Bullet length to twist rate

[WisconSniper]

As far back as i can remember I've been told at twist rate is determined by bullet weight. However, im beginning to think that it is more relevant to the bearing surface of the bullet, not the weight. I understand that the weight is almost directly related to the length (duh) but to get the most accurate calculation of the twist rate desired im curious if this is in fact what the twist rate should be figured off of. Maybe I'm splitting hairs, but from a benchrest shooters perspective it may be helpful.
Also, would it be better to have the slowest possible twist rate that will stabilize the bullet to minimize spin drift?

~WS

 

[Deleted member 46119]

I can't comment on the spin drift part of the discussion but as I understand it.

It is much more the aspect ratio as in length vs. diameter. Weight is part of it but I believe aspect ratio is very important.

A heavy for diameter bullet will of course be longer. Mono/copper bullets will be longer for the same weight.

In Example: the .308 Nosler CC 168 is 1.175" long for an aspect ratio of ~3.815. To get a copper bullet with the same aspect ratio would be the Barnes TTSX BT of 130 grains with a length of 1.173".

Weight is clearly important but there is a lot to it.

 

[WisconSniper]

I kind of thought that's how it worked. I was shooting groups with the same load and weight of bullet from different brands with similar BC's and was getting some interesting grouping results and thought I'd ask some of you guys about it.

~WS

 

[Deleted member 46119]

I can't comment on the spin drift part of the discussion but as I understand it.

It is much more the aspect ratio as in length vs. diameter. Weight is part of it but I believe aspect ratio is very important.

A heavy for diameter bullet will of course be longer. Mono/copper bullets will be longer for the same weight.

In Example: the .308 Nosler CC 168 is 1.175" long for an aspect ratio of ~3.815. To get a copper bullet with the same aspect ratio would be the Barnes TTSX BT of 130 grains with a length of 1.173".

Weight is clearly important but there Is a lot to this stability thing

 

[Mikecr]

Consider 'twist rate' first for what it is; inches of displacement per turn -to overcome that displacement.
Your destabilizing factor is center of pressure to center of gravity length(overturning arm) with Cp forward of Cg, and the force applied to that length.
Now consider stability; given fixed bullet construction, as center of pressure changes (more/less, forward/rearward), stability changes with it. The center of pressure position and amplitude, again given fixed bullet construction, are influenced by air density. This is your displacement, let's call it relative displacement, which is destabilizing.

Gyroscopic forces established by each turn can overcome some amount of destabilizing forces(the overturning moment Cp:Cg), but only just so much. When we apply more relative displacement than each turn can overcome, we go unstable.

Bullet construction as a variable influences both Cp and Cg. You can change all kinds of things to change the overturning arm. You can change drag amount and locations to influence effective CP, and watch stability change with BC, and you can make stability follow or counter varying BC.
This is why rules of thumb cannot pass all tests. Then you get into dynamic stability which cannot be predicted at all, but must be tested for. Then you have other local/equipment challenges to stability..

Seriously, don't even bother with an endeavor to simplify bullet stability. It really really comes down to a bunch of things combined.

 

[WisconSniper]

Great read man. Actually makes a lot of sense. Everything ballistic is scientific and you can learn something new about bullet flight every day and not know everything.

~WS