Subsonic means "slower than the speed of sound" so a subsonic
bullet is any bullet that travels slower than the speed of
sound.
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Transonic being past the speed of sound and subsonic is below
the speed of sound.
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subsonic is moving below the speed of sound, supersonic is
faster than speed of sound
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Subsonic is below or slower than the speed of sound, and
supersonic is above or faster than the speed of sound.
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Subsonic means any speed below the speed of sound. (Mach 1). So
subsonic aircraft range from 0 - 661 knots at sea level. (Mach 1 is
661 knots at sea level.)