BC question.

[Linefinder]

I'm having a discussion that has got me second-guessing my own understanding of BC. In particular, the impact that the material from which the projectile is made has upon BC.

It is my understanding:

Given two bullets of the same weight, shape, and caliber, the longer bullet will have the poorer BC, all things being exactly the same except the length.,Or, put another way, density).

For example, a bullet made from a composite substance,such as the Barnes Varmint Grenade)which is lighter than lead has to "make weight" by adding length, when compared to a lead-based bullet. This results in more surface area of the composite projectile upon which external forces can react, without the benefit of any additional offsetting weight.

IOW, in my mind, the "less dense,and longer), but otherwise equal" projectile will have less BC.

Am I completely missing the boat here?

Thanks

Mike

 

[Bryan Litz Ballistics]

Mike,
As I understand it, you're saying:

Same ogive, same boat-tail, same weight. One bullet has a longer bearing surface because it's less dense.

In this case, the longer bullet would have a slightly lower BC, but it would be very slight, and difficult to measure. The increase in drag is due to the 'skin friction' component of drag. You see, there's different components of drag that add up. Wave drag,from creating a shockwave with the nose), base drag, and skin friction drag are three big ones, skin friction being the smallest,something like 10% of the total drag). Skin friction is proportional to surface area, so adding length to the bullet shank will increase the surface area, hence increasing drag. If you lengthened the bullet 20%, increasing surface area the same amount, you would increase skin friction by 20%. But skin friction is only 10% of total drag. In other words, increasing length by 20% only increases overall drag by 2%.
*disclaimer* these numbers are approximations meant to convey a sense of proportion, not to be taken as exact.

For the above description, it's important that nose and boat-tail length is held constant and just the shank is stretched. If you lengthen the ogive, and/or the boat-tail, these things will reduce wave drag and base drag more than the added length will increase skin friction drag.

A bigger deal with such a lower density projectile is it requires a faster spin rate to stabilize. Bullet dispersion is related to spin rate, so you may pay a precision penalty for the longer bullet that needs the faster twist.

I think a better way to go is with increased density cores like compressed tungsten powder. In this way, you can have a 190 grain .30 caliber bullet the same size as a 155 grain bullet. The smaller, heavier bullet leaves more room in the case for powder,if that's an issue), and requires very slow twist. There are problems with this approach also. Expense for one. Another problem is creating a core that's balanced by compressing a column of powdered metal.

Every design is a compromise.

Take care,
-Bryan

 

[Linefinder]

Thank you very much!

That cleared it up very nicely for me. I appreciate you taking the time.

Mike

 

[Linefinder]

Thanks, Cat.....

That figures. Just about the time I think I understand a subject pretty thoroughly, somebody throws a wrench in the works.

It's pretty hard to reason away those Scenars...

Mike