TOWARDS A DROP-IN REPLACEMENT SUBSONIC CAPABILITY FOR NATO SMALL ARMS

Abstract

While subsonic ammunition has been of military interest for years, particularly for covert operations, significant limitations of current offerings have prevented its adoption including the inability to cycle an automatic action. This and other limitations were investigated through a design exercise to establish the feasibility of a drop-in replacement relevant to the Canadian Armed Forces using commercial off the shelf components. A survey and stability analysis of commercially available 5.56 mm projectiles led to the selection of the 90 grain Sierra Matchking for this design. Three different cartridge configurations were investigated in order to evaluate the effect of reducing cartridge volume on key internal ballistic variables. A cycling impulse model was developed in order to rank candidate designs on their ability to cycle a prescribed automatic weapon. Cartridge volume, in terms of standard cartridge, straight internal bore, and custom internal diameter to realize 95% load density, was explored as a means of increasing the load density of subsonic ammunition in order to reduce muzzle velocity variance and thus increase accuracy. However, the accuracy gained by reducing cartridge volume is shown to cause significant tradeoffs in cycling impulse. Live fire testing was performed in order to determine the minimum cycling impulse required to cycle the test weapon. The results also indicate that there may exist a threshold load density above which load density does not play a significant role in muzzle velocity variance. Thus, an objective of maximizing load density may overconstrain the design for minimal benefit.