The rise of urban warfare against individuals and small groups suggests that a source of reliable, accurate, and (most of all) quiet subsonic rounds is, to SOCOM, a highly desirable goal. However, to date, subsonic ammunition performs reliably only in small caliber silenced pistols, which are unsuited to many missions for which subsonic ammunition would be favored, even transformational. Favored, perhaps, but not easily achieved.
Past experience with subsonic rounds reveals significant accuracy problems, which drives a considerable reduction in effective range. Achieving subsonic velocities requires igniting a very small propellant charge in a rather large cartridge, leading to unsteady burning and erratic chamber pressure. Small chamber pressure can lead to gas leakage from the chamber, and also to a greatly increased chance of having a bullet become stuck in the barrel, rendering the rifle inoperable. Such low pressures are also insufficient to reliably cycle the action on current gas-operated semi-automatic and automatic rifles, raising a strong possibility of jamming during use.
It seems clear that new cartridge technologies must be invented to satisfy these conditions. These might include polymer-based cartridges for improved gas sealing, and sabots for sealing of the barrel while insuring predictable barrel friction and highly accurate performance. The feasibility study will produce an outline of the general approach to developing subsonic rounds.
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