Conclusions and Recommendations

An experimental campaign consisting of 18 shots against Perma-Gel which have been exposed to increasing number of melt/recast cycles has been undertaken to investigate the potential for this material’s performance to degrade within a ballistic setting. Depth of penetration measurements were taken pre and post melt cycle, alongside a comprehensive review of high-speed video footage to examine differences in cavity diameter maxima.

The main conclusions from this work are that careful consideration to the application of Perma-Gel within wound ballistics studies should be given due to notable degradation in depth of penetration (30.40% maximum observed difference), when utilising the parameters of this study. Additionally, assessment of temporary cavity indicates a 5.36% increase in maximum diameter post melt cycle further evidencing material degradation, for which it is proposed that a limit be placed on the number of melts cycles the material can undergo before disposal is required to mitigate against falsified data being produced.

Further, concerns remain surrounding its wider use, and the ability for its data to be easily referenced back to 10% gelatine or human cadavers. It is therefore recommended that researchers using Perma-Gel should:

Calibrate each new batch/block with a reference projectile (steel BB or a projectile with well-known behaviour in 10% gelatine).Limit remelts - record number of melts; retire block after a conservative reuse threshold (establish threshold from pilot tests).Control temperature during testing and report it (block temperature and lab temperature).Run side by side tests with 10% gelatine or biological proxy for at least a subset of shots if you must claim clinical relevance.Document optical condition (clarity, discoloration, bubbles) that might affect imaging/interpretation.Document batch number, date received, number of recasts/melt cycles and each shot location for traceability.

To ensure the recommendations provided above are investigated, additional work should be undertaken on the environmental impact of incorrect disposal of Perma-Gel to explore severity as its constituent parts contain non-biodegradable materials such as mineral oil. Additionally, emphasis should be placed on undertaking additional mechanical property characterisation (density/viscosity/elasticity) for pre and post melt cycles focussing on microstructural changes by measuring mass/volume changes. Verification of rebound/elasticity tests (compression tests) should also be undertaken to observe recovery over time (melt cycle) with large changes indicating mechanical drift.