Thread: Retained velocity

Might be having a blond moment here, but can someone please try to explain the physics behind the reason higher BC projectiles have higher retained velocity ?

They slow down more slowly due to less air resistance.

Its all aerodynamics, you are def having a blonde moment.

Imagine throwing a small chunk of lead, and a screwed up piece of paper that weighs the same. Five metres out, the chunk of lead will still be travelling at a hefty speed but the paper will have pretty much stopped.

It's all to do with one Krupp cannon round isn't it? That's the standard velocity so that all projectiles can be compared as a persentage of. That's why the higher bc bullets go further (have higher retained velocity). Not sure how old this is but I'm sure it's on Wikipedia someplace

Errr I kinda compacted history a bit there. There's about 6 people involved with that over about 400 years (not just mein krupp). The short story is that it's not a real round. It's a theoretical norm for the boffins to have calculator fights over. There I go again haha

What they said... more efficient/less drag so it takes longer to slow down.

Had a look at Litz's chapter on BC again, he simply states that the heavier a bullet is, the better it can penetrate the air.

BC = (weight/7000) / (cal X cal X form factor). So from this formula, it is obvious that for a given form factor, a heavier projectile will have a higher BC.

All good, but I want to know WHY ?

Inertia, a heavier bullet has more inertia than light one, therefore with the drag being equal it will take longer to lose its velocity.

If you then have a lower drag coefficient (shape) you have less force slowing it, so less drag, more inertia means it takes longer to be slowed.

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Possibly inertia. A heavier projectile will have more inertia and therefore require a larger force to slow it down. Because the atmosphere effectively exerts the same force on all projectiles a heavier projectile will slow down slower.

Bugger Spudattack is a faster typer than me.

Aha, thanks spud & stug. I knew it was something fundamental that I just could not get my head around

I was confused with this for a bit as well.... a higher drag coefficient Cd, means it will slow down faster, a lower ballistic coefficient means it will slow down faster than a higher one.

@ebf
Eben - thanks for the 'blonde moment' mate.
I've been trying to correlate something similar.
My 'rough seat of pants' guess went like this:
SD + initial velocity + best aerodynamic shape = better downrange retained velocity.
spud & stug explained it much better

If SD = Sectional Density that is another animal again and is a calculated figure related to a projectiles penetration ability. Used to be used by some as a rough guide to killing ability but of course that is dependant on jacket and core design as well.
Chuck Hawks on SD Sectional Density

Originally Posted by Spudattack

Inertia, a heavier bullet has more inertia than light one, therefore with the drag being equal it will take longer to lose its velocity.

If you then have a lower drag coefficient (shape) you have less force slowing it, so less drag, more inertia means it takes longer to be slowed.

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Momentum I think, as inertia amount of energy it takes to make an object move, or a measurement of how hard it is to move something.
Kinetic energy = 1/2 mass x velocity squared
So a heavier object has more kinetic energy than a lighter one at the same speed. Bullets are slowed down by friction which reduces the kinetic energy of the bullet. A high BC bullet has low friction and therefore it takes longer for that bullet to lose its kinetic energy/momentum. A heavier high BC bullet has more kinetic energy and therefore takes longer to lose it, sort of. I think BC has weight built into it so it might be more accurate to talk about a heavier bullet with the same drag coefficient.

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