Bc's in relation to caliber

[LeftyJason]

It seems to me that lots of people say that bc's are higher in smaller calibers. Say bc's are higher in 7mm vs 300 for the belted magnums. Is this right thinking for the same basic design just scaled for the caliber? Doesn't make sense to me if it is.

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[308pirate]

It seems to me that lots of people say that bc's are higher in smaller calibers. Say bc's are higher in 7mm vs 300 for the belted magnums. Is this right thinking for the same basic design just scaled for the caliber? Doesn't make sense to me if it is.

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It makes sense once one understands the theory and calculations that go into the ballistic coefficient.

Applying a layman's knowledge is typically not enough to understand physics and aerodynamics.

 

[LeftyJason]

if that is so then why does it seem the further you go up in caliber the bc's go up?
http://www.bergerbullets.com/products/target-bullets/

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[morganlamprecht]

weight and diameter matter

same bullet design in a 180 in 7mm is going to have a higher bc than a 180 in 30 cal...squeezing 180 grains into a smaller diameter makes for a longer bullet

as you go up in caliber, you can also go up in weight which adds length, which can give you BC gains as long as you can actual push them in your cartridge

its all relative to bullet design/weight/bullet diameter

 

[LeftyJason]

But keeping the weight the same isn't scaling the design for the caliber. That changes the ratios of diameter to length or diameter to weight.

To me scaling the design keeps proportions the same which does add weight. Just doing some post it note ratio calculations and it doesn't appear that grain weight over diameter scales rationally.

Bottom line my thinking was right and I'm not going nuts.

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[Sheldon N]

Heavy for caliber bullets have higher BC, basically because long/thin bullets fly better. Shooting the "heavies" is just way more accessible in smaller calibers. No one bats an eye at a 105gr or 115gr bullet in 6mm and you can get them going fast easily which adds to their ballistic advantage. A heavy 7mm bullet with a similar BC takes a lot more powder behind it to get going the same speed, and even more so in 30 cal. So basically it's just easier to get good ballistics by shooting a moderate capacity 6/6.5mm cartidge with a high BC bullet rather than trying to launch a 230gr bullet from a 30 cal rifle with a magnum case.

Ultimately it seems like total weight matters too since the heaviest of the large caliber bullets actually have the best BC overall. Not sure how the math works but big 338 and 50 cal rounds have BC's that a 6mm can't touch.

 

[ballison]

If you're going to try and compare bullets by "scaling for caliber", you really should use the cross sectional area of the bullet (Pi x r^2), not diameter. So for a .308, the area would be .0745 sq in. For a .223, it would be .0391. And so forth...

 

[LeftyJason]

Sheldon Very good points. That is the way I've understood it so far. I've listened to all of the Precision Rifle podcast and Precision Rifle Media podcast episodes as well (love the Nick Vitalbo and Bryan Litz episodes).

ballison Thanks. I knew there had to be an easy way to scale better. Just couldn't think of it at the moment.

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[strikeeagle1]

The relationship of BC's to caliber does not need to "scaled" for approximation, as there exists a specific relationship, i.e. inversely related to the square of the caliber




Form factor "i" is an absolute magnitude for a specified velocity.

The form factor changes with velocity because the aerodynamic drag of a specific bullet changes at different velocities, particularly in the transonic zone of flight.




PSA.........don't try to type a simple division formula as the algorithm of the forum syntax screws it up !!!!!!
Type it in a Word document and screenshot it, then upload it as an attachment. You are welcome.

 

[LeftyJason]

I was playing around a little with ballisons cross sectional area on excel a little. I was able to get basic grain weights to stay at the same bc (ish) in different calibers. Checked it and it was pretty close for existing actual bullets. Numbers are for bergers vld target bullets. I only used the heaviest for caliber. Sorry picture not the best. Took picture of screen with my phone.

Couldnt quite figure out how heavy a 338 or 50 vld target bullet would be based off the existing bullets that I had in that design.

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[Fursniper]

You guys are giving me a headache. All I know is nobody is shooting 3000 yds at ELR matches with a 6mm

 

[308pirate]

Is there any useful, practical information in all of this or is this just an academic discussion?

 

[SubOptimal]

Well, it is partially academic and partially practical. The form factor is the big decider, that and the G1 bullet being a flat based bullet makes a boat tail a completely different critter.

 

[Graywolf.260]

I think the practical take away is that compared to a larger diameter bullet, you can get the same BC with significantly less weight with a smaller diameter bullet.

 

[diverdon]

A bullet of the same shape and density will have an increasing BC as the diameter increases.

https://library.med.utah.edu/WebPath/TUTORIAL/GUNS/GUNBLST.html

BC = SD / I SD is the sectional density of the bullet, and I is a form factor for the bullet shape. Sectional density is calculated from the bullet mass (M) divided by the square of its diameter. The form factor value I decreases with increasing pointedness of the bullet (a sphere would have the highest I value). Since drag (D) is a function of velocity, it can be seen that for a bullet of a given mass (M), the greater the velocity, the greater the retardation. Drag is also influenced by bullet spin. The faster the spin, the less likely a bullet will "yaw" or turn sideways and tumble in its flight path through the air. Thus, increasing the twist of the rifling from 1 in 7 will impart greater spin than the typical 1 in 12 spiral (one turn in 12 inches of barrel).

 

[Graywolf.260]

A bullet of the same shape and density will have an increasing BC as the diameter increases.

If its the same shape and density, thats right, but the mass increases faster with increasing diameter than BC does (mass increases with the square of the diameter). I really only want enough mass to do the job at the end of the bullet path, and no more.