When a person fires a rifled weapon toward a paper target, the usual result is a ragged round hole in the paper, roughly the diameter of the bullet that was fired. However, sometimes a larger weird-looking hole appears, the hole may appear longer in one direction than the other.
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Often, the larger holes look like a keyhole. Because of that, we say the offending rifle is “keyholing.” By looking at the odd shapes of some of the holes, you will no doubt notice that some of those holes look distinctly like the shape of a bullet going through the target sideways. In fact, that is exactly what has happened.
When I began shooting, I was not aware of the keyholing phenomenon. In fact, I knew nothing much about it or gave it much thought until the publicity concerning the M16 first appeared. Prior to that, most of my firearms were limited to factory new that behaved as advertised.
I had a Ruger 10/22, Mark 1 pistol, Single Six revolver, and a Model 77 in .243. At that time, Ruger offered affordable prices that a student working part-time could afford. With shooting time confined to weekends, I never shot enough to develop any problems that would cause any of those to act erratically. I had just started reloading and was not experienced enough to get into trouble.
As I started to mention, the publicity regarding the M16 went on to say that the destabilized tumbling of the .22 caliber projectile enabled it to punch above its weight class making it a devastatingly effective weapon. Rumors abounded about its unparalleled destructive power. I remember one in particular that went like this, it hit a V.C. Guerilla in the finger and tore his whole arm off, and we of course believed it. The problem with a projectile that unstable, is that accuracy is awful. So, McNamara ordered changes that increase accuracy but almost eliminated lethality. No more awful wounds just nice clean holes.
That type of publicity is what put destabilized projectiles and keyholing on my radar. As my shooting interests and knowledge improved, I began to understand the effect that even a slightly unstable bullet had on accuracy. At that time, I was all about accuracy.
A Brief History of Rifling
We have one of two German-speaking inventors to thank for the invention of rifling. According to The Gun and its Development, one was Gaspard Kollner of Vienna, sometime in the 15th century. Others allege that his grooves were straight in nature and the first spiral grooves came from Augustus Kotter of Nuremberg in 1520.
Regardless of who was responsible, a lot of the early rifling development came from German-speaking areas. The Germans were aware of the basics of rifling and its benefits, even before they started manufacturing firearms. The Germans already had a history of manufacturing crossbows that would spin their bolts in flight.
During the American Revolutionary War of Independence, British troops found out — to their peril —that the American volunteer militias using their trusty Kentucky Long Rifles, (the key word is rifles) had deadlier accuracy than the British Brown Bess. So, the effect of rifling to stabilize the bullet was well known and once spitzer bullets came into general usage, the need for stabilization became even more important.
Importance of Rifling
Under normal shooting conditions, the rifling in the barrel imparts a spin on the bullet, which stabilizes it in the air and makes it travel with the nose always pointed forward. (Think of your favorite quarterback throwing a perfect spiral pass.) However, if the bullet is not stabilized properly when it comes out of the barrel, it will wobble in the air. If the wobble becomes too great, it will begin to tumble over itself and may curve off course while traveling in the general direction of the target.
When it hits the target, it can strike at any position of its rotation. It’s not likely that the nose of the bullet would be pointed precisely forward in its tumble. Therefore, it leaves a larger hole. Keyholing is a sign that the bullets are not being stabilized properly.
If a gun shoots one keyhole in 500 shots, it may just be due to a bad bullet. However, if it regularly shoots keyholes, that means there is a problem with the barrel or bullets or both. In that case, the problem needs to be fixed.
So, what causes bullet instability as it flies through the air? Conventional wisdom holds there are several reasons for this to occur and they are.
1. The 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. Therefore, it is not engaging the rifling properly.
3. The rifling twist rate may not be correct for the weight of the bullet.
4. Damage to the barrel near the muzzle or the crown may cause the bullets to wobble or tumble as they come out.
Once again, an unstable bullet will fly unpredictably through the air and that instability affects the accuracy, which in and of itself, is enough of a reason to want maximum stabilization. An unstable bullet also loses velocity faster. Additionally, it may not transfer as much energy to the target when, or if, it strikes the intended target.
That was my understanding of tumbling bullets and its causation. That is until recently. I’m guessing about 28 years ago I attended my friends, Mike Dalton and Mickey Fowlers ISI shooting school with my wife — even though I am an instructor. You all must know that if you are married, you can not teach your wife anything (regardless of your bona fides). Anyway, during our lunch break we were talking, and Mickey mentioned he had a Mini-14 that shot ½-inch groups. I replied, “Impossible.”
He eventually provided the information about who modified Mini-14, so off mine went. When I got it back, it also shot “lights out!” About a year ago, I took it out to shoot it, as it had not seen the light of day for many years. I wanted to try some new loads that I was working up for long-range ground squirrel shooting.
Much to my chagrin, it would not shoot the new loads. It was keyholing with the same bullet weight it had previously shot very well. After several tries with the new bullet and loads, I was quite perplexed and decided to send the rifle back to the gunsmith for repair, after all, something had to be wrong with it.
As luck and time would have it, because of age and illness the original owner had retired and sold the business. What to do, what to do? I had tried everything, and the problem persisted. Out of frustration, I looked for (and found) the original invoice where I specified I wanted to shoot light-for-caliber bullets to shoot ground squirrels. The gunsmith installed a barrel with a 1-in-14 rate of twist. Perfect for the light bullets I intended to put through it.
So why would it not now shoot? Then, it came to me. 30 years ago, I was shooting conventional 45-grain bullets. My recent reloads were 45-grain homogenous copper bullets (I live in Kommiefornia). Upon close examination, I realized the copper bullets were longer for their weight than jacketed lead bullets. All these years everyone said, it was the bullet weight that dictated the correct twist, but it was not.
Bullet weight was a relative shorthand for the length of the bullet, when they were all made with lead cores. Armed with that, I went to a still lighter and shorter (36-grain) all-copper bullet. Its length matched the conventional heavier bullet. The keyholing disappeared and the small groups returned.
Remember, it’s the bullet’s length — not its weight — that determines the correct twist for stabilization and accuracy. That was a frustrating learning experience at my age. Hopefully, I can save other folks from having to go through the same frustration.
Have you ever suffered from a gun that was keyholing? How did you resolve the problem? Share your answers in the comment section.