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Ballistic Coefficient
Discussion regarding the reloading of ammunition and tuning of loads for accuracy
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MacD
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PostPosted: Fri Nov 28, 2014 10:58 pm    Post subject: Ballistic Coefficient Reply with quote

In trying to decide which bullet to use in reloading for paper punching with my 6.5x55 I naturally looked for one with a high BC. This led me to the Hornady A Max 140 with a BC of. 585. I then found a Berger bullet with a BC of. 629. The Berger is a much more costly bullet. The question? Is the higher BC worth the extra $?

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dhc4ever
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PostPosted: Sat Nov 29, 2014 12:21 am    Post subject: Re: Ballistic Coefficient Reply with quote

At ranges up to 300m probably not a lot in it.
If on the other hand you'll be paper punching at 500 to 1000m then the extra money will definately be worthwhile.

Feed your data into point blank and see what it says, use same speed, range etc just change the cd figure.

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Azar
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PostPosted: Sat Nov 29, 2014 2:16 pm    Post subject: Re: Ballistic Coefficient Reply with quote

You will also want to measure the twist on your 6.5x55 and check that against the manufacturers recommendation. Lots of high B.C. bullets require a pretty tight twist. Older Swedish Mausers are typically 1:7.87" and will stabilize just about any bullet in 6.5mm, but you may want to measure first just to verify...

Also, to reiterate what dhc4ever it also depends on how far you're planning to go.
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Elvis
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PostPosted: Sat Nov 29, 2014 4:22 pm    Post subject: Re: Ballistic Coefficient Reply with quote

B.C and S.D and bullet weight/shape and construction dont matter a flying fig if your particular shooting stick doesnt like them...thats why they now sell sample packs of to or so projectiles...pick a mid range load and try a few different projectiles till one seems to give better results than others then play with it till it hums is the theory....I dont try to shoot tiny wee even groups as my hunting style doesnt NEED it.
have fun.
I mean realistically if the old girl likes a mildly loaded 160grn roundnose moose intended load and groups them better than all others well mate just go with it...the opposite may apply and say a 120grn ballistic tip may tickle her fancy..trial and error is the only way to tell for sure.I know the old .303s are notorious for liking one load over another.

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slimjim
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PostPosted: Sat Nov 29, 2014 6:07 pm    Post subject: Re: Ballistic Coefficient Reply with quote

I have found the precision of the Berger bullets to be outstanding and an excellent target bullet. Flat-based target bullets may give you better accuracy if that is your primary goal. Beware, Berger may list some as Hunting bullets but the 6.5 and .277 bullets share the same copper jacket. I can't get their .277 bullets to expand or fragment regardless of the velocity (kinetic energy) so I don't use them for hunting.

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Pumpkinslinger
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PostPosted: Sat Nov 29, 2014 9:59 pm    Post subject: Re: Ballistic Coefficient Reply with quote

Run the two bullets through Pointblank to see how much difference it makes ...

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Elvis
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PostPosted: Sat Nov 29, 2014 10:39 pm    Post subject: Re: Ballistic Coefficient Reply with quote

Pumpkinslinger wrote:
Run the two bullets through Pointblank to see how much difference it makes ...
if the 160grn round nose does 3/4" groups and the 125s 3" groups it doesnt matter that the 125s shoot faster and flatter and have a less rainbow like bullet path.once you have drop worked out it doesnt matter.and the theoretical 160 is the better bet.

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SingleShotLover
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PostPosted: Sun Nov 30, 2014 9:56 am    Post subject: Re: Ballistic Coefficient Reply with quote

Also remember that ballistic coefficient of any given bullet isn't an absolute. It can vary substantially with velocity changes - even while in flight.

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PaulS
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PostPosted: Sun Nov 30, 2014 10:39 am    Post subject: Re: Ballistic Coefficient Reply with quote

SingleShotLover -
The ballistic coefficient doesnt change but the resistance of the air does change with velocity, barometric pressure, humidity, altitude and temperature. The air resistance changes geometrically as the velocity changes - double the velocity and the air resistance increses at the square of the velocity change. Most ballistics software takes that into account in the calculations done. This can be translated into cars or anything else that moves through the air. In a car it take four times the power to double the speed. Say it takes only 25 HP to go 60 mph - it takes 100 HP for the car to go 120 mph. The bigest change is air resistance. The same is true for bullets, with the exception of the transonic velocities. In this region the drag increases substantially because of the bullets tendancy to corkscrew around its nose when the center of pressure moves forward on the bullet. In the transonic region the drag on the bulet is at its highest and the coning of its flight produces unpredictable flight patterns that result in scattered groups. With high powered rifles we rarely shoot at ranges where this happens but the venerable 22 rimfire goes through this range at far closer distances than most other cartridges.

My point is that the ballistic coefficient doesn't change - just the air drag on the projectile.

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SingleShotLover
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PostPosted: Sun Nov 30, 2014 2:31 pm    Post subject: Re: Ballistic Coefficient Reply with quote

Sierra lists their bullets with differing ballistic coefficients depending on initial velocity. Regardless of what causes it, be it velocity, barometric pressure, humidity, altitude or temperature, they acknowledge that BC does change with velocity. Since their resources exceed mine, I'll trust their data.

www.sierrabullets.com/...-rifle.pdf

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slimjim
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PostPosted: Sun Nov 30, 2014 5:49 pm    Post subject: Re: Ballistic Coefficient Reply with quote

Typical BC values are referred to as G1 BCs. For long-range, low-drap bullets, the G7 BC provides a more reliable prediction of bullet performance. Berger has G7 information on their website.

