RPM VS Velocity

[Vaino]

Reading all about the cats kinda "waylaid" my thinking and just thought that some words on dogs would be appropriate....afterall, fair is fair and I'm peeved that the cats are getting all the hoopla.

Back to the topic...the projectile spins at a rate according to the twist of the rifling and velocity....and that's a fact......Fred

 

[coloradoclyde]

the projectile spins at a rate according to the twist of the rifling and velocity....and that's a fact......Fred

Indubitably .... :grin:

 

[necchi]

Back to the topic...the projectile spins at a rate according to the twist of the rifling and velocity....and that's a fact......Fred

Exactly,
Guy's will experiment with let's say a Hornady Great Plains bullet, in a fast twist,, they get great groups at 70grns,, but they just know they need at least 100grns to kill a Deer, so they bump it up,, and now they're getting "key holed" targets,,
,,it doesn't matter, it's hitting kinda close to =center so it's gud-n-nuff for Deer.

 

[coloradoclyde]

Another way to visualize and demonstrate the relationship between RPM, twist rate and velocity would be to look at something like a grain auger.
The rotation of the auger is your RPM. the twist rate is the screw angle of the auger vanes and velocity is represented by how fast the grain comes out of the auger.

Really we can make the same comparison with any thing that employs an Archimedes screw, drill bits, ice and earth augers. boat motors etc.....

 

[necchi]

Now here's the gig about that key holed target,
The projectile is still rotating and it still has forward momentum,(velocity),,
but it's now in "yaw", it's rotation has twisted from the central axis developed by the barrel and rifling,, it's still spinning,, but,
When that spinning bullet hit's a solid target like a deer,, something with mass, not only does it have forward momentum, but that spinning wants to move the thing along (like a drill) in the direction of the rotational axis.
:shocked2:
So the turned bullet hits a rib, and suddenly decides to use the energy of the spin and moves up into the shoulder,, when what you want is for it to bust through the rib and keep going straight to the other side.
Key holed target = a bullet in yaw,, no longer stabilized.
A ball doesn't care.
p.s. a boat motor (or a propeller in water or air) won't work for the demonstration, both are moving a mass differently then a projectile

 

[coloradoclyde]

p.s. a boat motor (or a propeller in water or air) won't work for the demonstration, both are moving a mass differently then a projectile

Explain please?

 

[smoothshooter]

I don't know; I've had a few cats over the years that were as good and as affectionate a friend as any dog I ever had or saw.
My elderly parents have a 15 year old outside cat that they got when he was a kitten that seems to think he is part dog and part human, and they have several friends and neighbors who say he is the only cat they ever saw that they liked.

It will be a sad day around there when he dies.

Cats and dogs, like children, tend to develop better personalities the more you spend time playing and having loving physical contact with them.

To have a friend, you have to be a friend.

BTW, I like dogs too.

 

[azmntman]

Cats are for micing alone, no other need or usefulness IMHO.

However as my three German shepherds also mice well I have no cat (or mice).

As for the topic, I dont care how fast or slow my balls spin as long as the group well. When shooting conicals I was pleased with the group as long as was not keyholing and didnt ever consider any of the above....until now? Now I will unconsider the above, keep the dogs, forget about cats and continue to try to get the best round ball groups I can without worry of RPM.. But I'm funny that way :idunno:

 

[Jim C]

Cats are for micing alone, no other need or usefulness IMHO.

However as my three German shepherds also mice well I have no cat (or mice).

As for the topic, I dont care how fast or slow my balls spin as long as the group well. When shooting conicals I was pleased with the group as long as was not keyholing and didnt ever consider any of the above....until now? Now I will unconsider the above, keep the dogs, forget about cats and continue to try to get the best round ball groups I can without worry of RPM.. But I'm funny that way :idunno:

:rotf: :thumbsup:.....you ARE funny too, man!

 

[Stumpkiller]

What's the optimum RPM for whitetail deer? .54 caliber.

And where does a smoothbore fit in the equation?

Now I have to learn how to use a darned abacus so I can shoot Period Correct.

 

[leslie]

You need to make sure you don't shoot monday or friday cast balls in a smoothbore.

 

[coloradoclyde]

What's the optimum RPM for whitetail deer? .54 caliber.

Deer aren't part of the equation per say. However if you want to hit a deer then the proper rpm is determined by the projectile used and the velocity of said projectile to obtain desired accuracy.

