BC and twist rate

A place to discuss subjects related to those who prefer to take those longer shots. Sponsored by European Long Range Hunting and EuroOptic

Moderators: deerhunter338mag, stokesrj, trazman

Post Reply
User avatar
Corjack
Administrator
Posts: 9711
Joined: Thu Jun 11, 2009 2:28 pm
Location: Missouri
Location: Booger county Missouri
Contact:

BC and twist rate

Post by Corjack » Sun Feb 22, 2015 8:59 pm

Stokes,ttrazman, and deerhunter338, do not answer this question.

Does twist rate have any affect on the ballistic coefficient of a bullet?


There are no fleas on the 9.3s


Booger county time and temp
Image
Click for weather forecast

User avatar
Corjack
Administrator
Posts: 9711
Joined: Thu Jun 11, 2009 2:28 pm
Location: Missouri
Location: Booger county Missouri
Contact:

Re: BC and twist rate

Post by Corjack » Fri Feb 27, 2015 8:18 am

Well evidently no one cares to expand their thinking.
There are no fleas on the 9.3s


Booger county time and temp
Image
Click for weather forecast

User avatar
Vaughan
Moderator
Posts: 2498
Joined: Wed Jun 17, 2009 12:43 pm
Location: Sweden
Location: Sweden

Re: BC and twist rate

Post by Vaughan » Fri Feb 27, 2015 8:20 am

I just don't happen to know the answer and had simply decided to keep quiet rather than flaunt my ignorance.... ;-)
/Vaughan

User avatar
Corjack
Administrator
Posts: 9711
Joined: Thu Jun 11, 2009 2:28 pm
Location: Missouri
Location: Booger county Missouri
Contact:

Re: BC and twist rate

Post by Corjack » Fri Feb 27, 2015 8:38 am

Well, I hope someone will research it, because when I did, I found it fairly interesting.
There are no fleas on the 9.3s


Booger county time and temp
Image
Click for weather forecast

dieselten
Posts: 487
Joined: Sat Jul 07, 2012 3:01 pm
Location: USA
Location: Dallas, TX

Re: BC and twist rate

Post by dieselten » Fri Feb 27, 2015 10:32 am

I will take a stab at it to hopefully start a discussion. I will say that the published BC's are actually the "potentially optimum BC's". If the twist causes a bullet to de-stabilize then the actual BC is effectively lower.

Chief
Posts: 334
Joined: Sun Dec 18, 2011 12:03 am
Location: Europe

Re: BC and twist rate

Post by Chief » Fri Feb 27, 2015 11:31 am

Nope.
As long as the bullet is stable that is.

dchamp
Meister der jagd
Posts: 914
Joined: Mon Jul 14, 2014 10:42 pm
Location: USA
Location: Bakersfield, CA

Re: BC and twist rate

Post by dchamp » Fri Feb 27, 2015 11:57 am

As I understand it BC is governed by weight, diameter & shape. The higher the weight for a given diameter generally equates to a longer bullet. Longer bullets for a given diameter require a faster twist rate. Thats the only connection I can see. So as I see it twist rate does not have an affect on BC as it cannot change it. But you could possibly say BC can have an affect on the twist rate required since high BC bullets are longer than lower BC bullets of the same diameter but that maybe a bit of a stretch.

I think I am right in my thinking, hopefully I have not made a complete fool of myself, but if I have please sort me out so I don't go on thinking wrongly.
I have never met a man so ignorant that I couldn't learn something from him. Galileo

The fact that Jellyfish have survived 650 million years, despite not having brains, gives hope to many people. sun-gazing.com

challer
Posts: 29
Joined: Tue Jan 20, 2015 12:18 pm
Location: United states

Re: BC and twist rate

Post by challer » Fri Feb 27, 2015 2:02 pm

BC is influenced by rotational stability and so twist has an impact (sorry). Side yaw also varies with spin and will influence measured BC. It's been some time for me on this, certainly this is over simplified.

dchamp
Meister der jagd
Posts: 914
Joined: Mon Jul 14, 2014 10:42 pm
Location: USA
Location: Bakersfield, CA

Re: BC and twist rate

Post by dchamp » Fri Feb 27, 2015 7:48 pm

Thanks challer, your comment spurred me to look further so I grabbed my sons copy of Bryan Litz's Modern Advancements in Long Range Shooting and there it was in Chapter 2; Stability and Drag and it has to do with Gyroscopic Stability Factor or (SG). So now I have more reading to do which may take awhile as I can be thick headed and slow to understand. I'm just not that :geek: y. Anyone wishing to explain the how and why of it please do so as I am interested.
I have never met a man so ignorant that I couldn't learn something from him. Galileo

The fact that Jellyfish have survived 650 million years, despite not having brains, gives hope to many people. sun-gazing.com

User avatar
Corjack
Administrator
Posts: 9711
Joined: Thu Jun 11, 2009 2:28 pm
Location: Missouri
Location: Booger county Missouri
Contact:

Re: BC and twist rate

Post by Corjack » Fri Feb 27, 2015 8:08 pm

Very good guys! Yes, a bullet not properly stabilised, may not live up to its advertised/potential BC, as the aberrations in flight, increase drag, and lower its BC to various degrees. A little article by Mr. Litz is where I read about this.