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PaulS
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PostPosted: Mon Dec 01, 2014 5:43 pm    Post subject: Re: Ballistic Coefficient Reply with quote

Berger also rates their bullets using the G7 table. While you can convert a typical hunting bullet G1 coefficient to a G7 equilivant it is unlikely that you will get a better prediction of trajectory. You are usually better off using the drag table upon which the bullet was rated.

The form of the G7 "ideal" bullet is just as far from the flat based spitzer bullets as the spitzer is from the G1 "ideal" bullet.

The flat based spitzer is closer to the "ideal" G8 projectile than to any of the other "ideals" but unless you can convert from the G1 coefficient to the G8 then you have the same problem.

The reason Sierra uses multiple BC's is because they work with "standard" atmospheric conditions in their software. It doesn't compensate for temperature, humidity, barometric pressure and altitude. Most software won't let you enter a different BC for different velocities so you are still stuck with a single BC anyway.

I feel like I am coming off as a know-it-all and believe me I do not. I am learning a great deal as I rewrite an old DOS ballistics program of mine to bring it up to a full 6 degrees of freedom prediction program.

Don't get excited because I am writing it in ANSI C (C99) so there are no fancy pull down menus, pop-up windows or even printer routines. I had to write formatted text files that can be printed from a word processor. Everything is saved to a file, including the input that you might want to save and the ballistic tables. It will be most useful for calculating a shot in the field just before it is fired.

The program has a lot of input and even more calculations than anything I have written before. I run it (to test it) in a terminal window on my computers (linux). All the input begins at the bottom of the page because the only way to clear the screen in C99 is to print blank lines - which leaves you at the bottom of the page.

I don't expect many people to even want to use it for the reasons stated above but it will be great for porting to portable devices which is easy enough if you have a compiler for the device you want to use. It will be released under the GNU-GPL open software copywrite so the code will be available to anyone who wants to modify it for use in Windows, Mac, or even the Linux front ends.

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MacD
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PostPosted: Mon Dec 01, 2014 7:51 pm    Post subject: Re: Ballistic Coefficient Reply with quote

Paul,
I have looked for a good, quick electronic range card that can be used in the field on a phone or even a wearable. It woud be calibrated in Mils. Put in the range from your scope or finder, estimate wind and get a mildot hold for a preselected round or rounds if you are hunting in the type of mixed topography like we have here. Not perfect but probably better than rough estimation. No paper to get wet or torn and no gummy residue on stocks. The key attribute is that it has to be fast to use.

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hunterjoe21
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PostPosted: Mon Dec 01, 2014 7:59 pm    Post subject: Re: Ballistic Coefficient Reply with quote

PaulS wrote:
Berger also rates their bullets using the G7 table. While you can convert a typical hunting bullet G1 coefficient to a G7 equilivant it is unlikely that you will get a better prediction of trajectory. You are usually better off using the drag table upon which the bullet was rated.

The form of the G7 "ideal" bullet is just as far from the flat based spitzer bullets as the spitzer is from the G1 "ideal" bullet.

The flat based spitzer is closer to the "ideal" G8 projectile than to any of the other "ideals" but unless you can convert from the G1 coefficient to the G8 then you have the same problem.

The reason Sierra uses multiple BC's is because they work with "standard" atmospheric conditions in their software. It doesn't compensate for temperature, humidity, barometric pressure and altitude. Most software won't let you enter a different BC for different velocities so you are still stuck with a single BC anyway.

I feel like I am coming off as a know-it-all and believe me I do not. I am learning a great deal as I rewrite an old DOS ballistics program of mine to bring it up to a full 6 degrees of freedom prediction program.

Don't get excited because I am writing it in ANSI C (C99) so there are no fancy pull down menus, pop-up windows or even printer routines. I had to write formatted text files that can be printed from a word processor. Everything is saved to a file, including the input that you might want to save and the ballistic tables. It will be most useful for calculating a shot in the field just before it is fired.

The program has a lot of input and even more calculations than anything I have written before. I run it (to test it) in a terminal window on my computers (linux). All the input begins at the bottom of the page because the only way to clear the screen in C99 is to print blank lines - which leaves you at the bottom of the page.

I don't expect many people to even want to use it for the reasons stated above but it will be great for porting to portable devices which is easy enough if you have a compiler for the device you want to use. It will be released under the GNU-GPL open software copywrite so the code will be available to anyone who wants to modify it for use in Windows, Mac, or even the Linux front ends.

Cant even read that without getting a headache

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SingleShotLover
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PostPosted: Tue Dec 02, 2014 5:37 am    Post subject: Re: Ballistic Coefficient Reply with quote

Mac - Hawke scopes make just such an app for "smart phones" that works with their scopes and various reticles, including their mil-dots. It even uses the camera in your phone to determine incline! I have two of their rimfire versions with mil-dots and it really works pretty well. I'm even thinking of trying one of their varmint/target scopes for my varmint rifles.

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