Reducing a rifle barrel from 16 inches to 9 inches while shooting subsonic ammunitiong enerally has two effects:
1. Velocity (Speed)
A shorter barrel usually results in a slightly lower muzzle velocity because the expanding gases have less time to accelerate the bullet. [americanrifleman.org]
For ammunition that is already loaded to stay below the speed of sound (typically around 1,000-1,050 fps), the change is often modest. The exact amount depends on the cartridge, bullet weight, powder type, and barrel. Some subsonic loads are specifically optimized for shorter barrels and may lose very little velocity. [americanrifleman.org], [silencershop.com]
That said, with subsonic ammunition the difference is often much smaller than the difference between subsonic and supersonic ammunition itself. The absence of the sonic crack remains the dominant factor in perceived noise. [en.wikipedia.org], [silencershop.com]
In practical terms
9" barrel: Usually a bit slower and a bit louder at the muzzle.
16" barrel: Usually a bit faster and a bit quieter because more gas energy is used inside the barrel before the bullet exits. [snipershide.com], [americanrifleman.org]
The magnitude of the change can vary dramatically by caliber. For example, .300 AAC Blackout subsonic loads are designed to work well in short barrels, while other cartridges may show larger velocity differences. If you tell me the caliber, I can give a more specific estimate.
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REDUCING A BARREL FROM 16" TO 10" or 9"
Reducing a rifle barrel from 16 inches to 9 inches while shooting subsonic ammunitiong enerally has two effects:
1. Velocity (Speed)
A shorter barrel usually results in a slightly lower muzzle velocity because the expanding gases have less time to accelerate the bullet. [americanrifleman.org]
For ammunition that is already loaded to stay below the speed of sound (typically around 1,000-1,050 fps), the change is often modest. The exact amount depends on the cartridge, bullet weight, powder type, and barrel. Some subsonic loads are specifically optimized for shorter barrels and may lose very little velocity. [americanrifleman.org], [silencershop.com]
2. Sound
Subsonic ammunition avoids the sharp supersonic crack because it remains below the speed of sound. [en.wikipedia.org], [silencershop.com]
However, shortening the barrel from 16" to 9" can make the shot somewhat louder at the muzzle because:
The bullet exits earlier.
The propellant gases have less barrel length in which to expand and cool.
Higher-pressure gas escapes from the muzzle. [snipershide.com]
That said, with subsonic ammunition the difference is often much smaller than the difference between subsonic and supersonic ammunition itself. The absence of the sonic crack remains the dominant factor in perceived noise. [en.wikipedia.org], [silencershop.com]
In practical terms
9" barrel: Usually a bit slower and a bit louder at the muzzle.
16" barrel: Usually a bit faster and a bit quieter because more gas energy is used inside the barrel before the bullet exits. [snipershide.com], [americanrifleman.org]
The magnitude of the change can vary dramatically by caliber. For example, .300 AAC Blackout subsonic loads are designed to work well in short barrels, while other cartridges may show larger velocity differences. If you tell me the caliber, I can give a more specific estimate.