by any head covering and/or scalp hair) is impacted at the front end of the bolt and carried along into the cranial cavity. If the bacterially contaminated skin is deposited in the depth of the brain, inflammatory sequelae are to be expected in cases of primary survival [4, 5, 10, 15, 41, 56].

Ever since the forties of the last century, some authors maintained that the distance between the entrance wound and the final position of the imprimatum may be longer than the bolt’s penetration depth [4, 18, 25, 34]. In this context, the imprimatum was labeled as a “secondary projectile” [4, 14, 25, 31, 34, 46] responsible for a lengthening of the wound channel beyond the way traveled by the bolt. In the context of gunshot wounds from conventional firearms, the conception of bone fragments acting as secondary missiles is met with skepticism [28, 49].

To answer the question, whether the skin-bone imprimatum moves further as well after the bolt has finished its forward motion, test shots with various types of captive-bolt guns were fired to simulants using video-documentation with a high-speed camera.

For the test shots, 2 composite models (a 2- and a 3-component model) as well as 3 different captive-bolt guns were used.

The 2-component composite model consisted of a synthetic bone plate and a gelatin block as soft-tissue simulant. The 3-component model was composed of artificial skin, synthetic bone, and gelatin (Fig. 1).

As a skin simulant, a 5-mm thick, 2-layer synthetic material (Ecoflex 0030, Synbone®, Zizers, Switzerland) was used, which has elastic properties similar to human skin. The plate-shaped 3-layer bone simulant made from polyurethane (Synbone®, Zizers, Switzerland) was 6 mm thick in all. The gelatin blocks were 25 × 25 × 40 cm in size; they were prepared from a 10% solution (250 Bloom, type A, grain 20/60) according to current recommendations for experiments in wound ballistics.

The following captive-bolt guns were used (Fig. 2):

With the “Schermer KR“ gun, the bolt does not automatically return to its original position after firing the shot, whereas the captive-bolt models “Schermer KS“ and “Pfeiffer“ are equipped with a bolt retractor system.

The shots with the narcotic devices were fired using either “Schermer” cartridges cal. 6.8 × 15 mm (Schermer GmbH, Ettlingen, Germany) or “Dynamit Nobel“ cartridges cal. 9 × 17 mm (Fürth, Germany), both with a red color marking, which is the most powerful cattle cartridge type (“for heavy bulls“).

With each of the 3 livestock stunners, the following test shots were fired:

The 9 test shots fired in all were filmed with a high-speed camera (Photron Limited, Fastcam SA-X2, Tokyo, Japan) with documentation at a rate of 5000 frames per second (Fig. 1b).

The medico-legal literature is abundant of case reports and reviews concerning fatalities from captive-bolt stunners. Most of them (about 85%) are suicides committed by males experienced in slaughtering procedures [3, 22, 29, 32, 33, 38, 45, 60]. Besides, a remarkable number of homicides [2, 7, 13, 20‐22, 24, 27, 29, 32, 38, 40, 46, 48, 51, 53, 55, 61] and several accidents (survived or with fatal outcome) [22, 26, 38, 41, 54, 56, 62] have been published.

In some suicides, apart from the slaughterer’s gun, another suicide method such as hanging is applied [18, 23, 29, 31, 32, 35, 37, 38, 40, 46, 51, 59, 62]. Exceptionally, even shots to the neurocranium need not cause immediate unconsciousness so that the victim may be capable of reloading the device and firing a second shot [1, 11, 16, 38, 40, 50, 57, 62]. Furthermore, two-gun suicides committed by shots from different livestock stunners have been reported [30, 39]. In slaughterer’s guns supplied with rubber bushings and/or recoil springs, the removal of these constituents prior to firing [31, 34, 42] can lengthen the penetration depth and even cause the bolt to break so that it becomes a real projectile [42]. If the bolt does not rebound into the barrel, it gets stuck in the skull [31, 32, 34, 35, 45, 46, 54, 59].

Up to the nineties of the last century, it was thought, that shots to the head inflicted with a slaughterer’s gun do not involve an abrupt increase in intracranial pressure followed by indirect fractures or cortical contusions. Rabl and Sigrist [47] were the first to describe skull fractures away from the bolt’s track. Later on it was shown, that indirect fractures may occur due to a hydraulic burst effect even in shots from captive bolt stunners [17, 18]. Another fundamental and still open question is related to the punched out skin-bone complex which has been regarded as a secondary projectile by some authors [4, 14, 25, 31, 34, 46].

