Deformation of bullet surface effect on BC?

[Changeling]

This would be a question for Mr. Litz.
When you figure out the BC for bullets, is deformation taken into consideration? I ask this because supposedly/maybe different styles of rifling and number can cause different amounts of damage to the bullet surface.
It would make sense (to me) that BC differences would exist because of the rifling affecting the surface of the projectile but I just don't know to what extent and the magnitude of change would be, especially since it is a variable.

 

[JarheadAZ]

With deference to Brian, I think the question has a range of answers corresponding to both the physics and the manufacturing process.

The physics require that no two bullets will be precisely alike, and manufacturing requires that QC intends to exclude deviances that exceed an acceptable amount of deviance.

In essence, it is feasible and affordable for both manufacturer and user to accept that the QC specs result in a product that meets sporting standards. For users who require more uniformity, smaller manufacturers can produce a more uniform product, but the cost to the consumer must be higher, and reality dictates that perfect uniformity is humanly impossible.

In practical terms, taken in sight of human frailties, the vast majority of consumers are incapable of a level of marksmanship that exposes the flaws inherent in the sporting standards. Further, there is a far better way to determine a meaningful BC than measuring individual bullets; and that comes from firing the bullets and inferring the BC from the performance.

To be pragmatic, the true BC of a bullet type can only be concluded in combination with the shooter's skills at the time of testing.

No single component in the shooting process can be accurately assessed except in light of the entire system, which includes components like ammunition, rifle, support, the environment, and the shooter. No single component can deliver perfection; it is the harmony of those components that reflects real world performance.

Greg

 

[Bryan Litz Ballistics]

The effect of riflings is one of the reasons why it's so difficult to estimate BC based on bullet shape; measurements of bullet shape are almost always taken on unfired bullets.

All of the BC's reported in the Applied Ballistics literature and software applications are derived from live fire testing, so they are 'real world', fired BC's, warts (riflings) and all.

Beyond that, the question is; does the type of riflings affect the BC? To answer simply, I can say that the difference in BC produced by different types, profiles, and number of riflings doesn't matter very much directly. I've fired the same bullets from conventional riflings vs. non-conventional and found their BC's to be the same within my ability to measure (+/-1%). However, there is another affect which is what Greg is alluding to.

Different rifling profiles can affect the rate of RPM decay for a bullet. In other words, if a bullet is engraved with rough and aggressive riflings, it can have a noticable affect on how quickly the spin rate of the bullet drops off. This isn't important thru normal long range, supersonic flight. However, when the bullet slows to transonic speed (roughly 1340 fps and slower) the stability of the bullet becomes challenged, and it can begin flying with some wobble. This wobble can erode the effective BC of the bullet. Now, a bullet that's spinning faster when it encounters transonic speed will have a more rigid axis and, all else being equal, will 'wobble' less than a faster spinning bullet in the same conditions. So it's possible to fire the same ammo thru two 1:10" twist barrels and have one exhibit a higher BC thru transonic, if the riflings in the barrels engrave the bullets differently, affecting their RPM decay.

All of the previous paragraph was tested and observed thru live fire. I made an apparatus to measure a bullets rate of spin downrange and learned that bullets fired from 1:10" twist barrels do not all retain the same RPM's at long range if the nature of the 1:10" riflings are different. This research is all published in: "Modern Advancements in Long Range Shooting - Volume 1".

-Bryan

 

[Changeling]

Thank you Bryan, I believe I should get your book.

 

[Ketosan]

Brian,

In your previous post you said:

Now, a bullet that's spinning faster when it encounters transonic speed will have a more rigid axis and, all else being equal, will 'wobble' less than a faster spinning bullet in the same conditions. So it's possible to fire the same ammo thru two 1:10" twist barrels and have one exhibit a higher BC thru transonic, if the riflings in the barrels engrave the bullets differently, affecting their RPM decay.

Just to keep my head "spinning" in the right direction on this subject--did you mean to say "will wobble less than a "slower" spinning bullet in the same conditions"???

I've got your book and never knew there was so much "math" involved in the fine art of shooting--

Thanks for all you do--

 

[Bryan Litz Ballistics]

You are correct, that was a mistake on my part. Hopefully I didn't confuse too many people with that.

Thank you for pointing it out.

-Bryan