Twist Rates Advice please

[Ken3134]

I acquired some 6mm 95 gr Berger bullets that say on the box 1:9” Minimum. The rifle I have is a .243 Savage with a 1:9.25” twist rate. I understand the lower the number the faster the twist, and that these are close. Is this close enough that I should not expect any difference?


Thanks!
Ken

[Jordan Smith]

https://bergerbullets.com/twist-rate-calculator/

Enter the parameter values and you'll get an idea of what to expect.

[obsessed1]

Gotta shoot them to see
My savage 243 shoots 103 eldm under an inch if I runt them FAST. Slow them down and the spread out quite a bit.
Give them a try. ..wont know for sure until you do.

[aardvaark]

It might and it might not.

Best way to find out is to try, as suggested above. And if you find they shoot good at 100, you need to confirm that at 200 and 300 when you’re in a marginal situation such as this. I’ve had a few loads now that shot really nice at 100 and then opened up to 4 or 6 inches by the time they got out to 200.

[pikergolf]

I am betting they will be just fine. My understanding is that Berger is a little conservative on those recommendations.

[pikergolf]

wrong thread.

[pikergolf]

Find your bullet on the drop menu and then fill in the Altitude, Temp and velocity.

https://bergerbullets.com/twist-rate-calculator/

[Ken3134]

Looks like it should stabilize according to the calculator. I’ll load some up and give them a try.

Thanks for the help everyone!
Ken

[Dean2]

Even if you can push them fast enough, the issue with running too long a bullet for your twist rate is it may be stable up close and then lose that stability as it slows down. You will need to shoot test groups at 100 yards past the furthest you intend to hunt with these to be sure they are still stable. Personally, I would find a shorter bullet so I wasn't right on the twist rate edge, but then I am not a Berger fan at all so that probably colours it some.

[Dick284]

I tried these bullets in a .243 Savage I once owned(same ROT), and the results were less than stellar.
I ended up giving the remainder of the box away, and settled on 87gr V Max’s.

[Jordan Smith]

Originally Posted by Dean2

Even if you can push them fast enough, the issue with running too long a bullet for your twist rate is it may be stable up close and then lose that stability as it slows down. You will need to shoot test groups at 100 yards past the furthest you intend to hunt with these to be sure they are still stable. Personally, I would find a shorter bullet so I wasn't right on the twist rate edge, but then I am not a Berger fan at all so that probably colours it some.

Stability increases as linear velocity decreases, until the bullet's velocity reaches the transonic regime.

[Dean2]

Originally Posted by Jordan Smith

Stability increases as linear velocity decreases, until the bullet's velocity reaches the transonic regime.

No it doesn't, at least in terms of yaw in the bullets rotation. A bullet needs to spin at a certain rate to maintain its ability to not keyhole.

[Jordan Smith]

Originally Posted by Dean2

No it doesn't, at least in terms of yaw in the bullets rotation. A bullet needs to spin at a certain rate to maintain its ability to not keyhole.

It absolutely does. Gyroscopic stability is a function of rotational speed and translational speed. Drag force on the bullet increases quadratically with translational velocity, and destabilizes the bullet. Rotational speed is responsible for the angular momentum that leads to gyroscopic stabilization. For a bullet to be stable, the rotational speed has to be sufficient such that the rotational axis is not overturned by the torque applied by the drag force. In other words, if rotational speed is high enough, and forward velocity is low enough, the bullet will be stable. But as the bullet travels downrange, translational velocity decreases much faster than rotational speed does, resulting in an increase in stability as the bullet travels downrange.

The bullet is least stable at the muzzle, and increases in stability from there. At least until the transonic zone. So if the bullet is stable at 100 yards, it will only be more stable at 200 yards. This doesn’t mean that it’ll shoot precisely and group well, but it’ll be stable and won’t ‘keyhole’.

[Dean2]

Originally Posted by Jordan Smith

It absolutely does. Gyroscopic stability is a function of rotational speed and translational speed. Drag force on the bullet increases quadratically with translational velocity, and destabilizes the bullet. Rotational speed is responsible for the angular momentum that leads to gyroscopic stabilization. For a bullet to be stable, the rotational speed has to be sufficient such that the rotational axis is not overturned by the torque applied by the drag force. In other words, if rotational speed is high enough, and forward velocity is low enough, the bullet will be stable. But as the bullet travels downrange, translational velocity decreases much faster than rotational speed does, resulting in an increase in stability as the bullet travels downrange.

The bullet is least stable at the muzzle, and increases in stability from there. At least until the transonic zone. So if the bullet is stable at 100 yards, it will only be more stable at 200 yards. This doesn’t mean that it’ll shoot precisely and group well, but it’ll be stable and won’t ‘keyhole’.

Cool, that actually makes perfect sense. Learned something new. Doesn't explain however why bullets that make nice little groups at 100 often don't shoot worth a darn at 300, and I have seen that many times. Also, by this explanation how does a bullet not keyhole at 100 but does keyhole at 200, which I have also seen more than once. While the rotational speed does not slow as quickly I think it is possible that it slows enough to no longer have sufficient rotational speed to stabilize the long bullet that was already at the edge of what the twist could handle, or do you have a better explanation for these two occurrences.

[Jordan Smith]

Originally Posted by Dean2

Cool, that actually makes perfect sense. Learned something new. Doesn't explain however why bullets that make nice little groups at 100 often don't shoot worth a darn at 300, and I have seen that many times.

Correct, stability doesn’t necessarily explain group precision. A lot of people conflate the two, but stability is evidenced by the shape of the bullet holes in paper, not their relative dispersion.

I’ve seen scope parallax, wind, and a change in shooting rest setup all account for small groups at 100, but larger groups out further.

A marginally stable bullet will continue to pitch and yaw as it travels downrange, so has a lower effective BC, and is pushed around more by the wind.

Originally Posted by Dean2

Also, by this explanation how does a bullet not keyhole at 100 but does keyhole at 200, which I have also seen more than once.

Aerodynamic drag is not only a function of translation of velocity, but also of air density. Colder, dryer air is more dense than warm, humid air. Did you shoot the 100 yard target on a different day than the 200 yard target? A difference in environmental conditions can cause the same load and bullet to go from marginally stable to unstable.

Originally Posted by Dean2

While the rotational speed does not slow as quickly I think it is possible that it slows enough to no longer have sufficient rotational speed to stabilize the long bullet that was already at the edge of what the twist could handle, or do you have a better explanation for these two occurrences.

It’s true that rotational speed does slowly decrease due to rotational friction between the bullet and air, but forward speed decreases at an even faster rate due to drag force. A bullet that is marginally stable at the muzzle can eventually become completely stable as it travels downrange, going from an SG of ~1 to 2+.

[Bushrat]

I think we've all seen the youtube videos of spinning bullets shot into ice that are not moving but remain spinning for a long time. The only effect velocity has on bullet spin is in the barrel, upon exit from the barrel the bullet spins with very little reduction in RPM, the bullets foreward velocity has little to no effect on rate of bullet spin.

[Pathfinder76]

This is moving away from the original intent of the thread. But, given where I have lived all my life, I have little choice but to shoot in the wind. I used to hate it, but I don’t anymore. I have learned to shoot in it for load development. What I have found, is that a bullet that groups well in the wind at 100 yds is a very consistent bullet (I believe this has to do with stability but not necessarily stabilization). Or is a bullet that maintains consistency in POI and group size across a wide variety of conditions at a very wide range of distances.

[Beeman3]

For target I think they’ll be ok. For hunting look for higher stability. Stable bullets track true and straight. Unstable can turn or tumble.