30 cal twist rates

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30 cal twist rates

Post by Jamesb570 » 27 Sep 2017, 7:59 pm

Hi all, Just trying to tap into the collective brains trust for some knowledge - I have an Eddystone M17 that had been converted back to .308 for range work. I have all the gear to take her up to 300 H&H and that was the direction I was going until I measured the twist rate on the retro-fitted barrel and a little snag came up. The barrel fitted has a 1:10 twist.
I already have a Remington .308 as a general purpose hunting rifle and so was planning to go up to 300 H&H and use heavier projectiles for bigger game.
Conventional wisdom states that for heavier bullets i.e. 180 - 220gn in .30 cal then a 1:12 barrel is recommended.
My question is this - Would the 1:10 twist barrel work with 180-220gn projectiles? Has anyone had experience with doing this and did it work for you? As I have stated I know what the theories say but was looking for some real world experiences?
Any help is appreciated. Thanks James
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Re: 30 cal twist rates

Post by in2anity » 27 Sep 2017, 8:21 pm

Yes - do some reading about 300blk rifles
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Re: 30 cal twist rates

Post by bladeracer » 27 Sep 2017, 8:28 pm

Jamesb570 wrote:Hi all, Just trying to tap into the collective brains trust for some knowledge - I have an Eddystone M17 that had been converted back to .308 for range work. I have all the gear to take her up to 300 H&H and that was the direction I was going until I measured the twist rate on the retro-fitted barrel and a little snag came up. The barrel fitted has a 1:10 twist.
I already have a Remington .308 as a general purpose hunting rifle and so was planning to go up to 300 H&H and use heavier projectiles for bigger game.
Conventional wisdom states that for heavier bullets i.e. 180 - 220gn in .30 cal then a 1:12 barrel is recommended.
My question is this - Would the 1:10 twist barrel work with 180-220gn projectiles? Has anyone had experience with doing this and did it work for you? As I have stated I know what the theories say but was looking for some real world experiences?
Any help is appreciated. Thanks James


Tighter twist is required for heavier bullets. 300BLK runs 7" twist for very heavy bullets.
Easiest way to see if it'll stabilise heavy bullets is simply to load some up and shoot them. If they work in .308 you won't have a problem when it's rechambered.
What twist rate is the Remington .308?
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Re: 30 cal twist rates

Post by Apollo » 27 Sep 2017, 10:51 pm

Minimum twist rates are not only about bullet weight but also bullet design and one factor is say for a 180gr bullet how long the bearing surface is and if it is a tangent or secant design, boat tail or flat base. One may be 1:12 and the same weight but longer may call for a 1:10 twist.

Another factor is also the velocity that can be achieved depending on the actual calibre / cartridge design being used.
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Re: 30 cal twist rates

Post by straightshooter » 28 Sep 2017, 7:47 am

Jamesb570 wrote:Conventional wisdom states that ..........s


By 'Conventional wisdom' I expect you mean opinions.

The twist rate is purely dictated by physics.
1 in 10 will adequately stabilize up to 220 grain projectiles in 30 caliber chamberings from 308 and larger.
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Re: 30 cal twist rates

Post by BRNO_Bigot » 28 Sep 2017, 1:13 pm

To be pedantic - it has nothing to do with bullet weight as such - it's bullet length, and most formulas are for a bullet as a cylinder, making them just approximations for real-world bullets.

The other factor is rotation rate, which depends on the barrel twist rate and muzzle velocity.

This means that you can have a certain bullet stabilising beautifully, and you change to a Barnes or similar weight, shape and muzzle velocity and the length causes it to become unstable .... exterior ballistics can be such fun. :)
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Re: 30 cal twist rates

Post by Jamesb570 » 28 Sep 2017, 4:03 pm

Thanks for all the responses , guess it is either back for more reading & research or just build it and go bang. Fun times either way!
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Re: 30 cal twist rates

Post by bladeracer » 28 Sep 2017, 7:21 pm

straightshooter wrote:By 'Conventional wisdom' I expect you mean opinions.

The twist rate is purely dictated by physics.
1 in 10 will adequately stabilize up to 220 grain projectiles in 30 caliber chamberings from 308 and larger.


While it is dictated by physics, the weight of the bullet is not a constant, it's length that matters. A long VLD bullet of 220gn might not stabilise in a barrel that stabilises a 220gn RN bullet. Similarly, a monolithic brass or copper 220gn bullet will be much longer than a composite one and might also not stabilise.
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Re: 30 cal twist rates

Post by straightshooter » 03 Oct 2017, 7:22 am

Now I might be a weirdo but when I go into a gunshop to buy projectiles I generally ask for them by caliber and weight. I think I might get a blank stare if I asked for them by length.
So I think the original poster was simply looking for straightforward guidance in his barrel twist decision and not obtuse references to the Greenhill formula.
Anybody can do a search for the Greenhill formula and there will no doubt be online calculators even though the algebra is exceedingly simple. This is because the formula is a massive simplification which has both positive and a negative aspects. Nonetheless for general use at short and medium ranges it gives a perfectly adequate answer to the question.

