a question for the ballistitians

swarovski

Well-Known Member
Hi all, in the sierra data they give different ballistic coefficients for different velocities for every one of there bullets, nobody else seems too, would these velocities be relative or very similar for other bullets. Swaro
 
Hi all, in the sierra data they give different ballistic coefficients for different velocities for every one of there bullets, nobody else seems too, would these velocities be relative or very similar for other bullets. Swaro


There was a time when Dupont put out sheets of bullet "forms" so that a shooter could match his or her bullet to the form as closely as possible and plug in a BC so, in that sense, yes: It's relevant. Sierra A 150 grain Game King flat base probably has a very similar BC to a Speer 145 grain Hot Cor 7mm bullet based on a fairly similar form and weight.

It will be close, but not exact. For coarse work it's an OK starting point. For serious work, you won't even take Sierra's word and you'll calculate your own BC.~Muir
 
The Sierra figures, I think, reflect the "G1" model of the bullet which is a cylindrical flat based thing with a rudimentary point that does not properly reflect the more modern (since about WW1) streamlined designs. You find figures from subsonic, supersonic and high-supersonic to reflect this. Most other manufacturers just quote the lowest drag value. If, like Sierra, you're flogging a software package with trajectory calcs then knowing the BC at various velocities should allow a better model to be built.

Have a look here: Ballistic coefficient - Wikipedia, the free encyclopedia
 
I think the 6.5 cal 142gr smk is a fairly modern bullet, one of the highest bc 6,5 cal bullets you can get,2860fps and above is 0.595bc, lower than that velocity the bc deminishes
 
Ive tried the g7 bc on my bal cals and they are way off, g1 bc's are very very close to actual as I shoot out to 1000yds thats a good yard stick to measure with.unless I am doing something badly wrong with the g7 figures which I cant see what as all I am doing is changing from one to the other on the bal cal, not changing anything else.
 
There was a time when Dupont put out sheets of bullet "forms" so that a shooter could match his or her bullet to the form as closely as possible and plug in a BC so, in that sense, yes: It's relevant. Sierra A 150 grain Game King flat base probably has a very similar BC to a Speer 145 grain Hot Cor 7mm bullet based on a fairly similar form and weight.

It will be close, but not exact. For coarse work it's an OK starting point. For serious work, you won't even take Sierra's word and you'll calculate your own BC.~Muir

The man is correct again. Best to work it out yourself you will be surprised how far off it will be. To be honest it's the last thing I look for in a hunting bullet.

http://www.jbmballistics.com/cgi-bin/jbmbcv-5.1.cgi
 
Ive tried the g7 bc on my bal cals and they are way off, g1 bc's are very very close to actual as I shoot out to 1000yds thats a good yard stick to measure with.unless I am doing something badly wrong with the g7 figures which I cant see what as all I am doing is changing from one to the other on the bal cal, not changing anything else.

At one time Sierra used a C1 drag coefficient as did Speer, iirc, which is very similar to the G1. There are dozens of different drag factors out there but most ballistic calcs are for G1. The G1 works for you so the answer is plain....~Muir
 
Ive tried the g7 bc on my bal cals and they are way off, g1 bc's are very very close to actual as I shoot out to 1000yds thats a good yard stick to measure with.unless I am doing something badly wrong with the g7 figures which I cant see what as all I am doing is changing from one to the other on the bal cal, not changing anything else.

I tried comparing the Berger model with my more usual Sierra V5 and the difference isn't that much in my opinion.

This is Berger's G7 variant
Berger_77_223_3000_G7.jpg

This is Berger's G1 version
Berger_77_223_3000_G1.jpg

And Sierra V5 (not a slightly different BC but not much IMO)
Sierra_77_223_3000_G1.jpg

To me, close enough. One thing I thought about and checked: if fiddling with G7 the bullet is streamlined (i.e. boat tail) as the the G1 is more like a conventional flat base. In fact Berger only quote G1 BCs for the FB bullets.
 
surely the only reliable way is to work out the BC for your chosen bullet at your chosen velocity over a suitable range
the further away from the muzzle the better
 
surely the only reliable way is to work out the BC for your chosen bullet at your chosen velocity over a suitable range
the further away from the muzzle the better

Possibly but as the drag on the projectile (bullet) isn't a "fixed equation" and depends on the velocity itself it could be fraught with issues.
You'd need to measure a series of velocity / range data for each velocity zone.

If the projectile stayed inside a velocity zone (such as well supersonic) then one could do the estimate given that the drag would be proportional to some fixed power of the velocity: make a few measurements at, say, muzzle, 50m, 100, 150m and some simultaneous equations.

However, if the projectile transitioned to a low sub-sonic or into the transonic / subsonic then that'd be a nightmare as one doesn't, without masses of measurements, know precisely where the bullet transitioned from each velocity zone and whether the drag is, for instance a v^6 or a v^2 etc.

It'd be a lot of fun to measure it but I think the Artillery types in the 19th / 20th Century produced a lot of paperwork on the subject.
 
Possibly but as the drag on the projectile (bullet) isn't a "fixed equation" and depends on the velocity itself it could be fraught with issues.
You'd need to measure a series of velocity / range data for each velocity zone.

If the projectile stayed inside a velocity zone (such as well supersonic) then one could do the estimate given that the drag would be proportional to some fixed power of the velocity: make a few measurements at, say, muzzle, 50m, 100, 150m and some simultaneous equations.

However, if the projectile transitioned to a low sub-sonic or into the transonic / subsonic then that'd be a nightmare as one doesn't, without masses of measurements, know precisely where the bullet transitioned from each velocity zone and whether the drag is, for instance a v^6 or a v^2 etc.

It'd be a lot of fun to measure it but I think the Artillery types in the 19th / 20th Century produced a lot of paperwork on the subject.

This is exactly what Sierra do. I've skimmed through the (95) pages of explanation in the Sierra #V manual from time to time.

They measure velocity over 5 velocity ranges using a series of chronographs, then throw in correctional data from other sources such as Doppler Radar readings, known variations from standard atmospheric conditions/height above sea level/temperature constants,etc.

They calculate a different BC for each velocity range by relating the results to the standard G1 drag coefficient model for which there is a mass of computational data from the military. Apparently all commercial bullet makers quote BC's based on the same G1 model as it's the industry standard. I can't say the chapter was rivetting reading, but when my eyes uncrossed and the headache cleared I was quite impressed with the thoroughness of the research Sierra put in. This is incorporated in their Infinity software.
 
This is exactly what Sierra do. ...the thoroughness of the research Sierra put in. This is incorporated in their Infinity software.

And builds on about 300 years of history.

The real headache for me would be looking at Bernouli's equaltions.

The whole things is a French (I'd love a copy of Trité Balistique Extérieure), German, British, Italian and US hotchpotch with interactions between many of the people in the 19th / 20th C. A model perhaps?
 
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