Bullet flight time


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The period of time that occurs between the time you get the reticle in your scope aligned on the target animal, press the trigger and hit the game can be measured literally in fractions of one second. Let’s look at each step and try to calculate exactly what happens.

You’ve been sitting glassing across a wide gully and picked up movement. Your pulse quickens and your heart beats faster as a deer steps out into the open. You work the bolt to load your rifle and then steady it by lying prone with the forend rested on your pack. You take aim with the crosswires on the game’s chest. Reaction time varies. It takes some hunters longer than others, often several seconds, to get ready to shoot, so we’ll cocnetrate on what happens after.

Now!
(1) From this moment on, all your attention is focused on a sequence of events with which you are familiar, but seldom think about and are inclined take for granted. These events, a series of short time intervals, include lock time, ignition time and barrel time; lock time starts when the trigger releases the sear and the sequence ends when the bullet either hits or misses the game. And all this takes place in less than a second!
After the time it took to adjust to a steady aim, until the moment you pressed the trigger and released the firing pin, takes a fifth of that second. (2) Ignition time starts after .005 seconds, when the firing pin impinges on the primer, and the cartridge is driven into the chamber, and comes to halt when the case shoulder contacts the chamber shoulder.

In the same instant, the firing pin pushed the primer forward until it bottoms in the pocket. This also took thousandths of a second. The cup is indented, and the priming compound crushed against the anvil ignites due to friction. However, it is incorrect to consider the primer flash alone as ignition, although it certainly starts it. A certain high temperature is required, normally in the region of 199.4°C. This takes up to .0004 seconds! Now we’re up to .2014 seconds. (3)

The hot flame of burning primer compound jets through the flash hole onto the powder. The pressure forces the primer cup backwards against the bolt face. The powder ignites and starts to form hot gases. When the pressure of the hot gases is high enough it starts the bullet moving from the case. It also exerts pressure against the walls and bottom of the case. (4)

The short span of time between when the pin hits the primer and gas pressure causes the bullet to move forward (5) and the case backwards is normally about .0003 – .0004 seconds. At the same time as the case expands and seals against the chamber wall, the primer is pushed back against the case.

Barrel time is next and it starts where ignition ended. It is the time from the bullet’s first movement out of the case until it exits the muzzle. The pressure peaks after a further  .0005 seconds. The bullet has moved about 10cm. The velocity is already 460 fps (6)

Let’s activate the stop watch for a moment. We’ll assume that we have a .30-06 cartridge firing a 180gn bullet. The bullet and the powder charge weigh approximately as much as the case and primer together. If there was no bolt to retain it, the case would now be blown back toward your face at about the same speed. Not a very desirable outcome, therefore we’ll quickly go back and restart our clock.

The bottom of the case pushes against the bolt face and the whole gun is moving backwards from the same moment that the bullet began to move forward. Only much slower as the gun is many times heavier than the bullet and the gases.

After the pressure has peaked, the bullet continues moving down the barrel and increases its velocity, but now it leaves behind it an increasingly larger volume than the burning powder is able to fill with more gas of the same pressure. This increase in volume causes the pressure to drop!

Pressures of 55,000 psi are common at the peak but may drop to 6,000 psi by the time the bullet reaches the muzzle. The bullet leaves the muzzle at 2,700fps and its average barrel time is around .0012 seconds.(7) The rifling causes it to rotate at about 3,000 r/second to keep it flying point-on towards the target. If the distance is 100 yds, the clock will run for a further .15 second (8). The bullet has actually taken .36 second to reach the target from the moment you decided to fire.

The expanding gas of the burning powder causes recoil. A heavier bullet will increase recoil because it will require a heavier force to move it out of the case and down the barrel and exerts an equal force back against the rear of the chamber, breech, and stock. The force is the same in both directions, but the gun is heavier than the bullet so it is easy to understand why the bullet is moved much faster and further than the gun.
Actually, the bullet’s weight and its velocity have a larger influence over recoil than the gun’s weight. Firing a 180gn bullet in the .30-06 from a 3.6kg rifle produces recoil energy of 20ft/lbs. How far to the rear has the gun travelled? How soon do you feel the recoil? This depends on how light or how hard you hold the stock against your shoulder when you press the trigger. The average speed of the gun is generally about 3fps and the 30- 06 moved back about 1.52mm before the bullet leaves the muzzle. And by the time the bullet left the barrel, the gun had moved back perhaps 5mm. That little movement is not enough to be felt. But what if you hold the stock against your shoulder so that the gun moves a total 12mm backwards in recoil before you feel it. That takes about .01 second. After that, your reaction time to register is as long as when you press the trigger, .2 second (9).

Hence, before you are able to feel that recoil about .4 second has elapsed. The bullet has already hit the game and the butt stock will push on your shoulder a little while longer as the gun continues to come back in recoil.

We should clarify something here – recoil is the technical and mathematical amount of rearward movement that results from the bullet’s forward movement. Kick is the amount that is felt. They are not the same. Kick can vary a great deal due to stock design and other factors.

But now you are feeling joyful. Before the barrel flipped up and the target was lost to view in the scope, you were able to call your shot as hitting where it was aimed. You see the game lying still on the mountainside and that picture will loom large in your thoughts. But I’ll bet that you won’t spare even a brief moment to think about the internal ballistics and combination of short time intervals that ensured your success.

This article was first published in Sporting Shooter, May 2012


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