Selecting the best bullets based on twist rates

[coreydb]

Like so many hunters, I bought a rifle, then searched for the best/most accurate ammo for my caliber. Some brands didn't shoot as well as the others..... WTF!
I decided to build a custom rifle , but based on my experience with my manufactured rifle, I didn't know which barrel twist was optimal for my caliber. I searched the web, looked into different forums, and got many opinions based on their own successes. They all got me to thinking which rifling would actually maximize my bullet selection with my caliber, so I could expand its usage in the field. Then I came across this website: https://bergerbullets.com/twist-rate-calculator. Just fill in the blanks, and it determines the stability of the bullet based on:
-bullet weight (grams)
-ballistic coefficient
-bullet length (different lengths for different weights)
-barrel twist rate
-ft per sec of the manufactured ammo or desired velocity if hand loading

I calculated each bullet in my caliber from the lightest weight (grams) to the heaviest based on the different twist rates offered by the custom barrel makers. I discovered (Forest Gump moment here) that the accurate ammo from my rifle was loaded with the bullets best suited for the rifling....(duh). The different brands weren't the problem, I shot good ammo from a rifle that didn't have the the rifling that optimized all the bullets.

I know many, many hunters are re-loaders and already know this, but for those who aren't that knowledgeable, this will pre-empt a lot of frustration.
Best to everyone in our community, F.G.

 

[Mekaniks]

Yes sir you are spot on...

I built a custom rifle and started with the caliber and specific bullet that I wanted to shoot 1st. Then I selected the barrel and twist to go with it... Turned out to be the most accurate hunting rifle I have...

I always just used this formula- Bullet diameter squared, times 150, then divided by the bullet length = optimum twist in inches.

The Berger formula you posted is excellent and takes many more factors into consideration. I like it and wish it had been available when I needed it .

 

[coreydb]

Yes sir you are spot on...

I built a custom rifle and started with the caliber and specific bullet that I wanted to shoot 1st. Then I selected the barrel and twist to go with it... Turned out to be the most accurate hunting rifle I have...

I always just used this formula- Bullet diameter squared, times 150, then divided by the bullet length = optimum twist in inches.

The Berger formula you posted is excellent and takes many more factors into consideration. I like it and wish it had been available when I needed it .

Ain't it the truth!

 

[sgt_zim]

Like so many hunters, I bought a rifle, then searched for the best/most accurate ammo for my caliber. Some brands didn't shoot as well as the others..... WTF!
I decided to build a custom rifle , but based on my experience with my manufactured rifle, I didn't know which barrel twist was optimal for my caliber. I searched the web, looked into different forums, and got many opinions based on their own successes. They all got me to thinking which rifling would actually maximize my bullet selection with my caliber, so I could expand its usage in the field. Then I came across this website: https://bergerbullets.com/twist-rate-calculator. Just fill in the blanks, and it determines the stability of the bullet based on:
-bullet weight (grams)
-ballistic coefficient
-bullet length (different lengths for different weights)
-barrel twist rate
-ft per sec of the manufactured ammo or desired velocity if hand loading

I calculated each bullet in my caliber from the lightest weight (grams) to the heaviest based on the different twist rates offered by the custom barrel makers. I discovered (Forest Gump moment here) that the accurate ammo from my rifle was loaded with the bullets best suited for the rifling....(duh). The different brands weren't the problem, I shot good ammo from a rifle that didn't have the the rifling that optimized all the bullets.

I know many, many hunters are re-loaders and already know this, but for those who aren't that knowledgeable, this will pre-empt a lot of frustration.
Best to everyone in our community, F.G.

In general, most mfgs cut their barrels for the heaviest/longest bullet made for that caliber. Exceptions I know about are 270 Win and certain 284 caliber cartridges.

For 270, standard twist rate is 10. That should stabilize up to 160 gr bullets, but no heavier.

A lot of 7mm barrels are cut with a 9.5 twist, which will stabilize the vast majority of 175 gr bullets, with 3 notable exceptions: Accubond Long Range, TSX/TTSX, and Hammer bullets designed for optimum stability - having the least bearing surface (the part of the bullet, the shank, actually in contact with the rifling) with long, secant ogives (thinking specifically about Hammer Hunter and HHT). If you want to shoot the heavier Hammer bullets out of a 7mm, the Stone Hammer line is the best option. They have the longest bearing surface and I believe they've all tangent ogives, or hybrid ogives.

This "issue" with Hammer bullets isn't unique to 7mm, though. If you want to shoot their heavier HHT or Hammer Hunters in any caliber, you're probably going to need a barrel with a faster twist rate than standard. I'd have to go take another look, but I believe the heaviest 6.5 mm Hammer I can shoot out of my 8.5 twist 6.5 Swede is their 130 gr Stone Hammer.

Twist rate is entirely about bearing surface, bullet weight, and dwell time (the amount of time the bullet is in the barrel). A heavy bullet with a long secant ogive, little bearing surface, and fired at high velocity needs a faster twist.

FWIW, most hunting bullets have tangent ogives and long bearing surfaces.

Ballistic coefficient isn't really a factor, though most target bullets do have less bearing surface and long, secant ogives.

