Barrel twist rate and bullet weight questions

[Quirt Evans]

Trying to equate velocity and "RPMs" of a certain twist rate is a chasing down a rabbit hole.
The twist rate "Is what it is", no matter the velocity.
A hundred yards=300ft=3600in.
In that 3600in. with a 1:7 twist barrel the bullet only revolves ~514 times.
In a 1:9 barrel the bullet only revolves ~400 times.
And the velocity doesn't matter!,, slow or fast, the turns are the same.
The idea of a bullet spinning like a top as it strikes the enemy soldier is a false perspective.
With perhaps a front-to-back travel of the bullet of ~12in. the bullet only makes a little over 1 revolution on its way thru,, if it makes it thru.
You can see that in your hunting rifle, with its much slower twist, the bullet might go thru the entire deer without revolving even one turn.

 

[DizzyJ]

Trying to equate velocity and "RPMs" of a certain twist rate is a chasing down a rabbit hole.
The twist rate "Is what it is", no matter the velocity.
A hundred yards=300ft=3600in.
In that 3600in. with a 1:7 twist barrel the bullet only revolves ~514 times.
In a 1:9 barrel the bullet only revolves ~400 times.
And the velocity doesn't matter!,, slow or fast, the turns are the same.
The idea of a bullet spinning like a top as it strikes the enemy soldier is a false perspective.
With perhaps a front-to-back travel of the bullet of ~12in. the bullet only makes a little over 1 revolution on its way thru,, if it makes it thru.
You can see that in your hunting rifle, with its much slower twist, the bullet might go thru the entire deer without revolving even one turn.

Velocity combined with spin rate can absolutely make a difference on how a particular bullet stabilizes and therefore performs on target. That's what I'm trying to get across. I believe this is pertinent to what the OP is trying to determine.

 

[ferret427]

You are correct. I didn't explain myself well. The rpm rate of a projectile is a function of the twist rate of a barrel and the velocity of a projectile.

What I meant to convey was that the length of a barrel, and by extension, the number of times a projectile is spun in the barrel ( like a 1:7 twist in a 21" barrel would be 3 revolutions to impart the spin, or 1:7 twist in a 3.5" barrel would be 1/2 revolutions to impart spin) will not affect affect the rpms, velocity being equal. Either would stabilize a given projectile to the same degree. I admit the differences between the 2 barrels is extreme, and that it would be neat to find 2 rifle loads that would give the same velocity out of a 3.5" barrel as a 21". I just picked them to easily illustrate the idea.

Differing twist rates can affect muzzle velocity, but only a small amount between something like identical cartridges fired from a 20" barrel one with a 1:7 and one 1:9 at rifle velcities is only a few fps, but the effects on stability could be a lot different depending on the projectile and distance shot.

 

[FordPrefect]

You're backwards. 1:9 is SLOWER than 1:7. Heavier projectiles do like faster twist rates.

I may have it backarsewords. It's the one confusing thing when I was building my CMP AR. I'll have to take a look at it again when I wake up in the morning.

 

[baker3gun]

Trying to equate velocity and "RPMs" of a certain twist rate is a chasing down a rabbit hole.
The twist rate "Is what it is", no matter the velocity.
A hundred yards=300ft=3600in.
In that 3600in. with a 1:7 twist barrel the bullet only revolves ~514 times.
In a 1:9 barrel the bullet only revolves ~400 times.
And the velocity doesn't matter!,, slow or fast, the turns are the same.
The idea of a bullet spinning like a top as it strikes the enemy soldier is a false perspective.
With perhaps a front-to-back travel of the bullet of ~12in. the bullet only makes a little over 1 revolution on its way thru,, if it makes it thru.
You can see that in your hunting rifle, with its much slower twist, the bullet might go thru the entire deer without revolving even one turn.

Yah. This is totally wrong.

Three things happen at the very instant in time that a bullet leaves the muzzle of the barrel: it starts slowing down, it starts falling, and it's spin rate starts to slow down.

It starts slowing down because it is not in a vacuum.

It starts falling because gravity is a constant.

The spin rate starts slowing because no force is acting to increase or maintain the muzzle spin rate.

Take it easy man. No one is born an expert on anything. We all have to learn. Your observations defy laws of physics. Slow down a bit.

