This study aimed to expand upon the work done by Taylor and Kranioti , Smith et al.  Thali et al. [10, 11], and Mahoney et al. [7, 14] on testing whether Synbone® spheres are good proxies for human crania in ballistic studies and to record different injury patterns observed at different distances. It was found that the entry points of the ammunition into the spheres exhibit similar morphological patterns to real forensic cases when shot from a distance less than 35 m while exit wounds were larger than documented forensic cases. The radiating fractures seen in the present study were also consistent with those seen when Synbone® spheres were shot with FMJ pistol rounds  and as expected in real cases. Contrary to this, entry wound patterns seemed little consistent with real cases in shots from distances of 50-100 m according to one study . This seems highly dependent of the distance, although other factors such as wind velocity and angle of the shot cannot be discarded. Synbone® sphere thickness was 1 mm smaller in the last study  but again this small discrepancy seems highly unlikely to be responsible for the diverse injury pattern observed in these rounds.
Circumferential delamination (CD) is another featured observed in the current study. It has been argued that CD is a feature indicative of FMJ rounds in previous research . The hardness of the FMJ rounds comparative to the bone it moves through, crushes the bone or proxy around the rim of the entrance wound, leading to the appearance of CD. This has been observed consistently in a previous experimental study using handguns and FMJ rounds . The present study employed two FMJ military rounds fired from distances of 0.3 to 35 m and it was observed that all Synbone® spheres exhibited CD at the entrance wound across all three shooting distances and caliber sizes (See Table 3). This also agrees with the study of Smith et al.  that described CD in a sphere shot with a 7.62 × 51 mm caliber from a distance of 2 m. Unfortunately studies by Thali and colleagues  at 10 m and Mahoney and colleagues at 50-100 m  do not describe any CD in the entry wounds, however, it is not clear if this is because the authors did not include this feature in their research protocol or because they feature was absent.
Also witnessed in this study was the hydraulic burst effect (Fig. 2) [20, 25]. The human head with the brain present is considered to be a hollow organ (brain tissue being classified as a liquid). High velocity rounds cause an immense amount of hydraulic pressure inside a hollow organ, and in the presence of liquid (which cannot be compressed) it will cause the organ, such as a human head, to rupture . Synbone® spheres filled with gelatin have been shown to replicate this effect better than other proxies, such as bovine scapulae , as not all of these can recreate the hollow organ nature of the cranium like the spheres can. Di Maio  stresses that “a high-velocity bullet fired through an empty skull produces small entrance and exit holes with no fractures” while “same missile fired through a skull containing brain causes extensive fracturing and bursting injuries”. The head proxies used in our experiment fulfilled the requirement of “hollow liquid-filled” organs, thus, all shots across the three shooting distances produced this effect. This was not the case in the handgun experimental study from 0.3 m where only three of the six pistol calibers produced this effect . This is probably due to lower energy produced in the smaller rounds. The hydraulic burst effect was not mentioned in other studies thus, no assumptions can be made on additional factors that may affect its appearance in the experiments using high velocity military ammunition.
Krönlein shots  were also observed in the 7.62 × 39 mm and 7.62 × 51 mm rounds and distances < 10 m while it exhibits an inconsistent pattern in 7.62 × 39 mm rounds shot from distances of 50-100 m. Krönlein shots occurred at 0.3 m for both 7.62 × 39 mm spheres with and without skin covering, which is likely related to the amount of energy of 2188 J produced by the round compared to 1700 J produced with 5.56 × 45 mm. Interestingly, Krönlein shot occurred at 35 m with the 5.56 × 45 mm. This shot could have happened due to the muzzle velocity of the round at 35 m  which is still past the threshold of the velocities needed for a Krönlein shot to occur (< 800 m/s) . Yet, this cannot not explain why the 5.56 × 45 mm did not cause Krönlein shots at the closer distances in our study. Discrepancies in the appearance of a Krönlein shot are also noted in the study of Mahoney and colleagues  at 50-100 m for with 7.62 × 39 mm rounds. Besides the longer distances in the latter study, Synbone® thickness is 1 mm thinner compared to ours but even considering these two factors the random appearance of Krönlein shots in the Mahoney study (2019) cannot be explained due to lacking information such as firing angle, wind velocity and other parameters that can affect wound morphology in high velocity military ammunition. Tangential hits, for example, may provoke a lift of the cranial vault while the brain remains undamaged . These observations most certainly pose the basis for further research on experimental ballistics with a larger number of controlled factors and multiple repetitions.
The advantage of Synbone® spheres is that they are easy to obtain and pose no ethical limitations on researchers, which is in contrast to both human and animal tissues when used for this purpose. They do not come though without limitations. Whilst studies have shown that Synbone® exhibits a satisfactory reaction to impact as a cranial proxy [10,11,12] and can exhibit realistic reaction to ballistic injuries as demonstrated in the current study (e.g., hydraulic burst, Krönlein shot) there are still questions as to its suitability as a proxy for long bones [7,8,9]. Also, when ballistic testing on Synbone ® spheres used 7.62 × 39 mm ammunition at 50 m and 100 m the resulting injury patterns were considered to be “too comminuted and fragmented” when compared to contemporary military injuries encountered by participating physicians .
The study itself exhibits several additional limitations. Due to the financial constraints of purchasing multiple Synbone® spheres the sample size was limited to five spheres per caliber for this pilot study. This limited the shot distances to three different distances which represent extreme versions (executions and longer shots) of realistic cases. We have chosen to include 0.3 m so that results can be comparable to our previous study on handguns fired from the same distance  and the remaining distances in order to fill the gap encountered in the literature complementing other experimental studies on military ammunition [6, 11, 14]. This drawback needs to be addressed in future studies by expanding the number of spheres used at each distance and the calibers used. Regardless, the present study offers new initial data on the behavior of Synbone® proxies in ballistic testing of military ammunitions for distances that have not previously been published, suggesting that efficient tests can take place under these conditions.
Taking into account the results of this and previous studies [6, 11, 13, 14] there is evidence that Synbone® spheres filled with ballistic gelatin with or without extra skin substitute can be efficient head proxies to simulate gunshot inflicted trauma using a variety of guns and ammunition. Yet, due to the number of uncontrolled factors and limited repetitions of the so far reported experiments, definite conclusions on wound ballistics cannot be drawn with confidence. Instead, it seems safer to use artificial head proxies to simulate specific conditions of a shooting incidents where multiple factors can be controlled according to the scene evidence. A good example of this principle is the experimental study by Mahoney and colleagues  that simulated two military combat inflicted injuries with the use of helmet for which a number of factors such as engagement distance, bullet manufacturer, batch and propellant load being controlled. Repetitive experiments (3 per case) gave however a range of bullet trajectories indicating the possible influence of more uncontrolled factors in the experiments. This leaves room for more experimentation in future studies.