Twist vs Bullet Weight Question

[Small Lady]

I know that generally speaking its important to match twist, with weight of the bullet used.
But that leaves me wondering why.
Is there a simple explanation?

 

[Justice1327]

I think truthfully, it has more to do with the length of the bullet, and the bearing surface of that length.

 

[Coyote Shadow Tracker]

I know that generally speaking its important to match twist, with weight of the bullet used.
But that leaves me wondering why.
Is there a simple explanation?

Has to do with the "Twist and RPM" of the bullet for it to stabilize and also hold together. Going too slow won't get correct stabilization. Going to fast can over stabilize and pull the bullet apart. There are articles posted from bullet manufactures with "Twist Rate Calculators".

EDIT:
There are other factors involved besides Twist, Weight. The twist will give you spin rate usually in RPM. But with the weight if the bullets can effect the Length of the bullet that can effect the Sectional Density, Bearing Surface also using BC G1/G7, ogive of the bullet
Bullets the same caliber don't always follow the same rule for weight and length. You can have two.357s one is short and stubby, and the other is long and skinny from the ogive out. Goes the same way with different calibers. you can have a small weight caliber Either long or heavy
and a heavy diameter with less with weight and short length, Then there is wind heat humidity

The easiest way is to use a Ballistic calculator,

Twist Rate Stability Calculator

Our newly updated Twist Rate Stability Calculator not only tells you what your SG is, but it also lets you know if you are reaching your optimized BC.

bergerbullets.com

 

[FEENIX]

I know that generally speaking its important to match twist, with weight of the bullet used.
But that leaves me wondering why.
Is there a simple explanation?

A heavier bullet means longer because it is the only way to increase the weight since it cannot increase the bullet's diameter. Longer bullet also requires faster twist for stability.

 

[del2les]

I know that generally speaking its important to match twist, with weight of the bullet used.
But that leaves me wondering why.
Is there a simple explanation?

Its not weight but length, as the usage of differing core materials like all copper makes the same weight bullet longer than lead core or powdered T heavier and shorter. Gyroscopic stability to keep the bullet stable and accurate in flight.
Think of a football in flight when thrown with a good spin verse one that is wobbling badly.

Simply Google a few articles on the subject.

 

[Calvin45]

I think truthfully, it has more to do with the length of the bullet, and the bearing surface of that length.

This ^^^^^

An extreme example perhaps, but Hawk makes a 250 grain 30 cal round nose…it is as blunt as can be. It's stable in a 10 twist. Despite being 50 grains lighter, the newer 200 grain matchking (there is an older design too) is listed as needing a 1:9. It is LOOOOOOOONG, like a lawn dart, and has very little bearing surface as well. Heck the 225 hornady eld m is just fine in a 10 twist, and it's actually about the same length and heavier. But has a lot of bearing surface.

 

[Calvin45]

Though I am working up a load for my 300 win 10 twist with that same 200 smk haha. Talked to the folks at Sierra via email. They think I'll be fine. The 1:9 twist is to ensure full stability at .308 velocity at sea level. 1:10 at .300 win velocity even at my modest 1100 feet elevation should have zero issues they tell me.

That's a whole other can of worms too haha!

Air Temperature, humidity, elevation, and muzzle velocity all play into whether a given twist rate is sufficient to stabilize a given projectile.

Hot air is less dense than cold, humid air is less dense than dry (yes, less: it feels thicker to us, but water vapour is lighter than air), the air is thinner the higher up you go, and the slower you run the bullet through the barrel to start with the lower it's RPMs are and this can become evidently problematic as distance increases (bullets don't really lose any rotational velocity in flight, not enough to matter: even if it's slowed down a lot in terms of horizontal velocity, it's still spinning about as fast as it was when it left the barrel. As velocity drops, higher rpms are a good thing usually.

 

[CaneBrake]

I know that generally speaking its important to match twist, with weight of the bullet used.
But that leaves me wondering why.
Is there a simple explanation?

Not quite simple. Sadly, nothing in physics is simple. It's more than just the weight. Length, bullet construction and atmospheric conditions all come into play. Essentially, the twist needs to be fast enough to gyroscopically stabilize the bullet. Imagine spinning a top on a flat surface. If the top is spinning fast enough (high angular velocity) its axis of rotation will maintain 90 degrees to the table. Once it slows down if begins to wobble. If the barrel twist isn't fast enough the bullet will wobble like a top that is slowing down. That wobble increases drag, decreases accuracy, and severely effects terminal ballistics. The heavier and longer the bullet the harder it is to stabilize. Weight is a little harder to visualize. Again, imagine spinning top. Easy right? Now imagine that same top weighed 20lbs. It probably would barely make a few rotations before falling. The heavier top has more mass and a higher moment of inertia. It's much harder to accelerate that bullet to reach the critical angular velocity to stabilize it.