 

[Quirt Evans]

Velocity combined with spin rate can absolutely make a difference on how a particular bullet stabilizes and therefore performs on target. That's what I'm trying to get across. I believe this is pertinent to what the OP is trying to determine.

You might re-read my post.
In no way did I say that stability is not influenced by velocity.
I was pointing out the fallacy of conflating velocity with "RPMs", (so to speak,).
And also the idea that bullets are spinning real fast thru a target.

Yah. This is totally wrong.

Three things happen at the very instant in time that a bullet leaves the muzzle of the barrel: it starts slowing down, it starts falling, and it's spin rate starts to slow down.

It starts slowing down because it is not in a vacuum.

It starts falling because gravity is a constant.

The spin rate starts slowing because no force is acting to increase or maintain the muzzle spin rate.

Take it easy man. No one is born an expert on anything. We all have to learn. Your observations defy laws of physics. Slow down a bit.

Of course the bullet slows down, of course the rotational speed slows down also,, and yeah, gravity works, and in many ways it's nice that we aren't in a vacuum.
Did you want me to spend a few paragraphs explaining the drag factors and perhaps the shifting turbulence of the parting air as the bullet drops below the speed of sound, or the effects a vacuum.
I didn't feel the need to discuss the physics of drag/gravity/vacuum.
What I posted is true,, without trying to be super-technical.

 

[Kruel J]

Temperature, elevation, Coriolis, blah blah blah. Come on guys we don't need anything this technical for the question at hand.

 

[Grizzly_A]

Listen to your barrel

Temperature, elevation, Coriolis, blah blah blah. Come on guys we don't need anything this technical for the question at hand.

 

[Quirt Evans]

The rpm rate of a projectile is a function of the twist rate of a barrel and the velocity of a projectile.

Only the first half of what you said.
I tried in a previous post to explain that.
Imagine two rifles with the same twist rate, (lets say 12in.,) one has a lead slug over some black powder,, going 1,500fps.
The other has some high-velocity smokeless load going 3,000fps.
If you fired the rifles at a target 100ft away, no matter that one got there twice as fast, each bullet still only revolved 100 times in that 100 feet to the target.
Revolutions Per Minute and Revolutions Per Distance are not the same.

 

[baker3gun]

Quirt, it's not okay for you to post wrong info in a thread where folks are asking questions to improve their own understanding.

All else the same, the faster you push a bullet through a rifled bore, the faster it will spin.

The spin rate of the bullet in rpms is a function of TWO things: the twist rate of the barrel, and the speed at which the bullet is pushed through the rifling.

Example:
A 90gr bullet is a heavy bullet for the .224 bore.

If you push a 90gr .224 bullet through a 20" 1:7 twist 5.56mm bore at 2600 fps, the 1:7 twist will spin that bullet fast enough to stabilize it in flight.

If you change nothing but the charge weight, reducing the charge so much that the muzzle velocity drops to 500fps, and change nothing else, that same bullet will be spinning much slower and will not stabilize. It will tumble and keyhole at 100 yards.

If you cut charge weight down to a fraction of a grain so that you achieve a muzzle velocity of 1 inch per second, you can catch the bullet in your bare hand. It won't be spinning at all.

Think about it.

From the same barrel, using the same bullet:
Faster velocity = faster rpms.
Slower velocity = slower rpms.

You keep taking about whether a bullet is spinning when it arrives at the target. Yes. It definitely is still spinning. Really fast. Not as fast as it was spinning when it left the muzzle, but still really fast. If you wanna see it, go to youtube and search for "bullet spinning on ice".

You keep talking about how many times a bullet spins in a complete revolution between the muzzle and the target.
No one cares.
No one ever talks about that.
No one ever calculates how many revolutions will occur between Point A and Point B because it doesn't matter.

What we talk about is whether the bullet is spinning fast enough to stabilize in flight.

Heavy-for-caliber bullets are fired at slower velocity.
You can't shoot a 90gr bullet from a .223 as fast as you can shoot a 40gr bullet from the same gun.
Slower velocity = slower spin rate.

Heavy-for-caliber bullets are longer bullets.
To make a bullet heavier, you can't make it fatter. You have to make it longer.
Longer bullets require more rpms than shorter bullets to achieve and maintain stable flight.

So that's two strikes against stability for heavy bullets. The solution is a faster twist barrel.

K then. Everyone have a great night. C yas.