Forensics on the brain too much…Part 2

A local swim school uses “foam” baf­fles to reduce the noise gen­er­ated in the swim­ming pool area. The baf­fles are shaped like dol­phins and cir­cles. I was look­ing at one of the cir­cles the other day, and noticed that there were tool­mark impres­sions left behind from the man­u­fac­tur­ing process.

It looks like the discs are cut out of a sheet of the mate­r­ial. When the harder “cookie cut­ter” tool, cuts out the shape of the disc, any imper­fec­tions on the cut­ting edge of the cut­ter, and some ran­domly gen­er­ated imper­fec­tions deter­mined by how homo­ge­neous the “foam” mate­r­ial is, will result in stri­a­tions on the side of the disc.

This anal­o­gous to how tool­marks are left behind on fired bul­lets. When the bul­let is being pushed down the bar­rel of the firearm, any imper­fec­tions on the inside of the bar­rel cre­ate slight scratches on the side of the pro­jec­tile called striations.

These stri­a­tions are unique to the firearm, and depend­ing on the qual­ity or quan­tity of the stri­a­tions impressed on fired bul­lets, it is pos­si­ble to deter­mine what firearm a par­tic­u­lar bul­let was fired from.