Twist requirements for same weight, different bullet?

[MakeItHappenCaptain]

Looking at 75gr Hornady A-Maxes and there is no minimum twist specified, whereas for example Speer and Sierra match rounds of 65+gr weight are all specifying 1-9" or higher. Is this just a function of VLD bullets or are Hornady missing something here. Looking to use in a S491 .223 Sako with 1-12".

Cheers in advance.

[deye243]

not with that twist 1:12 is not enough to stabilize a horn 75g AMAX

[Jack V]

It's not just about weight . it's also about length of the bullet . They just use weight as a rough guide. Velocity is also a factor and barrel condition and length can make a difference . I am thinking 1 in 8 is required.

[newsteadvic]

whereas for example Speer and Sierra match rounds of 65+gr weight are all specifying 1-9" or higher.

Do you mean lower (faster twist)?

As Jack V states the important factor is length, not mass as described in the Greenhill forumula. For the 1.07" long 75grain A-Max this suggests a twist of 8.6" as minimum:
http://kwk.us/twist.html

[MakeItHappenCaptain]

Newsteadvic,
Yup, apologies, faster twist.

Might be sticking with the 60gr for this barrel.
Thanks for the responses.

[VonKlitzing]

Actually, it's not about weight at all.... Taken from Patrick Sweeney's "Gunsmithing Rifles";
"Remember, it is the ratio of twist to bullet length, and not its weight that is the determining factor of stability and accuracy...."

How do you calculate twist? The method listed in the 1929 book British Textbook of Small Arms gives a formula plenty good enough even today. “The length of the bullet in calibers, multiplied by the twist rate in calibers per turn, is 150.”

Known as the Greenhill rule after Sir Alfred George Greenhill, Professor of Mathematics at Woolrich, England, this formula gives a twist that produces a gyroscopic stability between 1.5 and 2.0. Gyroscopic stability less than 1.0 means a serious loss of accuracy, while 1.3 is a marginal level of stability. The “sweet spot” for most-accurate stability is right in the 1.5 to 2.0 area, although the useful range goes from 1.3 up to 4.0 or 5.0. The higher GS figures do not begin to affect accuracy until it gets larger than 5.0.

[huccl]

Yep, it's about length (she said )

The incorrect term of "bullet weight" instead of length when talking has well and truly stuck though.

[Gwion]

Newsteadvic,
Yup, apologies, faster twist.

Might be sticking with the 60gr for this barrel.
Thanks for the responses.

As the guys are suggesting, a shorter heavier bullet will stabilise ok. You should be able to find something around 65gn that will stabilise well. Not sure if you will find anything heavier that will work too well, although i have heard of people shooting up to around 69gn in a 1:12 but details are sketchy and what may be "ok" for them may not cut the mustard for you. No harm in trying, other than wasting a few bucks on projectiles, etc. I'll soon be going through that process myself as i'm getting low on my current proj and wanting to go heavier for longer range target work.

[Arth]

The incorrect term of "bullet weight" instead of length when talking has well and truly stuck though.

It's those damn forums spreading misinformation I say!

[MakeItHappenCaptain]

75 gr Hornadys (surplus range rounds) keyholed as expected. Picked up some 60gr V-Maxes. Box actually specified 7-12". No mention on website though.

[Josh Smith]

Hello,

It's more about length, as was mentioned.

For example, a lead core 0.311" bullet that weighs the same as a 0.311" steel core or 0.311" monolithic copper bullet will be shorter than these latter bullets.

Regards,

Josh

[Prios]

Looking at 75gr Hornady A-Maxes and there is no minimum twist specified, whereas for example Speer and Sierra match rounds of 65+gr weight are all specifying 1-9" or higher. Is this just a function of VLD bullets or are Hornady missing something here.

All bullets have a twist requirement, whether it's mentioned or not.

It may be the case for some that the requirement is so slow that it's not worth mentioning as it'll work in everything. e.g. most .223s come between 1:12 and 1:9. If a projectile would theoretically work in a 1:15twist barrel there is no need to make a big point of it.

Others I guess if you're reloading match or VLD bullets they assume you have some understanding of twist and its requirements.

It would be nice if each brand listed the tested twist for each bullet though.

[Herdsman]

Maybe we should all go back to musket balls so it's nice and simple

[Gun Nut Gordo]

1:12 I wouldn't really go over 55gn

[Herdsman]

Found a good read here

http://www.6mmbr.com/223Rem.html

Scroll down to 'Twist Rates' about 1/4 way down.

[Tonysize]

I found these searching around the net, pretty interesting. It basically says that plastic tip and OTM's can stablize with a slower than normal twist for their length.
The PDFs link is in the top right.

Gyroscopic Stability of Open Tipped Match Style Rifle Bullets
http://arxiv.org/abs/1401.4187v2

A Stability Formula for Plastic-Tipped Bullets
http://arxiv.org/abs/1410.5340

[Wes]

University research on shooting, huh