And where does a smoothbore fit in the equation?
The lack of rpm's of a projectile fired from a smoothbore is why smoothbores are less accurate than a rifle....no rifling to impart spin, creating rpm's which create gyroscopic stability.

 

[Billnpatti]

...the projectile spins at a rate according to the twist of the rifling and velocity....and that's a fact......Fred

Uh....yep! You got it! :thumbsup: That was my original point. You know, the one before cats, dogs, mice, etc. :yakyak: That's the whole thing in a nutshell. It's mainly of concern when dealing with conicals. Not so much so when dealing with patched round balls and it does not apply when firing anything from a smoothbore. Oh sure, a projectile fired from a smoothbore can have a rotation but if it does, it is of relatively minor importance compared to firing a conical projectile from a rifled barrel.

Thank you for making my point. :hatsoff: :haha:

 

[coloradoclyde]

Oh sure, a projectile fired from a smoothbore can have a rotation but if it does, it is of relatively minor importance compared to firing a conical projectile from a rifled barrel.

I would just add that rifling allows the projectile to rotate on a singular axis there by creating stability. A smoothbore projectile can rotate on any or multiple axis, thus creating a tumble.

 

[hanshi]

I've owned both and love both. Both are deserving of a good home and affection.

 

[Gene L]

Unwisely, I'll jump in this morass of cats and RPM once again. I originally stated that a 12-twist bullet will rotate 300 times in 100 yards. I stick by that equation.

From Accurate Shooting, I quote (as I did before);

Bullet RPM Formula

Here is a simple formula for calculating bullet RPM:

MV x (12/twist rate in inches) x 60 = Bullet RPM

Quick Version: MV X 720/Twist Rate = RPM

Example One: In a 1:12”³ twist barrel the bullet will make one complete revolution for every 12”³ (or 1 foot) it travels through the bore. This makes the RPM calculation very easy. With a velocity of 3000 feet per second (FPS), in a 1:12”³ twist barrel, the bullet will spin 3000 revolutions per SECOND (because it is traveling exactly one foot, and thereby making one complete revolution, in 1/3000 of a second). To convert to RPM, simply multiply by 60 since there are 60 seconds in a minute. Thus, at 3000 FPS, a bullet will be spinning at 3000 x 60, or 180,000 RPM, when it leaves the barrel."

End of quote. I will note that this is RPM. To convert that to RPS, divide by 60, which gives you 3000 times per SECOND. Not Minute, as that's too gross a measurement to mean much.

Now the time of flight at 100 yards, we'll say for ease of math, is 1/10 of a second. (It's probably less than that.) Hence the bullet rotates 300 times at 100 yards.

But MV of a ML rifle does not go at 3000 fps. So you can use the above formula to figure out, if you want to, the actual amount of rotations the bullet makes. It's nothing like a lot of people think, however.

Please don't ridicule me for my stance on this. If there are arguments against it, or for it, let's get down to it.

 

[Gene L]

Where I was wrong was saying V doesn't affect twist. It does.

 

[coloradoclyde]

Where I was wrong was saying V doesn't affect twist. It does.

Yes, V= velocity.
You will notice all of the formulas are expressed in RPM and not RPS that is because it is easier to make calculations with whole numbers and then convert rather than vice-versa.
If we really wanted to we could calculate the revolutions in nanoseconds.

Knowledge is not always understanding.....But you're getting there.

To recap and keep your original quotes in proper context, here they are:

RPM is not how twist rate is measured. It's unimportant, really. Twist rate is measured in distance..inches, usually. RPM can be determined if you want to pursue it, I guess, but the figure is not important.

RPF (revolutions per foot) is what you're looking for and it's what the twist rate is for, at any velocity. It does not increase with higher velocity. For a 48 twist, as I said, one rotation every four feet. You can figure out how many times it rotates by measuring the distance to the target. Say 100 feet, the ball will rotate 25 times.

I make it a rule to never argue with a man who quotes Einstein. What you're saying, correct me if I'm wrong, is rate twists rates are not useful and are arbitrary based on the velocity of the bullet? The formula is D=RT, but that's forward velocity, not rotational velocity.

Source from bullet rotational speed website.

