Wednesday, March 25, 2015

What is Keyholing?

When a person fires a rifled weapon towards a paper target, the usual result expected is that a round hole corresponding to the diameter of the bullet will appear on the target. However, sometimes a larger hole appears, often the hole may appear longer in one direction than the other, such as shown in the image below:

An example of keyholing. Click on the image to enlarge.

These larger holes sometimes resemble a keyhole and therefore, the effect is called keyholing. We will study the causes of keyholing in today's post.

By looking at the shapes of some of the holes in the above target, the reader may notice that some of those holes look distinctly like the shape of a bullet going through the target sideways. In fact, this is exactly what has happened.

Under normal working conditions, the rifling of the barrel imparts a spin on the bullet, which stabilizes it in the air and makes it travel with the nose pointed forward always. However, if the bullet is not stabilized properly when it comes out of the barrel, it will wobble in the air or repeatedly tumble over itself while traveling to the target. Thus, when it strikes the target, it may not strike it with the nose precisely pointed forward and will therefore leave a larger hole.

In the above image, we see three holes. Observe that all three holes are somewhat larger than the diameter of the bullet. The top most hole is shaped like an oblong and was caused by a bullet not flying straight when it impacted the target. The middle hole is more round, but still has a pointed hole on one end showing that the bullet was wobbling in the air when it hit the target. The bottom hole clearly shows that the bullet was toppling end over end and hit the target sideways.

So what causes the bullet instability through the air? There are several causes for this:

  1. Rifling in the barrel could be worn out, therefore it does not impart enough spin to the bullet while it is leaving the barrel. 
  2. The bullet might be undersized and is therefore not engaging the rifling properly.
  3. The rifling twist rate may not be adequate for the weight, shape and profile of the bullet. For example, the M855 cartridge and the L110 cartridge are both designed for the M16A2. The bullet from a M855 (or SS109) cartridge can be adequately.stabilized by a barrel with a 1 in 9 twist rate (i.e.) 1 turn every 9 inches (228.6 mm.) of barrel length. On the other hand, the bullet from the L110 tracer round cartridge does not adequately stabilize at this twist rate and needs a twist rate of at least 1 in 7 (i.e. 1 turn every 7 inches (180 mm.)) for the tracer bullet to stabilize. This is because while the bullet diameters are the same, the weight,  distribution of mass throughout the bullet and the bullet profile shapes are different, which causes the instability. Therefore, M16A2 rifles come with a 1 in 7 twist rate barrel, so that they can be used with both bullet types.
  4. Leading in the barrel could also cause the bullets to not spin as much when they come out of the barrel.
  5. Damage to the barrel near the muzzle may cause the bullets to wobble or tumble as they come out.
  6. The bullet does not always immediately stabilize in the air as it leaves the barrel and needs to travel a little distance in the air before it gains stability. If the target is too close, the bullet may be still wobbling in the air a bit, by the time it hits the target.
  7. The bullet may have hit something on the way to the target, causing it to tumble in the air for the rest of its journey.
An unstable bullet is undesirable because it flies unpredictably in the air and therefore affects the accuracy of the firearm. An unstable bullet also loses velocity faster and it may not transfer as much energy to the target when it strikes it. 

Keyholing is a sign that the bullets are not being stabilized properly. If a gun shoots maybe one keyhole in 500 shots, it may just be due to a bad bullet, but if it regularly shoots keyholes, then that means there is a problem with the barrel or bullets or both, which needs to be fixed.

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