 

[Mikecr]

I doubt stability differences are significantly tied weight differences. That just fails too many tests.
Stability is tied to center of mass -vs- center of pressure. This, creating an overturning arm, the length of which depends on their spread.
Longer bullets (regardless of weight) tend to exhibit more overturning moment -because their center of mass falls further behind center of drag.
Higher drag causes the center of pressure to be higher w/respect to center of mass.

Both must be overcome with gyroscopic inertia per overturning displacement (not per time)(forget RPMs).
And consider the reality that gyroscopic inertia would not be needed for bullets in the vacuum of space.

 

[Calvin45]

I doubt stability differences are significantly tied weight differences. That just fails too many tests.
Stability is tied to center of mass -vs- center of pressure. This, creating an overturning arm, the length of which depends on their spread.
Longer bullets (regardless of weight) tend to exhibit more overturning moment -because their center of mass falls further behind center of drag.
Higher drag causes the center of pressure to be higher w/respect to center of mass.

Both must be overcome with gyroscopic inertia per overturning displacement (not per time)(forget RPMs).
And consider the reality that gyroscopic inertia would not be needed for bullets in the vacuum of space.

Agree with some of this. But if one is to forget RPMs how else does one explain the real and observed phenomena of a given bullet at a given twist rate having stability issues mitigated by greater muzzle velocity?


But absolutely how aggressively "rear heavy" a bullet is is way more important than how heavy it is.

 

[CaneBrake]

I doubt stability differences are significantly tied weight differences. That just fails too many tests.
Stability is tied to center of mass -vs- center of pressure. This, creating an overturning arm, the length of which depends on their spread.
Longer bullets (regardless of weight) tend to exhibit more overturning moment -because their center of mass falls further behind center of drag.
Higher drag causes the center of pressure to be higher w/respect to center of mass.

Both must be overcome with gyroscopic inertia per overturning displacement (not per time)(forget RPMs).
And consider the reality that gyroscopic inertia would not be needed for bullets in the vacuum of space.

Mass does play a big part. Center of gravity and Center of Pressure are things to consider but only once the bullet is not stabile and we do need to know them to figure out what the bullet needs to become stabile. You are correct, that is another reason longer bullets are harder to stabilize. We are trying to make sure the bullet is gyroscopically stabile so that the center of pressure offset from the center of gravity is nearly eliminated. Mass is going to factor into how much moment of inertia the bullet has. That's also why bullet construction matters because the distribution of its mass also changes its moment.

 

[Calvin45]

For the OP @Small Lady Here's some pictures to illustrate what we've been saying.

Here is a 225 eld m and a 200 SMK (.30 cal) beside each other, bases flush against a butter knife. The 225 is heavier obviously BUT as you can see, it is longer too.

HOWEVER….hornady lists the minimum stability twist as 1:10, Sierra as 1:9.

Here, I have them beside each other BUT with the transition points from shank to ogive being parallel (as crudely determined by etching a little ring into them with a resized .300 win mag case mouth. As you can see, the ogive on the 200 SMK is actually longer than the 225 eldm.

Finally, side by side with boat tail/shank junctions parallel. Again, the smk is just a more "severe" and less forgiving bullet: it's boat tail is notably longer than the eld too!

These things are why it actually does take more twist to stabilize this smk: longer ogive, longer boat tail, waaaaaaay shorter bearing surface, and definitely more aggressively rear heavy all means it's less stable at a given twist .

 

[Mikecr]

if one is to forget RPMs how else does one explain the real and observed phenomena of a given bullet at a given twist rate having stability issues mitigated by greater muzzle velocity?

Twist requirement is expressed with displacement per turn. This holds, regardless of velocity.
It IS NOT expressed in turns per time (RPMs). That absolutely fails all tests.
Higher velocities change where a bullet is in it's drag curve at any given moment. Drag changes the center of pressure, and Sg totally follows a bullet's drag curve.
It is VERY rare that velocity alone will bail anyone out of a stability issue, because drag does still go up (overall) with the square of velocity.

We are trying to make sure the bullet is gyroscopically stabile so that the center of pressure offset from the center of gravity is nearly eliminated.

Changing Sg does nothing to change the overturning moment arm. That remains.
What changes is the gyroscopic inertia to overcome the moments.

Mass is going to factor into how much moment of inertia the bullet has. That's also why bullet construction matters because the distribution of its mass also changes its moment.

This is true. Both a mass change, and center of mass change, can occur with either addition or deletion of weight.
So a simple change in bullet weight, in itself, is meaningless to stability.
You couldn't declare that a heavier bullet will always be more or less stable with a given twist rate. That will fail tests as Calvin45 has pictured.

 

[ARlife4me]

Velocity does play a part in stabilizing just as twist does.................some bullets more than others! When the 224V load data explains that you need a 90gr (#9290) 6.5T if less than 2,650fps and 7T if equal to 2,650fps. Bullet parameters didn't change, but twist requirements did.