Now the forum managers are free to comment.
There are no fleas on the 9.3s


Booger county time and temp
Image
Click for weather forecast

User avatar
trazman
Meister der jagd
Posts: 1011
Joined: Mon Feb 21, 2011 10:52 pm
Location: Slovenia - Koper
Location: Europe Slovenia Koper
Contact:

Re: BC and twist rate

Post by trazman » Sat Feb 28, 2015 5:10 am

A lot of people think that a bullet needs full stability to shoot good from a certain barrel which is not necessarily true... You can have a marginally stable bullet that groups well. The problem is that only a fully stable bullet will have all of its BC.

Lets compare the berger 210vld and 215hybrid. The hybrid has a bigger bc but when shot from a barrel with 1-11" twist will loose some of its bc. The grouping maight be perfect but the bc can be worst than with 210vlds...

All of this can be compared with the online Berger stability calculator:
http://www.bergerbullets.com/twist-rate-calculator/
Tomaž Ražman
European Long Range Hunting
http://eu-lrh.com

User avatar
stokesrj
Moderator
Posts: 4976
Joined: Mon Jun 17, 2013 12:41 pm
Location: USA
Location: Mesa AZ, USA

Re: BC and twist rate

Post by stokesrj » Sat Feb 28, 2015 8:41 am

While coaching rifle teams, we use a huge spotting scope and tripod, a Unertl that weighs about 75 pounds. It is very stable and you can watch the bullet or trace of the bullet all the way from the rifle to the target. It is quite easy to see this issue when a bullet is only marginally stabilized it will look like a cork screw in flight. This means that during the yaw, more of the bullet surface is presented to air resistance, therefore placing more drag on the bullet.
At long range you can actually see the more pronounced arch as compared to a fully stabilized bullet. The shooter is often not aware because they have adjusted their sights so as the point of aim and point of impact coincide. Because the bullet is slowing down more quickly the wind drift is also more pronounced as well, you can also see this through the scope.
That is part of the reason you hear me complain or question Blaser's wisdom in choice of twist rates. They seem to be transfixed on 0-200 yard/meter performance.
Bob
Robert J Stokes

User avatar
Vaughan
Moderator
Posts: 2498
Joined: Wed Jun 17, 2009 12:43 pm
Location: Sweden
Location: Sweden

Re: BC and twist rate

Post by Vaughan » Sat Feb 28, 2015 9:29 am

So to clarify all of the above - twist rate per se has no effect on BC but poor reloading choices do? ;) :lol:
/Vaughan

User avatar
Corjack
Administrator
Posts: 9711
Joined: Thu Jun 11, 2009 2:28 pm
Location: Missouri
Location: Booger county Missouri
Contact:

Re: BC and twist rate

Post by Corjack » Sat Feb 28, 2015 12:08 pm

Vaughan wrote:So to clarify all of the above - twist rate per se has no effect on BC but poor reloading choices do? ;) :lol:
No, wrong twist rate for the bullet you need to be shooting, can result in a lower BC for that said bullet.
There are no fleas on the 9.3s


Booger county time and temp
Image
Click for weather forecast

User avatar
Vaughan
Moderator
Posts: 2498
Joined: Wed Jun 17, 2009 12:43 pm
Location: Sweden
Location: Sweden

Re: BC and twist rate

Post by Vaughan » Sat Feb 28, 2015 2:19 pm

Corjack wrote:
Vaughan wrote:So to clarify all of the above - twist rate per se has no effect on BC but poor reloading choices do? ;) :lol:
No, wrong twist rate for the bullet you need to be shooting, can result in a lower BC for that said bullet.
Well, I was being a bit facetious but even so, I wasn't trying to pick an argument just understand. Twist rate or bullet rotation does not directly affect BC, right? But if your twist rate is not high enough to stabilize your bullet of choice then the yaw etc induced by the below optimal rotation cuts your BC. Yes? That makes sense. I'd still say the cause of lower than optimal BC is the choice of an unsuitable bullet for your bbl and twist rather than twist directly affecting BC - but I take the point that it might not always be obvious from the way the bullet groups that its flight is sub-optimal and thus its BC is sub-optimal. Not something I had properly taken into account.
/Vaughan

Post Reply

Return to “Long Range Hunting and  Precision Shooting”