"Then you "cross" the rotation vector with the wind, or gravity, or coriolis (earth's rotation) vector using the Right Hand Rule again, to see what is the resultant force vector acting on the bullet."

Agreeded and the above is where you get the resultant force. All summed forces will not be on X,Y axis of the flight path. Correct??

quote:

X,Y axis of the flight path

If I assume the Z axis is vertical, then you are right. For a right-hand twist rifle shooting with a right-to-left crosswind, the resultant force would be... UP! (This is in addition to the aero force of the wind pushing the bullet sideways, which is much more significant.) Edited: a crosswind will not have this effect unless it is actually trying to yaw the bullet (the CP and CG not coincident). If the CP is ahead of the CG, that bullet would behave as described above. If the CP is behind, it will yaw right and the force will be downward, while actually being blown (translated) to the left. Sorry! End of Edit

FYI, consider race cars with spinning masses... Imagine the crank, flywheel, trans, driveshaft and their vector... Now yank all that into a hard left at the end of a straightaway... Many engine builders have reverse camshafts ground so that they can have the resultant force pushing the car down harder instead of trying to lift the car.

BTW - Gravity, coriolis effect, wind, etc. only affect bullets this way if the CG is not coincidental with the CP.

That is why most match bullets are hollow points or Tupperware points - to get the CG and CP closer and minimize those effects!

quote:

Originally posted by 1 Shot Hunter:
IMO, there's nothing intuitive about supersonic (compressible) flow! That's exactly the reason I didn't take the AE (aerospace engineering) fluids course in college....

Stop it! You're giving me flashbacks....

quote:

I just don't know if it would be true while supersonic!?

If you don't want to look at data that's out there or do your own tests, I guess you'll just have to take my word for it.

A big ol' flat point on a bullet will kill its BC, I don't care what the tail looks like.

Easy, Jon!
I'm looking for help, not makin' trouble.

The numbers you quoted are shown on the Sierra site for .308 cal bullets... However, I look at the round nose and Spire point and see another significant difference, aerodynamically... Their lengths. Spire points are a bit longer, which also generally increases a BC.

So, since we would probably agree that weight has no bearing on BC, I wonder how a round nose vs. spire point of the same length would compare, to see if it is the shape of the nose or the length that is more significant?

I have no round noses on hand and only a few spire points to measure - all my bullets are boat tails... Hey, at least I'm not a hypocrite!

I'm trying to answer my own question. From Sierra's site:

150 SPT = .336 BC
180 SPT = .407

150 RN = .200
180 RN = .240
220 RN = .310

If I can assume that the 220 RN is longer than the 150 SPT , then it does look like the shape of the tip is more significant than the length, in terms of better BC. Looks like you may have shown me to the answer, Jon. Thanks.

quote:

Originally posted by 1 Shot Hunter:
Their lengths. Spire points are a bit longer, which also generally increases a BC....So, since we would probably agree that weight has no bearing on BC,

Actually you have it backward, it's a common misconception. Length, by itself (the skin drag portion) has very little to do with BC. Weight is very important to BC (think about what BC really is for a moment--not just a representation of drag, but a representation of the effects of drag). Things that increase the BC usually do make the bullet longer (boattails, sharp points, increased weight) but the added length is a side effect, not the reason for the BC.

If you want to compare bullets of different weights, you need to extract form factor or cd out of the BC's for apples to apples comparison.

Jon -

I agree about the length thing. When I said "generally," I should have said "as a general side effect." Indeed it is not always so, but almost always, because the shape factors that generally make a better BC (such as pointy ends) also generally tend to make bullets longer (for a given weight).

I think I disagree about weight having anything to do with BC. Are you saying that two bullets identical in every way (size, shape, etc.) but density will have different BC's???

Maybe I didn't understand BC... I always just kinda thunked it was same as Cd (which is strictly based on form factors). Hmmm, unlike Cd, if it is a measurement of the effects of drag, I can see it!

That also explains why the BC is different at different velocities. Or would differ with humidity or ambient temp. changes, too.

quote:

Are you saying that two bullets identical in every way (size, shape, etc.) but density will have different BC's???

Yes. They will both suffer the same force of drag. But A=F/M. This force will decelerate the heavier bullet more slowly, it will retain its velocity better--that's what BC represents. The effect drag has on the bullet.

In terms you should be familiar with: BC is the ratio of SD to form factor.

Think of form factor as analogous to Coefficient of drag. SD contains the bore diameter squared, which is proportional to frontal area.

From those two things you can calculate the drag the old fashioned way. Then, once you know the drag, the SD also contains mass so you can figure out what effect this force will have on the bullet.

Make sense?

"Si", said the blind man as he picked up his hammer and saw!

Lernt a coupla things, like I was trying to do. Thanks again.

Hey 1 Shot Hunter, If you can get ahold of the 2 Volume Set of P.O. Ackley's Handbook For Shooters & Reloaders, you will find an interesting surprise in them.

Between the two books is an envelope with a couple of Charts. Chart #2 is for Determining the Ballistic Coefficient of a Projectile. You take a Bullet and lay it on the Chart to see which Profile it best fits. That would help explain what you saw when you looked at the Manual with the different Bullets.

The information in the two Books is very "dated" since it was written in the 1950s-60s. So, you would need to bear that in mind if looking at some of those "Old Loads" and rely on more current data. But some of the stories in the two books are worth getting them, just for the historical perspective and Ballistic Concepts of those times.

Best of luck to you.

Guess I'll need to keep a lookout for more reading material - I just don't know all there is to know, after all... I appreciate the recommendation, Hot Core.