Apparently there is not a single recorded case of a LEO killed by a round of M855. However I wanted to check what kills cops and specifically what rounds defeat body armor. I found the answers on table 41 of the FBI’s 2013 LEOKA (Law Enforcement Officers Killed And Assaulted.) It does not specify the type of body armor but from 20104 to 2013, 18 officers have died by rounds that defeated their body armor. One was a 9 mm (yeah, I wanna see the details on that one) and a grand total of 3 officers were killed with .223/5.56 ammunition. The rest? 30 calibers in different flavors: from 30-30 to 7.62×39. During the same time period, 3 officers were feloniously killed by knives and 28 were feloniously killed by vehicles (Table 27).
But racket must be made even if it achieves nothing regarding the safety of the officers….because they care!