Help! - It just doesn't make sense to me

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michaelsabre

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I was play with on-line ballistics calculator.

I found for different bullets at same speed, as long as their BC is the same, the wind drift is the same.

And then I found, 150gr Hornady 308 will have more wind drift than Hornady 75gr 223 bullet (assuming they are at same speed).

How come? Wind will blow 150 gr further? Just so not "common sense".
 
...and then you'll go out and shoot with those two bullets and discover that the bullets don't do what the calculator says it will...:)

Ballistic coefficient measures the relative amount of friction as the bullet travels forward.

The sectional density comes into play in two different axis... the frontal sectional density (Which is what we tend to obsess over) and the lateral sectional density.

Sectional density = mass divided by area. The lateral sectional density can be different if the area is different in proportion to its mass. In other words, if the 223 and 308 bullets were in exact scale in terms of weight, length and total surface area, then they should exhibit the same degree of wind drift, but if there is even a slight variation in any one of these parameters, then the lateral sectional density would be different and thus they should have different drift characteristics.
 
Thanks for the replies.

I can see 75gr 223 is longer than 150 gr 308, thus it will cut through air easier. But the wind drift is from different direction. It's easier to blow away a needle than a lead block, isn't it?
 
a good analogy would be putting up the sail on a sailboat. the wind will still blow a boat without a sail, but if you increase the area that the wind will affect then it will move the boat more. The boat weighed the same with the sail down, but because the sail catches the wind better, the boat moves more.

a .308 will have a larger amount of surface area exposed to the wind, which will act like a sail.
 
A byproduct of a lower BC is that the bullet slows down faster over a given distance, taking longer to get to the distination. More drag - air resistance.

This means that stubbier bullet is influenced by the wind more thus it drifts more. Look at the different velocity values over time/distance. Also, compare time of flight.

The 75gr Amax has a higher BC then the 150gr 30cal. If both are launched at the same velocity, the 30cal will slow down faster, take longer to get to the target, and be blown off course more.

To match the two, the lower BC bullet would have to travel faster to overcome the drag.

The calculation for BC is supposed to take out the effects of bullet mass. Only the aero shape is considered (in relative terms of course). That is why BC is dimensionless.

Where things get really complicated is there are several drag models (ways of calculating a bullets drag) which spit out a different BC value.

The most common algorithm is the G1 drag function. Garden variety FB bullets seem to follow this drag curve well. VLD or secant ogive bullets don't. These tend to follow a G5, even a G7 drag curve.

So a 0.45BC using a G1 curve is vastly different then a G5 bullet of the "same" BC value. Play around on JBM and you will see what I mean.

There will be many times where you will shoot a new gen bullet and it seems to defy gravity. Flying way flatter then expected. The most common reason is that the G1 drag function is chosen in the ballistics program which doesn't agree with how the bullet actually flies. Changing to a G5, 7 or one of the other options will likely bring the ballistics table and what you observe inline.

SOOOOOO, when comparing bullets to see which will slip through the air better and drift less, you need to know not only the BC value but what drag (G) function was used to generate that number.

A 0.6BC G1 bullet has almost twice as much drift as a 0.55BC G7 bullet when launched at the same velocity.

Simple no????

Jerry
 
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