Ballistics question

declan1980

Ok, I've got a 1:8 twist rifle, and I'm pretty sure it's a right hand twist which is normal I think.
Does this mean that the bullet is inclined to drift off to the right even with no wind?
I'm using a ballistic calculator which tells me that a +8 in the twist denotes a right hand twist. But when I calculate it tries to tell me that the bullet is drifting left. Is this me or is it a fault in the program

FISMA

declan1980 wrote: »

Ok, I've got a 1:8 twist rifle, and I'm pretty sure it's a right hand twist which is normal I think.
Does this mean that the bullet is inclined to drift off to the right even with no wind?
I'm using a ballistic calculator which tells me that a +8 in the twist denotes a right hand twist. But when I calculate it tries to tell me that the bullet is drifting left. Is this me or is it a fault in the program

Not sure, but here's the way I understand it.

Clockwise rifling drifts right and counterclockwise drifts left.

Isn't clockwise (as viewed from receiver to barrel front) a right hand thread and clockwise left hand?

I guess if you want to remember it, you could always think that clock's are right (that is, correct). That's a little strange since in Physics and Math the positive direction is usually counterclockwise. Hence we say clocks are negative.

I am no expert here, but it appears a drift due to yaw, like an airplane. The rest is just aerodynamics and net Forces.

It should happen in calm air as well. As long as there is relative motion, you will have this effect.

dev110

FISMA wrote: »

Isn't clockwise (as viewed from receiver to barrel front) a right hand thread and clockwise left hand?

Which is it FISMA

One of the lads off the F/TR team was telling me about this.
He was saying if you fired a rifle, in a vacuum or with no wind at all, at 1000 yards it would hit right because most rifles have a twist rate spinning clockwise or to the right. But if you had an left hand twist rate it would hit left.

I found this amazing but if you think about it it makes sense because if you kick a football and put spin on it it will curl in that direction.

rowa

Gyroscopic drift , i thought it was bullet yaw but thats something different.

http://en.wikipedia.org/wiki/External_ballistics#Long_range_factors

Mountainy Jack

It due to the yaw of repose. As the bullet spins right handed it tends to strike slightly right and high. If it spins to the left it hits low and left. This is due to the bullets longitudinal axis tilting in those directions due to the spin.

FISMA

dev110 wrote: »

Which is it FISMA
He was saying if you fired a rifle, in a vacuum or with no wind at all, at 1000 yards it would hit right because most rifles have a twist rate spinning clockwise or to the right.

Yep! Clockwise is a right hand thread - clocks are right, so the bullet drifts right.

I didn't want to bring up the part about it occurring in a vacuum because: (a) I don't understand why and (b) I have no solid sources to cite that this is true.

If it occurs in a vacuum, I would suspect internal Forces to be at work.

Any good links to this effect? Maybe a post in the Physics forum...

Cass

The vaccuum part would be more aptly described as shooting in perfect conditions. ie - No wind, humidity, heat, cold, etc. IOW anything that could take from or add to the variances of the bullet once fired. To get its true performance.

I read an article on this a while back. Must try and find it.

Tackleberrywho

Clockwise right
Anti Clock left

Be it Rifle twist or screw threads.

FISMA

Ezridax wrote: »

The vaccuum part would be more aptly described as shooting in perfect conditions. ie - No wind, humidity, heat, cold, etc. IOW anything that could take from or add to the variances of the bullet once fired. To get its true performance.

I read an article on this a while back. Must try and find it.

EZ,
If you find that article, please advise.

I have found some links on line where people are indeed saying that drift can happen in the absence of air. Unfortunately, I cannot cite a single credible source for this phenomena.

OP/Declan,
From what I have read, this phenomena is difficult to predict as there are so many variables and that spin drift may only account for 3" at 1000yards. If so, I doubt there will be much a ballistics calculation could do.

Cass

Whatever about what any article may say i can vouch for one thing.

When i am going to shoot on the range i must do a 25yd zero card and produce it to the RO before being allowed to shoot. Most people have the bullet splitting the vertical line exactly half and half. I prefer to have my bullet breaking the line in such a way that 3/4 of the bullet hole is to the left of the vertical line, and the other 1/4 of the hole is to the right.

When i went out to the range the other day i lay down at 800 yards. Conditions were perfect. No wind and what little was blowing from time to time was from 6 O'Clock. I fired and struck the bull. The next 4 successive shots also stuck the 5 ring/bull. Meaning that even though my windage zero was "off" at the distance and with its natural right twist the bullet did drift by 4-6 inches. So at 1,000 yards the drift could be up to 7.5 inches.

Norwayviking

declan1980 wrote: »

Ok, I've got a 1:8 twist rifle, and I'm pretty sure it's a right hand twist which is normal I think.
Does this mean that the bullet is inclined to drift off to the right even with no wind?
I'm using a ballistic calculator which tells me that a +8 in the twist denotes a right hand twist. But when I calculate it tries to tell me that the bullet is drifting left. Is this me or is it a fault in the program

Hey Declan

I ordered this book from the states called Pejsas handbook of new precision ballistics a while back.
According to this book theres two effects on a bullet that should taken into consideration when shooting.

The first is the coriolis effect,the effect from the earths rotation.
just to give an example from the book.
For a shooter at the North Pole,a target at 1000 yards is moving leftwards at 2.62 in/sec.
So if shooting at 1000 yards with 1,75 sec of flighttime,the target moves 1,75x2,62=4,6 inches.
So the drift will 4,6inch at the north pole.
This is because of the earth rotaion is eastwards at 1520 fps.
Because the target is moving eastward(left) the bullet will always drift right on the northern hemisphere.
This lateral drift changes with the sin of the Latitude.
So if you are shooting in the midlands at 53 degrees North Latitude.

Sin53=0,798

The drift at 1000 yards will be 0,798x4,6inch=3,67 inch of drift in the midlands.

On the the southern hemisphere,like Australia,this drift is to the left.

The other effect on the bullet is called the gyroscopic drift error.

When firing a high power rifle the bullet leaves the muzzle with around 180000 rpm.
As such it acts as a free gyroscope.
When a torque or twist is applied to the spin axis of a gyroscope,it tends to rotate to the direction of the torque.

So right twist will rotate clockwise(to the right)_
And left twist will rotate anti clockwise(to the left)

The precise amount of deflection is difficult to predict accurately because many variables are involved.
The shape of the bullet is critical,especially the precise distance between its center of gravity and airdrag center of pressure.(BC and VLD bullets)

And this also states Ezridaxs test shooting,so you probably looking at 7-8 inches of drift or more on our latitudes.

Drift tables determined mainly from well controlled army and navy tests in the US,indicates that this drift is roughly equal to the lateral drift due to the earth rotation(coriolis effect)
Interestingly ,if we used a left twist barrel rifling in the Northern hemisphere,these two effects would cancel each other.

And the effect of air temperature,pressure and altitude is not taken into this account.(which effect the drop mainly)

Something to think about when buying a barrell.:rolleyes:

Anyway Declan hope this explains a bit more regarding the ballistics computer,that is obviously wrong.:D

sfakiaman

From an article on the artillary model Luger

"Sometimes in examination people think the leaf site is bent. However, when raising the site to the full 800 meter position reveals a 7° left movement. This design compensates for the rifling on the 200 mm barrel which gives the bullet a twist in flight and if properly adjusted will permit a corrected point of impact. Another example of the Luger's engineering".

dev110

Go to 8.30 mins in.