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# BC Question?

#1
09-16-2005, 06:12 AM
 Gold Member Join Date: Oct 2004 Location: Northamptonshire England Posts: 676
BC Question?

To those guys who have a greater knowledge of ballistics,

Is the weight of a bullet relative to its BC?
Before i state my own opinions i would like to hear from those who know a lot more about ballistics then me. Thanks.

Ian

"I meant to shoot the pike but the duck got in the way"
#2
09-16-2005, 06:29 AM
 Platinum Member Join Date: Aug 2003 Location: NC, oceanfront Posts: 3,972
Re: BC Question?

A bullet's sectional density goes up with weight. So if all else were kept equal, BC would go up with weight. Problem is, rarely is all else equal(shape length, etc) with more weight. To keep it equal specific gravity would need to change rather than dimensions. Use of Tungsten, etc.
This is why a general statement that heavier bullets are higher in BC is incorrect. That may not be the case at all.
Just compare pistal bullet BCs to lighter rifle bullet BCs for an example.
#3
09-18-2005, 10:27 AM
 Gold Member Join Date: Oct 2004 Location: Northamptonshire England Posts: 676
Re: BC Question?

Good explanation Mike and thanks,

My question is not quite answered though.
For the purpose of comparison, lets say that i had 3 bullets of the same shape, length, dia and ogive etc, but 1 made out of aluminium, 1 made out of copper and 1 the convential copper jacket/lead core construction. Because they are all of different materials they do of course have different density's but does density have an effect on BC?

Will they then all have the same BC based on bullet shape or will the convential bullet have a better BC because it is heavier?
If launched at the same velocity will the aluminium bullet bullet shed velocity quicker then both the copper bullet and the convential bullet? Theoreticly it should do, because it will have less momentun then the heavier bullets.
The way i look at it is, that weight is relative to BC.
That would mean a bullet made out of depleted uranium would have much higher BC then a conventially constructed bullet for the same shape, dia. etc.

Or am i wrong?

Ian.

"I meant to shoot the pike but the duck got in the way"
#4
09-18-2005, 11:50 AM
 Platinum Member Join Date: Aug 2003 Location: NC, oceanfront Posts: 3,972
Re: BC Question?

Sectional density (SD) is a bullet's weight, in pounds, divided by the square of it's diameter, in inches. The higher the SD, the higher the weight per cross-sectional area. Shape makes no difference in the sectional density. For example, all 30 caliber (.308) bullets that weigh 180 grains have the same sectional density, .270, regardless of their shape.

A ballistic coefficient (BC) is the ratio of the sectional density of a bullet to it's form (I).
(I) is based on a ratio of drag coefficients(this bullet's to a standards bullet's). Its affected by air density, mach#, frontal cross section, shape, etc.

Ultimately, a bullet with the same shape as any standard bullet, weighing 1 lb. and 1 inch in diameter will always have a B.C. of 1.000. If the bullet is the same shape, but smaller, it will have an identical C.D, with a form factor of 1.000 and a B.C. dependant on just it's sectional density.

So to answer the question, best I can, a denser bullet with all else equal, will fly flatter(higher BC). If someone makes an equal weight solid bullet that is longer to due the material, all bets are off.
#5
09-18-2005, 07:02 PM
 Platinum Member Join Date: Jan 2003 Location: The rifle range, or archery range or behind the computer in Alaska Posts: 3,740
Re: BC Question?

Weight of course effects BC. More than weight is material which of course affects weight and velocity and therefore BC. Something else to consider is many differant materials have differant specifec gravity values. I think the reason Barnes publishes BC's so high is because I doubt they calculate the BC including specifec gravity and most likely base it on weight and form factor, most here will agree that their claims are quite optomistic. Solid copper has a very low specifec gravity value where jacketed lead has one of the best. a 178 AMAX will put a 180 X to shame despite Barnes claim of .552 for their 180 XBT and Hornaday's claim of .496 for the 178 AMAX. Any BC calculator you use calculates multiple factors for determining fairly close numbers. Any calculator worth its salt has a place for weight. A bullet with more weight with the same form, shape and size factors at same velocity as a lighter bullet should fly flatter due to increased energy.

Most of this idea would be because of differing specifec gravity values of various metals which of course changes weight quite drasticaly.

Hope that helps some.
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Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.
#6
09-18-2005, 11:31 PM
 Gold Member Join Date: Jul 2005 Location: East TX Posts: 773
Re: BC Question?

[ QUOTE ]
A bullet with more weight with the same form, shape and size factors at same velocity as a lighter bullet <font color="blue"> should </font> fly flatter due to increased energy.

Hope that helps some.

[/ QUOTE ]

It may seem that way in logic, but in my experience, bullet wieght means squat when it comes to putting the bullet on the paper. On killing game, the mass and energy/momentum of the bullet matters, but not in trajectory.

200 gr .338 BT (bc .414) @ 2950 fps- sighted in at 200 yards hits the exact same place at 87 gr .243 VMAX (bc .415 - in my @ 2950 fps at 400 yards. the .338 has 1970 ft lbs vs. .243 has 858 ft lbs, but both print 20 inches low.

This is just one case and my \$0.02.
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Stephen
#7
09-22-2005, 05:06 AM
 Gold Member Join Date: Oct 2004 Location: Northamptonshire England Posts: 676
Re: BC Question?

I am more enlightened,
Thanks for that info guys your explanations have helped a lot with my own interpretations of the BC/weight factor.
Thanks.

Ian.

"I meant to shoot the pike but the duck got in the way"

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