At this point we now turn up the pedanticism to level 8.
What we really need to know is the Gyroscopic Stability Factor where if GS is less than 1 then the bullet is unstable, if GS is greater than 1 then it is stable. But stability breaks down into areas which are marginally stable or too stable (for long range use but not so for short or even medium ranges). The sweet spot is a GS in the region of 1.5 to 2.
A simplified calculation (but far more complex than the Greenhill Formula) that gives a reasonable approximation of GS takes into account
Bullet length
Tip diameter
Ogive type
Ogive length
Boattail length
Boattail diameter
Density
Velocity
Twist
and is solved for the desired GS.
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Re: 30 cal twist rates

Post by Daddybang » 03 Oct 2017, 7:42 am

My bloody head hurts!!! :lol:
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Re: 30 cal twist rates

Post by bladeracer » 03 Oct 2017, 7:47 am

straightshooter wrote:At this point we now turn up the pedanticism to level 8.


I wasn't being pedantic :-)
When you are pushing the limits of stability one of things you need to look at when researching the bullet you want to run is its length. Otherwise you are just gambling on them working, which is fine also. I have bullets that have zero possibility of working in barrels I currently own. The 7mm 175gn ELD-X is 42mm long!
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Re: 30 cal twist rates

Post by Prios » 03 Oct 2017, 9:13 am

Daddybang wrote:My bloody head hurts!!! :lol:


I'm going back to a topic that's just full of pictures instead :lol:
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Re: 30 cal twist rates

Post by Jake peralta » 04 Oct 2017, 6:49 pm

Some really intersting thoughts here. Speed helps.......but not as much as you think. A projectile can be stable, but have a percentage of bc lag/tradeoff etc. Length v weight is the key.
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Re: 30 cal twist rates

Post by Jamesb570 » 04 Oct 2017, 8:01 pm

Good for you Prios, If someone could explain this in Pics it might help me? :unknown: No really, I think I just let the numbers fake me out and didn't really get that 1:10 is better suited to longer / heavier projectiles than 1:12. I will explore the formulas provided etc to work out what projectiles might be the most suitable but the project will continue ( given all the crap I have acquired to complete the build that is just as well). Thanks for all the help
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Re: 30 cal twist rates

Post by marksman » 05 Oct 2017, 5:38 pm

Jamesb570 wrote:Good for you Prios, If someone could explain this in Pics it might help me? :unknown: No really, I think I just let the numbers fake me out and didn't really get that 1:10 is better suited to longer / heavier projectiles than 1:12. I will explore the formulas provided etc to work out what projectiles might be the most suitable but the project will continue ( given all the crap I have acquired to complete the build that is just as well). Thanks for all the help


well done, I was sort of scratching my head thinking that I didn't get the question as a 1:10 twist is usually what factory rifles will have to suit the heaviest projectiles that can be loaded for that round
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Re: 30 cal twist rates

Post by straightshooter » 06 Oct 2017, 6:58 am

marksman wrote:well done, I was sort of scratching my head thinking that I didn't get the question as a 1:10 twist is usually what factory rifles will have to suit the heaviest projectiles that can be loaded for that round


Well put marksman
To explain it any simpler it would need to be written in crayon.
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Re: 30 cal twist rates

Post by Member-Deleted » 06 Oct 2017, 9:43 pm

HERE goes ;;;;;;;; I have stated on this forum a while back that i'm going to rebarrel a 30-06
Now what twist rate is perfect for 162--175gr bullets not boarder line not over stabilising one that will do it comfortably
Your help would be appreciated

Cheers
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Re: 30 cal twist rates

Post by bladeracer » 07 Oct 2017, 7:04 am

grandadbushy wrote:HERE goes ;;;;;;;; I have stated on this forum a while back that i'm going to rebarrel a 30-06
Now what twist rate is perfect for 162--175gr bullets not boarder line not over stabilising one that will do it comfortably
Your help would be appreciated

Cheers


10" twist.
12" would probably work if you don't delve into VLD's or monolithic bullets.
I would go 10" though.
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Re: 30 cal twist rates

Post by Member-Deleted » 07 Oct 2017, 11:58 am

Thanks bladeracer
No its only going to be my go to rifle for wild cattle the 25-06 is a bit light on and shot placement is crucial and can't always be achieved
resulting a loss
but some times I might do some long range to see if I can better my capabilities but no fancy bullets
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