2.1. Ammunition and rifle

Gun firings were conducted at the U.S. Army Research Laboratory (ARL), Aberdeen Proving Ground, MD (U.S.A.). An M4 carbine rifle was selected for study due to its prevalence with U.S. military and North American Treaty Organization (NATO) forces for the last 20 years. The barrel length is 36.8 cm plus the length of “A2” muzzle, or “birdcage” device which is used to block/suppress light emission from incandescing gases/particles immediately following bullet exit. The M4 weapon was tested in operational configuration and was not modified for the testing. It was capable of being switched between semiautomatic and 3-shot burst mode but was operated in single-shot mode. M855 ammunition was selected because it is available in multiple compositions with the same base propellant formulations (nitrocellulose ((C₆H₇(NO₂)₃O₅)_n)/nitroglycerin (C₃H₅N₃O₉) “double-base”) and contains Pb in the projectile, affording a chance to study the fate of metals. Three types of M855 ammunition were tested, each of which used a M41 primer containing Pb, aluminum (Al), Ba, Sb and a Cu jacketed lead bullet, 5.56 mm in diameter. The ammunition consisted of a 1990s era M855 round, its “salted” counterpart which contains K for flash suppression, and an older M855 Vietnam-era (1960s) “legacy” round. While the M855 is in common sport use, this is not the same formulation as in the current military-fielded M855A1 “green” lead-free ammunition. A total of 20 shots were analyzed for the three ammunition types. The composition of the ammunition used here is described more fully in Supplemental Information (SI) Tables SI–1. The metal and carbon fraction for these rounds are found in Tables SI–2.