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

From the first half of the twentieth century to the present day, injuries and fatalities from captive-bolt livestock stunners are a major topic in forensic medicine. The vast majority of cases account for suicides with the frontal, temporal, parietal, and occipital regions being the most common sites of entrance (in descending order of frequency). Due to the limited length of the bolt, the corresponding wound channel within the braincase is only several centimeters long. It has been a controversial subject for a long time, whether the skin-bone complex punched out by the conically grooved end of the steel rod may act as a "secondary projectile" being propelled beyond the actual path of the bolt. To answer this question, experimental shots from various types of captive bolt-guns were fired to simulants. Video-documentation employing a high-speed motion camera showed that the punched-out pieces of skin and bone did not move further than the bolt. Thus, a secondary extension of the total wound channel could not be observed. However, the suction effect caused by the bolt's rearward movement may induce a slight retrograde displacement of the skin-bone complex.


Introduction
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 highspeed camera.

Material and methods
For the test shots, 2 composite models (a 2-and a 3component 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 3component 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 3layer 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"). 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).

Schermer KR
After firing the captive-bolt gun, the bolt was rapidly driven out from the barrel as far as it would go. At the same time, a dense cloud of gunsmoke with separate powder grains exited from the muzzle before the bolt returned completely to its original position with a subsequent slower full forward movement. This was again accompanied by a smaller cloud of gunsmoke. After a short backward movement of some millimeters, the bolt remained in its resting/final position outside the barrel.

Schermer KS
After pulling the trigger, the bolt was rapidly driven out from the barrel as far as it would go followed by immediate retraction to its original position. After another slower forward movement of the stunning bolt (not quite to its maximum length), it was retracted until just before the muzzle and completely returned to its original position after a short delay. There was no visible cloud of gunsmoke.

Pfeiffer
After pulling the trigger, the stunning bolt exited through the hole in the muzzle end together with a dense cloud of gunsmoke. Then it was retracted into the barrel approximately half-way and after a short delay finally returned completely to its initial position.

Schermer KR
From the flat synthetic bone simulant, a round, 12-mm wide and 6-mm-thick fragment was punched out and displaced to the end of the 9.5-cm long penetration channel. In the video sequence, no secondary forward or backward movement of the imprimatum was observed. As after the dry-fire shot, the bolt returned into the barrel and-after another forward movement-reached its final position just behind the imprimatum. In the first section of the channel, for a short time, the maximum diameter was slightly wider than the cross-section of the bolt. In the permanent channel, some small bone particles were found away from the main imprimatum. In the first section of the channel, there were deposits of gunpowder residues visible to the naked eye. The hole in the synthetic bone plate showed cone-shaped widening in the direction of the shot. The outer edge of the entrance hole was covered with ring-shaped gunsmoke deposits.

Schermer KS
From the synthetic bone simulant, a round, 13-mm wide and 6-mm-thick fragment was punched out and transported by the bolt to the end of the 8.9-cm long channel. The high-speed photographs showed that a cone-shaped temporary cavity developed only in the initial part of the bolt track and that the secondary forward movement of the bolt did not cause any subsequent displacement of the imprimatum (Fig. 3a). Some small bone fragments had lodged in the initial section of the channel. Gunsmoke residues along the bolt channel were not visible to the naked eye.
At the entrance site, the bone plate showed a vague coneshaped extension in the direction of the shot. In the zone surrounding the entrance hole, minor soot blackening was seen on the outer margin.

Pfeiffer
The findings were similar to those of the Schermer KS device. Again the bolt punched out a fragment of synthetic bone with a maximum diameter of 13 mm and displaced it to the end of the 6.2-cm long bolt track. After the forward movement of the bolt, the imprimatum did not move either forward (i.e., in the direction of the shot) or backward.
The margin of the entrance hole in the bone plate showed ring-shaped smoke soiling (Fig. 4a) and fracture lines

Schermer KR
From the artificial skin, a round, 5-mm thick piece with a maximum diameter of 11 mm and two adhering bone fragments with a size of up to 8 × 6 mm was punched out. This skin-bone complex was displaced by the penetrating bolt to the end of the 8.6-cm long bolt track. The high-speed videodocumentation showed a brief cone-shaped widening of the channel in the initial section and tapering in the distal section. The entire length of the bolt channel was blackened by soot; it contained several bone fragments, especially near the entrance zone.
On the outer margin of the entrance hole in the artificial skin, there was an up to 8-mm wide ring of soot soiling. The synthetic bone showed a 12-mm wide circular defect with cone-shaped widening in the direction of the shot.

Schermer KS
From the artificial skin, a 5-mm-thick and 10-mm-wide piece was punched out and displaced to the end of the 7.2-cm long bolt track. The synthetic bone fragment had separated from the covering skin. As the bolt retracted, the imprimatum moved backward (in the direction of the entrance) for 8 mm (Fig. 3b). The channel showed temporary expansion near the entrance and contained several fragments of synthetic bone up to 8 mm in size.
At the margin surrounding the entrance hole, only minor soot soiling was discernible. The bone defect showed a slight cone-shaped widening in the direction of the shot with radiating fracture lines.

Pfeiffer
From the artificial skin, an 11-mm-wide and 5-mm-thick cylindrical piece with a synthetic bone fragment adhering to it was punched out and displaced to the end of the 6.6-cm-long channel. After the bolt had traveled to its maximum penetration depth, the skin-bone complex did not move any more. The whole bolt track was blackened by gunsmoke.
The zone around the entrance hole showed intense soot soiling over a width of up to 12 mm (Fig. 4b) and the underlying bone was shattered into small fragments. After removing the bone plate, the fragments were found adhering to the outer side of the gelatin block.

Discussion
The medico-legal literature is abundant of case reports and reviews concerning fatalities from captive-bolt stunners.
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 " s e c o n d a r y p r o j e c t i l e " d o e s n o t a p p e a r t o b e 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 socalled "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.
When discussing any additional wound lengthening produced by the "imprimatum," one has to take into account that a large-sized skin-bone-complex may protrude beyond the bolt's excavated end. Therefore, a minor extension of the wound path in order of the imprimatum's thickness does not prove a further onward movement in the sense of a secondary projectile. Under the terms of our test shots, the punched-out skin-bone imprimatum did not exceed the penetration depth of the bolt.
As described in the methodical section, the test shots were fired at composite models simulating the scalp, the underlying flat bone of the skull, and the brain. The bolt's path terminated in a block of translucent gelatin permitting a detailed video-documentation of the motion sequences inside. The experimental design did not completely correspond to the situation in shots to the head as there was no firm encasement comparable with the bony skull. Therefore, the lateral and forward acceleration of the simulant induced by the penetrating bolt was not limited as it is to be expected in a human brain case. Nevertheless, this variation would not inhibit any forward movement of the skin-bone complex exceeding the bolt's penetration depth. Accordingly, it seems justified to deduce from our test results that the skin-bone plug carried along by the bolt's end does not act as a secondary projectile.
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].

Conclusions
-In head shots from slaughterer's guns, the impacted skin and bone are punched out by the grooved steel bolt and forced into the depth of the brain, where the dislocated tissue complex can be found at the end of the wound track. -In the past, several authors held the opinion that the socalled imprimatum acts as a "secondary projectile" potentially lengthening the wound channel. -In order to verify these statements, test shots were fired to composite models and video-documented by a highspeed camera. The results confirmed that the imprimatum actually does not move beyond the penetration depth of the bolt.
Informed consent Not applicable.
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