In conventional gunshot injuries to bones, the energy transferred to osseous fragments is considered insufficient to produce separate wound channels. The splinters usually do not precede but follow the bullet as it passes through the soft tissues. Due to the development of a temporary cavity and the negative pressure in it, the bone fragments also move laterally before coming to rest, so that there may be some distance from the permanent wound channel [28, 49]. A final position far from the bullet’s track and embedded in soft tissue may suggest a propulsive movement of the splinter itself. Based on these considerations and confirmed by high-speed images, the term “secondary projectile” does not appear to be appropriate—at least in gunshot injuries from firearm bullets. Nevertheless, the opinions of authors are inconsistent even in this respect. So the monograph of Di Maio [8] contains the following statement: “Sometimes the bone chips create secondary tracks that deviate from the main path.”

Injuries from captive-bolt guns are comparable with those from conventional firearms to a minor degree only. The main differences concern the initial velocity of bolts in relation to bullets, the shape of the bolt’s anterior end (conically grooved, sharp-edged) versus common bullet configurations (round nose, hollow-point, wadcutter, etc.), as well as the bolt’s retraction movement after having reached its maximum penetration depth. Furthermore, the bolt acts like a punching tool as the sharp edge of its front cuts out a corresponding piece of the scalp. The underlying flat bone is broken out in a characteristic manner: The circular hole in the outer table of the skull has a clear-cut edge and roughly complies with the diameter of the bolt, whereas the inner table is beveled resulting in a funnel-like appearance. The punched-out pieces of skin and bone (the latter one mostly fragmented) often retain their anatomical connection so that they are jointly displaced into the depth. Only in very few cases (especially in acute-angled shots), the cut-out skin flap may remain linked to the edge of the entrance hole implicating that the “imprimatum” lacks a dermal component [10, 35, 40, 41, 44, 51].

After being punched out, the skin-bone complex gets embedded in the excavated end of the bolt. There the so-called “imprimatum” is accelerated up to the bolt’s velocity. When the bolt has completed its forward motion, the impacted skin-bone complex separates from the rod. A significant onward movement in the previous direction could not be observed in our experiments. Obviously, the kinetic energy of the “imprimatum” is not sufficient to overcome the penetration resistance of the simulant. The cross-sectional area of the skin-bone complex is large resulting in a minor sectional density considering the comparatively low mass of the punched-out tissue.

Mostly, the impacted bone is fragmented into pieces when it is punched out by the bolt’s anterior end. This explains that more than one osseous splinter may be present in the cranial cavity of victims injured with a livestock stunner. The term “bone imprimatum” in the proper sense only refers to that piece of the cranial vault which is wedged in front of the conically grooved bolt and displaced right up to the end of its track. Additional bone splinters may be located alongside the wound channel, sometimes just behind the entrance site [4, 5, 58, 59].

In our test series, different models of the Schermer and Kerner type slaughterer’s gun were used. The Schermer type is characterized by the absence of smoke outlets at the barrel’s muzzle end [19, 36]. In contrast, makes of the Kerner type have two opposite outlets for the combustion gases causing paired soot depositions on either side of the entrance hole. Even in contact shots, only a small part of the combustion gases is propelled into the depth of the bolt’s track [17, 52]. Nevertheless, in one of the test shots, an intense soot deposition was found in the initial section of the wound track (a similar observation was reported by Hagemeier et al. [19]).

Contrary to previous perceptions, it is known for several years that shots from slaughterer’s guns may be associated with a moderate radial displacement of the target medium. Accordingly, the first section of the bolt track is widened in a non-elastic simulant (ballistic soap) [17] and accompanied by short cracks in gelatine [17, 52]. The increase in intracranial pressure may result in a hydraulic burst effect causing secondary fractures in thin parts of the skull [17, 18, 47].

The high-speed video-documentation impressively visualizes the dynamic interaction between the bolt, the target medium (gelatine), and the skin-bone complex carried along up to its final position. Albeit far less marked than in conventional gunshots, there was a temporary widening of the bolt’s path. When the bolt moved rearward, the distal part of the track considerably reduced in diameter, obviously due to a transient negative pressure. This observation is in accordance with the findings in ballistic soap as previously reported [17] and also explains a potential retrograde shift of the skin-bone complex which was already described by Janssen and Stieger [25].