RIFLES
Super Subsonics
Slow down: For out-and-out accuracy, put away the high- and hyper-velocity ammo and load your favorite .22 with "below the speed of sound" stuff.
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Subsonic .22s are the loads to shoot for the best accuracy and meat-saving shots on small game. You might think high-velocity .22 Long Rifles, with a shorter time of flight, would drift less in the wind and be more accurate than subsonics. However, because subsonics are slightly slower than the speed of sound, they do not encounter a sharp increase in air resistance; that results in less wind drift and more stable flight.
A Not-So-Simple Explanation
The exact speed of sound depends on the temperature of the air. At sea level with a temperature of 70 degrees, sound travels at about 1,128 feet per second. As temperatures fall, air becomes denser and sound travels slower. When a bullet approaches and first passes this velocity, airflow severely buffets it. The outer edge of this transonic zone, if we go with the 1,128 fps at 70 degrees, is between about 902 and 1,466 fps.
Glen Weeks of Winchester Ammunition says a bullet that does not reach the speed of sound is much less affected by air turbulence. "A shadow graph of a subsonic bullet in flight shows a shock wave being pushed in front of the bullet and flowing around the bullet," Weeks says. "But as a bullet transitions from subsonic to supersonic, the shadow graph shows the bullet piercing the shock wave and the shock wave actually adhering somewhat to the bullet." Weeks says the shock wave created at these transonic velocities causes all sorts of turbulence. That turbulence can destabilize a bullet and throw it somewhat off course.
This transonic zone is about the speed of most high-velocity .22 Long Rifle bullets flying to out past 100 yards or so. A high-velocity .22 Long Rifle bullet leaves a rifle muzzle at the upper end of this zone, so it suffers little buffeting at its start. But somewhere around 75 yards the bullet has slowed enough that it is subject to the full pummeling of the increased air resistance at the speed of sound.
Now for the complicated part. Even though a subsonic bullet starts out several hundred fps slower than a high-velocity one, the subsonic drifts less in the wind. Logic dictates that the high-speed bullet should drift less because it has a shorter time of flight and thus wind has less time to push it off course. But that is not the case.
Allan Jones of CCI and Speer Bullets thinks subsonic bullets drift less in the wind than supersonic projectiles because of the characteristics of airflow.
"At subsonic speeds, the flow of air is detached from the bullet and more easily moves away from the bullet and that doesn't restrain the bullet as much," Jones says. That lower resistance gives a bullet a slightly higher ballistic coefficient compared to when it's going through the increased drag of the transonic zone.
When a bullet goes through the transonic zone, an increased amount of energy is required to push the shock waves. "When the bullet's going through the transonic zone, air compresses ahead of it and shock waves form up at different places on the bullet. This buffets the bullet and causes some loss of velocity and degrades the ballistic coefficient," Jones says.
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