# The Naked Scientists Forum

### Author Topic: The Acceleration Of a Bullet ?  (Read 32817 times)

#### neilep

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##### The Acceleration Of a Bullet ?
« on: 30/11/2007 18:18:17 »
Dear Ballisticologists !

Not too sure if this should be in Physics or Technology so..I chose the happy medium of General Science.

Here's Gerald my Guard Gopher !....

As ewe can see he is well armed !!...no one's gonna mess with him !!

After a hard day guarding  Gerald likes to unwind at the shooting range....now then..because the battery in my camera was low I was still able to photo-realistically sketch this bullet coming out of Geralds gun !

He then asked me this !

For how long does the bullet accelerate ?.....does it matter how fast the bullet is going ?...is there a formula that describes the point at which acceleration of a bullet stops ?...does the barrel length contribute to the acceleration ?

Well, I have to say I was flummoxed that he should ask me this ....considering he's a gopher and can't speak...and all of this whilst smoking a Havana too !!

#### another_someone

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##### The Acceleration Of a Bullet ?
« Reply #1 on: 30/11/2007 19:53:47 »
Excepting when a bullet in in freefall, under the influence of gravity, it usually ceases to accelerate after it leaves the barrel of the gun.

The force that accelerates the bullet is the pressure built up in the chamber, and within the barrel, caused by the combustion of the propellent charge (gunpowder, to use the vernacular - although it is not normally what is traditionally known as gunpowder, which is now regarded as black powder).

Exactly where the peak acceleration happens depends on the way the propellent burns, which depends on factors such as the grain size of the propellent (smaller grains burn faster, but you can also change the shape of the grains, so the they start by burning faster, and then get slower, or visa versa).  Generally, the longer the barrel, the more distance you have in which to build up speed, so you will use slower burning propellent.

In general, you want to be as efficient as you can in the conversion of propellent energy to building up speed in the bullet; so you want to make sure that when the bullet leaves the barrel, there is as little excess pressure behind the bullet as possible (as that excess pressure may make for a loader bang, but is otherwise wasted energy).  Since the acceleration of the bullet will be proportional to the pressure built up behind the bullet, it therefore follows that you wish to minimise the amount of acceleration that happens just as the bullet is about to leave the barrel (ofcourse, with short barrelled guns, you may not have too much choice in the matter, but if that is the case, then you are wasting a lot of energy).

#### lightarrow

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##### The Acceleration Of a Bullet ?
« Reply #2 on: 30/11/2007 22:02:51 »
The interval of time during which there is bullet acceleration in a common gun is around some milliseconds (I remember 4 milliseconds in a rifle but of course it varies). The gas push should stop before the bullet comes out of the barrel, as George said, but to increase the shot power the gas still have lots of pressure even outside the barrel, at least in most of the guns, so the bullet is still accelerated even for distances of the order of a metre outside the gun; in these cases the precision is reduced (without the barrel the bullet undergoes forces not exactly aligned with its trajectory)

#### lyner

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##### The Acceleration Of a Bullet ?
« Reply #3 on: 03/12/2007 21:08:20 »
How much is it accelerating whilst it is going down the barrel?
If it exits at 300m/s (a sub sonic bullet) and the barrel is 0.5m long, then, the good old equations of motion give the acceleration as 90,000m/s2 , assuming uniform acceleration . Quite a lot, don't you think? About 9,000G. Not a comfortable ride.

#### another_someone

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##### The Acceleration Of a Bullet ?
« Reply #4 on: 04/12/2007 01:45:24 »
The gas push should stop before the bullet comes out of the barrel, as George said, but to increase the shot power the gas still have lots of pressure even outside the barrel, at least in most of the guns, so the bullet is still accelerated even for distances of the order of a metre outside the gun; in these cases the precision is reduced (without the barrel the bullet undergoes forces not exactly aligned with its trajectory)

Are you talking about all guns (you seem to have said so), or just short barrel guns (pistols, etc.), and maybe shot guns, but not rifles or artillery?

#### lightarrow

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##### The Acceleration Of a Bullet ?
« Reply #5 on: 04/12/2007 14:16:33 »
The gas push should stop before the bullet comes out of the barrel, as George said, but to increase the shot power the gas still have lots of pressure even outside the barrel, at least in most of the guns, so the bullet is still accelerated even for distances of the order of a metre outside the gun; in these cases the precision is reduced (without the barrel the bullet undergoes forces not exactly aligned with its trajectory)

Are you talking about all guns (you seem to have said so), or just short barrel guns (pistols, etc.), and maybe shot guns, but not rifles or artillery?
I was talking about most of the pistols and rifles with a medium or high power and with medium or high caliber; I don't know about artillery and about high precision rifles.
Anyway, to explain better what I was saying (maybe it was not very clear, I don't know) I intended that they use more powerful charges to accelerate more the bullet inside the barrel, and not that they intend to accelerate it more by deliberating giving it a push even outside the barrel; this one is an unwanted consequence of the higher charge and limited barrel lenghts.
Anyway I read it somwhere, but don't remember exactly where.
« Last Edit: 04/12/2007 14:40:30 by lightarrow »

#### lightarrow

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##### The Acceleration Of a Bullet ?
« Reply #6 on: 05/12/2007 10:59:29 »
How much is it accelerating whilst it is going down the barrel?
If it exits at 300m/s (a sub sonic bullet) and the barrel is 0.5m long, then, the good old equations of motion give the acceleration as 90,000m/s2 , assuming uniform acceleration . Quite a lot, don't you think? About 9,000G. Not a comfortable ride.
In a rifle the bullet can exit at more than 900 m/s! Also consider that the acceleration (usually) decreases along the barrel (after some centimeters), so the peak value is even more than the average one.
« Last Edit: 05/12/2007 11:02:20 by lightarrow »

#### turnipsock

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##### The Acceleration Of a Bullet ?
« Reply #7 on: 08/12/2007 01:14:07 »
remember that acceleration can also be a negative value, so unless the velocity remains the same, the bullet is alway "accelerating".

I had a shot on a firing range recently and had a rebound that grazed he side of my head. That hurt.

#### another_someone

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##### The Acceleration Of a Bullet ?
« Reply #8 on: 08/12/2007 01:28:59 »
remember that acceleration can also be a negative value, so unless the velocity remains the same, the bullet is alway "accelerating".

I had a shot on a firing range recently and had a rebound that grazed he side of my head. That hurt.

If we are going down that road, you also have to take account of the acceleration of gravity.

#### lightarrow

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##### The Acceleration Of a Bullet ?
« Reply #9 on: 09/12/2007 14:51:34 »
remember that acceleration can also be a negative value, so unless the velocity remains the same, the bullet is alway "accelerating".

I had a shot on a firing range recently and had a rebound that grazed he side of my head. That hurt.
What gun did you use?

#### The Naked Scientists Forum

##### The Acceleration Of a Bullet ?
« Reply #9 on: 09/12/2007 14:51:34 »