We can look at my 6.5 Swede loads to understand why BC isn't really a factor. Hammer's 139 gr Stone Hammer has a G1 BC of .448 - it requires an 8 twist. The money load for my 6.5 Swede is a 160 gr Woodleigh Weldcore - it has a G1 BC of .51

 

[coreydb]

You are correct with your information, but BC is actually an important component along with bullet weight in determining optimal bullet performance from a specified twist rate. A low BC bullet won't fully stabilize if a barrel twist isn't enough to propel it with an adequate muzzle velocity. When a shooter sees punch marks like sideways pencil ends at 100 yds , it's because the bullet couldn't stabilize in time. Yep, this is an extreme example from hand loading. Try Barnes calculation program for the heck of it and enter both your bullets. More than likely both are more than stabilized with your 1:8 twist. It's actually kind of fun from a point of curiosity, how many different bullets you can actually use. I would love to have your 6.5 Swede. All I find are worn out rifles.

 

[sgt_zim]

BC is not a factor for calculating stability, other than a low BC bullet will reach transonic flight at a nearer distance than a higher BC bullet will.

I've used stability calculators published by Berger, JBM, Omni, and Bison. None of them uses BC as a criterion because it is only a byproduct of bearing surface, ogive shape, and overall bullet shape (boat tail vs flat base). A high BC bullet with a boat tail and a long, secant ogive will by necessity have less bearing surface because so much of the bullet's mass (and length) is in the ogive.

As I said previously, high BC bullets often do need faster twist rates, but it isn't due to the BC, it's due to the longer, more aerodynamically efficient secant ogive, and thus less bearing surface, not the BC.

Hornady ELD-X 143 gr, with BC of .625, will stabilize in my 8.5 twist 6.5 Swede. Stone Hammer 139 gr (actual weight is 135 gr, no idea why they label it 139 gr), with G1 BC of .448, will only marginally stabilize.

If BC were a factor, neither the Hornady nor the Woodleigh would stabilize in an 8.5 twist, but Stone Hammer should. But they both do, and Hammer won't because it has too little bearing surface for its weight. More bearing surface = more friction between bullet and rifling - it's physically easier to impart spin with a larger bearing surface.

The rifling has to overcome the inertia of a bullet not wanting to spin at all as it leaves the brass. A longer bearing surface necessarily means more friction to help overcome the inertia of not wanting to spin. Newton's first law is as important when it comes to imparting spin on a mass as it is to everything else.

 

[coreydb]

We will agree to disagree then.. The Barns calculator I referenced does in fact include the BC and it does have a part in determining a bullet's stability in flight. Take care my friend. It's ok to disagree, You have your proof and I have mine.

 

[mark-hunter]

I would build rifle for heaviest bullet per caliber, and then with fastest twist rate.
For example, 308 caliber goes from 150 grain, to possibly 190, 200 grains.
barrel twist for 308 is in average from 1/12 to 1/10.
So, I would build 1/10 twist.

It is possible to make faster twist rate from standard proven limits, but this is mainly done for rifles for specific use, like for example long range shooting rifle and hand made reloaded ammunition. In that case they want heaviest possible bullets, and in the same time not to loose too much powder volume in casing, so cartridge is made longer then standard SAAMI specs.
Then it will not fit in standard magazine, so the rifle will be single shot, etc...

 

[Hunter-Habib]

Yes, I 100% agree.

My 7x57mm Mauser Churchill Gun Makers Model Deluxe has a 1:8.7” twist rate. It shoots the 170Gr-177Gr bullets most accurately.

My grandson’s .30-06 Springfield Anschutz Modell 110 has a 1:10” twist rate. It shoots the 180Gr-220Gr bullets most accurately.

I’ve seen 7x57mm Mauser rifles with 1:9.5” twist rates which won’t stabilize any bullet heavier than 150Gr with any degree of accuracy. I’ve also seen .30-06 Springfield rifles with 1:11” twist rates which won’t stabilize any bullet heavier than 180Gr with any degree of accuracy.

 

[coreydb]

Yes, I 100% agree.

My 7x57mm Mauser Churchill Gun Makers Model Deluxe has a 1:8.7” twist rate. It shoots the 170Gr-177Gr bullets most accurately.

My grandson’s .30-06 Springfield Anschutz Modell 110 has a 1:10” twist rate. It shoots the 180Gr-220Gr bullets most accurately.

I’ve seen 7x57mm Mauser rifles with 1:9.5” twist rates which won’t stabilize any bullet heavier than 150Gr with any degree of accuracy. I’ve also seen .30-06 Springfield rifles with 1:11” twist rates which won’t stabilize any bullet heavier than 180Gr with any degree of accuracy.

Yep...same or similar results from my own rifles, which is why I wanted to share Barnes' calculator with the forum. It took the guess work out of bullet selection for maximum stability/accuracy, but playing around with it also showed me the different bullets I could shoot with my existing barrel twists. Open the link and play around matching your current barrel twist with the bullets you would like to shoot.

 

[Hogpatrol]

With the online programs, twist is easy to discern but I don't see any mention of freebore. If it's not the correct length, e.g. long enough for a particular bullet and it needs to be seated deeper, either to fit the mag or prevent a jam in to the rifling, powder capacity now becomes a factor.