Bullet weight/length vs twist rate

[Shankin McStaby]

Example cartridge = .223/5.56

So we know that as bullet weight increases, generally, a tighter twist barrel is required

Examples:

1:9 twist is good for 55-70gn
1:7 or 1:8 is good for 70gn+

My question: Is this a function of the weight of the bullet, or the length of the bullet?

If you were to take 2 bullets at 70gn, same profile, but one is a lead core, and one is a copper solid, would the longer solid bullet stabilize better in a 1:9 barrel, or vise versa?

I ask this because I got into a discussion about it with my old man, and told him that my thinking was that the longer solid, having a longer bearing surface, would stabilize better..... he disagreed for reasons I can’t remember right now.

What are y’all thoughts/experiences?

 

[pjparker]

Example cartridge = .223/5.56

So we know that as bullet weight increases, generally, a tighter twist barrel is required

Examples:

1:9 twist is good for 55-70gn
1:7 or 1:8 is good for 70gn+

My question: Is this a function of the weight of the bullet, or the length of the bullet?

If you were to take 2 bullets at 70gn, same profile, but one is a lead core, and one is a copper solid, would the longer solid bullet stabilize better in a 1:9 barrel, or vise versa?

I ask this because I got into a discussion about it with my old man, and told him that my thinking was that the longer solid, having a longer bearing surface, would stabilize better..... he disagreed for reasons I can’t remember right now.

What are y’all thoughts/experiences?

Bullet length determines twist rate. Weight does not enter in to the calculation. Velocity can help in some cases when stability is marginal. Per caliber, as weight increases, bullet length increases requiring more twist to stabilize due to the increase in length. In your example, the copper 70 grn bullet may require a tighter twist than the lead core one.

 

[918v]

It’s more complicated than just length and weight.

 

[spife7980]

It’s where the weight is at.

Crudely: Ideally the bullet is spinning perfectly along its axis of motion.

But the real world isn’t perfect so any tiny bit that bullets center of mass is off of the perfect middle can cause the bullet to lose its stability. You’re restricted by bore size diameter so the heavier the bullet/the more mass for longer distance shooting the longer you are forced to make that bullet. The longer the nose the more it can decrease air resistance for better long range flight. The longer the bullet is the longer the fulcrum is that forces can cause instability can act upon. Twisting it faster can help to spread that force more evenly around the entire bullets rotational path so one side of the bullet doesn’t experience more drag that the other and begin the process of tumbling.

 

[Shankin McStaby]

Sweet! Thanks for all the info guys. Makes more sense now.

Another question:

.....Velocity can help in some cases when stability is marginal....

How does this work?

Thanks again.

 

[Sheldon N]

Speed helps bullet stability because it increases overall RPM.

You can mess around with this twist rate calculator and see what affects stability. Temp/air density is another big factor.

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

 

[Shankin McStaby]

Nice. I’ll check it out. Thanks.