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Ogive question.

 
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  #15  
Old 04-05-2005, 07:02 PM
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Join Date: Aug 2003
Posts: 126
Re: Ogive question.

Yes, the numbers should be 3,4 and 5.
Dan Lilja's site has a downloadable Excel sheet that lets you play with bullet design parameters. You can put extreme ogives on, but then you will see that there is no bearing surface left. If you make it longer, you will see that the required twist changes. I believe it uses McCoy's code to compute stability. The code computes G1 BCs that are usually accurate to within about 2%. You can specify the density to reflect solid brass, or jacketed lead. Also of interest is changing meplat diameter. For a fixed length, a zero meplat is not optimal for BC. Of course when you trim meplats you change the length, so you should adjust the length in the calc.
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  #16  
Old 04-05-2005, 07:03 PM
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Join Date: Dec 2001
Location: Mukilteo, WA
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Re: Ogive question.

[ QUOTE ]
What is the difference between a secant and tangent ogive technically speaking?

[/ QUOTE ]
The sides of the tangent ogive are, well, tangent to the body of the bullet. [img]/ubbthreads/images/graemlins/laugh.gif[/img]

A secant ogive sets the nose of the bullet back so the intersection of the nose and the body forms a tight-radius intersection instead of a smooth transition. Think of it like this:

For a tangent ogive, you start with the cylindrical body of the bullet. You begin drawing the nose of the bullet from a point parallel to the top of the cylinder. So it smoothly rounds away from the body.

For a secant, you draw the nose from a point set back into the base a ways. Basically so the radius of the ogive isn't parallel with the sides of the bullet when it intersects.

Hmm, that may not be more clear than mud. Let me look for a pic....
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  #17  
Old 04-05-2005, 07:45 PM
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Re: Ogive question.

I re labeled the picture with the correct numbers. "A" would be the origination point of a secant ogive and B is that of a tangent if I understood. The secant is probably exagerated a bit.

The two bullets to the side illustrate what I believe you are talking about with a tangent blend and a nontangent blend of the ogive. They both have exactly the same ogive radius.

Am I getting it?
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  #18  
Old 04-05-2005, 08:47 PM
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Join Date: Nov 2004
Posts: 878
Re: Ogive question.

[ QUOTE ]
Among many other things BC is also a function of weight. I cant imagine a 42 gr .22 bullet approaching the BC of a normally shaped 168 grain .30 cal no matter how long the ogive is.

[/ QUOTE ]
BC is a number you can get from the formula sectional density times form factor.
Sectional density= bullet wt in pounds/diameter squared in inches.
if you used the .800 BC somebody was dreaming about, and used the same form factor as the 210bergervld(2.023765)...
it would be 2.023765X(139/7000/.224squared)=.800BC
Thats a 139gr bullet.
If you wanted the same BC as the 168SMK (.462) its possible, but you gotta drive up the weight a good bit to make it.
168SMK BC .462, Form factor1.82
80BergerVld BC .489, Form factor 2.147
75BergerVLD BC .448, Form factor 2.098
A 42gr, .462 BC .224caliber bullet is possible, the problem is that with a bullet that long, you need a super fast twist, and that super fast twist magnifies the inconsistancies in the bullet that make accuracy virtually impossible.
Now for a brief history lesson....
There were a few shooters that were experimenting with very high bc 30 caliber bullets back in the late 80s and early 90s. They only weighed 250gr, but had BCs of well.... really high.
Problem was, the only way to get workable bullets was to lathe turn them, then test them on a Juenke machine. With a high percentage of the bullets weeded out due to the fact that extreeeeemly low readings on the Juenke were nessicary to even hit paper due to the very high RPMs involved.It turned out to be cost prohibitive. From what i recall, rejection rates were about 80-85 per 100.

Thats right boys and girls, those expensive, custom, lathe turned bullets go into the trash at a rate of better than 80%

Like I said, its possible, just not really practical yet. Oh, then theres the custom barrel. No... really custom barrel, as in the tooling is custom. I think the barrel was a 30" 1:4.5..... something like that, memory kinda foggy.
One of these days we'll be able to buy bullets with those super high BCs, but they will have to be shipped in a shock absorbing container. Those pristine bullets that made it through the Junike with high scores(um.. technically low scores)... drop it on a hard surface from as little as 12" and its toast. What does the average package look like when the big brown truck drops it off?
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  #19  
Old 04-05-2005, 10:44 PM
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Join Date: Dec 2001
Location: Mukilteo, WA
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Re: Ogive question.

[ QUOTE ]
Am I getting it?

[/ QUOTE ]
Yes. Presactly.
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