"Quick Version: MV X 720/Twist Rate = RPM

Example One: In a 1:12”³ twist barrel the bullet will make one complete revolution for every 12”³ (or 1 foot) it travels through the bore. This makes the RPM calculation very easy. With a velocity of 3000 feet per second (FPS), in a 1:12”³ twist barrel, the bullet will spin 3000 revolutions per SECOND (because it is traveling exactly one foot, and thereby making one complete revolution, in 1/3000 of a second). To convert to RPM, simply multiply by 60 since there are 60 seconds in a minute. Thus, at 3000 FPS, a bullet will be spinning at 3000 x 60, or 180,000 RPM, when it leaves the barrel.

Example Two: What about a faster twist rate, say a 1:8”³ twist? We know the bullet will be spinning faster than in Example One, but how much faster? Using the formula, this is simple to calculate. Assuming the same MV of 3000 FPS, the bullet makes 12/8 or 1.5 revolutions for each 12”³ or one foot it travels in the bore. Accordingly, the RPM is 3000 x (12/8) x 60, or 270,000 RPM."

Keep in mind, however, that bullets seldom rotate for a full minute before hitting the target or falling to the ground. So you have to figure out the time it's exposed, which will probably be 1/5 of a second of exposure.

Edited by Gene L on 12-17-15 03:59 PM. Reason for edit: added information

I'm still awaiting an explanation of how twist rates change with velocity. Hmmm...not in my experience.

I've been shooting for almost 50 years and have argued this subject before. Although I don't often receive ridicule as a tactic to avoid science. And common sense.

A 48 twist will rotate the ball once ever 48 inches. If this is wrong, please point it out to me how it's wrong. RPM is only a side issue, like how many MPH the bullet rotates. A useless statistic with a round ball. I don't see how it's important.

Please explain how is that important? Bullets do not rotate for a minute, the time of flight is more like 1/4 of a second until the projectile reaches the target. It's probably a lot less. That's the T in D=RT (Distance is Rate x Time. If you want to know the Rate of velocity or the Time of flight, you can manipulate the formula to R/T=D or however.)

A round ball is the lightest projectile in bore diameter. It also has the lowest ballistic coefficient of any projectile. Which is one of the reasons why minie balls were adopted.

And finally your quote from above;

Unwisely, I'll jump in this morass of cats and RPM once again. I originally stated that a 12-twist bullet will rotate 300 times in 100 yards. I stick by that equation.

From Accurate Shooting, I quote (as I did before);

Bullet RPM Formula

Here is a simple formula for calculating bullet RPM:

MV x (12/twist rate in inches) x 60 = Bullet RPM

Quick Version: MV X 720/Twist Rate = RPM

Example One: In a 1:12”³ twist barrel the bullet will make one complete revolution for every 12”³ (or 1 foot) it travels through the bore. This makes the RPM calculation very easy. With a velocity of 3000 feet per second (FPS), in a 1:12”³ twist barrel, the bullet will spin 3000 revolutions per SECOND (because it is traveling exactly one foot, and thereby making one complete revolution, in 1/3000 of a second). To convert to RPM, simply multiply by 60 since there are 60 seconds in a minute. Thus, at 3000 FPS, a bullet will be spinning at 3000 x 60, or 180,000 RPM, when it leaves the barrel."

End of quote. I will note that this is RPM. To convert that to RPS, divide by 60, which gives you 3000 times per SECOND. Not Minute, as that's too gross a measurement to mean much.

Now the time of flight at 100 yards, we'll say for ease of math, is 1/10 of a second. (It's probably less than that.) Hence the bullet rotates 300 times at 100 yards.

But MV of a ML rifle does not go at 3000 fps. So you can use the above formula to figure out, if you want to, the actual amount of rotations the bullet makes. It's nothing like a lot of people think, however.

Please don't ridicule me for my stance on this. If there are arguments against it, or for it, let's get down to it.

You've come far pilgrim... :grin: :hatsoff:

 

[necchi]

But the math is skewed just a bit.
A projectile is stationary until the charge is ignited, it doesn't immediately move to it's highest rate of speed. It accelerates as it moves down the barrel and doesn't reach maximum speed until it leave the barrel and no longer has acceleration behind it.
Ya can't really "simplify" the formula assuming top speed as it traverses the barrel, it's always under acceleration until it leave the barrel.
The charge determines how fast it moves in the barrel and reaches it's terminal maximum velocity at exit.

 

[fishmusic]

This has to do with pump laws. The Archimedes screw is a positive displacement conveyor with no slip. A motor boat propeller has lots of slip. I can't go into specifics because I have forgotten most of my engineering classes.