Tuesday, July 05, 2016

Tim and the "Keyhole Effect"

I was about to turn off the radio and go back to work when I heard Tim, a caller to the Jim Vicevich Show on WTIC AM, mention the "keyhole effect" as it relates to a bullet fired from the AR-15/M-16 and my discourse the previous day on rifling and the spiral rotation of a bullet fired from a rifled barrel, "yada, yada, yada".

During more than three years as a USAF Combat Arms training instructor, I never heard the term, keyhole effect.  Tim went on to say that because the back end of the bullet was heavier than the front end, that it would tumble end over end, causing the keyhole effect when the bullet hits it's target.  (If I'm not mistaken, the day before, Tim had also mentioned the boat tail shape, or tapered heal, of the bullet.  Now that I think about it, he may have stated that that shape was intended to reduce the weight of the back of the bullet, which in the case of a .223/5.56mm bullet is a mere 55 grains.)

First of all, at some point in any bullet's travel, it will loose its inertia and therefore, its stability and fly erratically, throwing accuracy out the window.  Second, the maximum effective range of a .223 bullet fried from an AR type rifle is 550 yrds., well over half a mile.  That means that in the right hands, a shooter can consistently hit a specific target at that distance while the projectile retains enough energy to produce the desired effect.

The .223 cal. bullet has a maximum effective area range of around 800 yards, just a couple hundred feet short of half a mile.  This means that a shooter can consistently hit an area, i.e. an enemy encampment, at that distance while the projectile retains enough energy to produce the desired effect.

The maximum range is around 1.5 miles before all the bullet's energy is expended and it falls "harmlessly" to the ground.

If we engage in a little critical thinking, here, it stands to reason that any conical shaped object that travels end over end has no aerodynamic characteristic, whatsoever and therefore, cannot be expected to travel in a predictable, consistent trajectory.  It would be impossible to fire accurately.

Now, as far as the boat tail shape is concerned, most bullets designed to be used beyond 300 yrds are shaped such, not to reduce the weight of the bullet heal, but to reduce drag and create a higher ballistic coefficient.  This provides for a flatter trajectory.

And, as I stated the previous Thursday, the spiral rotation of the bullet created by the rifling aids in stabilization of the bullet's flight, increasing both range and accuracy.  The spiral rotation of a well thrown football analogy is accurate and enables the listener to better understand the effect of rifling on a bullet's flight.

Note:  Though I've seen what he is talking about, it was, in my experience, the result of a ricochet.  I double checked with some friends from my old USAF career field some 34 years ago, and they confirm that, while such a phenomenon occurs, it is the result of defective equipment, i.e. a shot out barrel, poor quality ammo or ammo that is incompatible with the barrel twist ratio.

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