Bullet BC theory- Have you proven it?

[jelenko]

I've read a tremendous amount on this subject and understand the physics behind it, and the theory itself. I know a bit about aerodynamics as related to aircraft, airflow, thrust vectors, drag, yaw vectors, etc.

I'm simply asking if anyone here has laid down with tow different calibers, different weight bullets but same BC, running the same speed, and fired them are side by side targets at exact same time - and compared difference in bullet velocity, drift, trajectory..in other words does the physics/math hold true? If BC is the same, they are both stable, and speed are the same, then they should be the same....

But, if it's already been thoroughly studied, tested, dissected by people with the time, money and equipment, what are you to take from anecdotes?

 

[jelenko]

Be truthful..... you did this to kick off the winter pissing match contest didn't you.

Completely agree! The topic has been studied and tested thoroughly by many. What's the point of asking for anecdotes?

 

[Steve Donlon]

If a lighter bullets sheds velocity faster wouldn't it spend more time in flight at 1000 yards ? I know a 6mm bullet and a .308 bullets will have the same elevation ,MOA at 1000 yards with the same velocity and BC, but how could time in flight be the same? If so, that should have more of an effect by the wind on the slower bullet. A 30 cal. 220 gr. bullet can have the same BC as a 107 gr. 6mm bullet and the same velocity.

 

[davidjoe]

We have to keep in mind that the lighter bullet with the “same” BC as a heavier bullet is not just going to be a scaled down version of the heavier bullet, it’s going to be far “longer and sleeker looking” if the BC is the same and the materials are similar.

If the lighter bullet was simply a scaled down version, then it would have a FAR lower BC even though its proportions are the same. So, mass already gets its due credit in the calculation.

Here’s a good example. The top bullet is the DTAC 115 .243 and the bottom bullet is the 95 SMK .224. Both are Sierra-made. Tap on the picture and it will expand fully.

Clearly, the 95 grain bullet is longer, pointier, heavier “for caliber” - and nearly as long in absolute value.

Yet the bottom bullet has a lower BC, not because the form isn’t more aggressive, it definitely is, but because it’s giving up 20 grains of weight. It’s arguably harder to make longer and heavier for caliber bullets agg, as well, (BR certainly doesn’t seek out extreme shapes for group size) so, there is still no free lunch in the choice, and when the different shapes are considered, that the paths match each other if BC is the same makes good sense.

 

[jelenko]

If a lighter bullets sheds velocity faster wouldn't it spend more time in flight at 1000 yards ?

The lighter bullet will only shed velocity faster if it's BC is less.

Gotta accept that the use of BC's is well established - been studied and confirmed by militiaries - because it really, really matters.

 

[falconpilot]

Completely agree! The topic has been studied and tested thoroughly by many. What's the point of asking for anecdotes?

Yes this topic has been kicked around many times. The point is that in real live testing(in most basic terms of what we do as F-Class shooters) seems to show different results sometimes as to what ballistic programs project, or what we expect according to the math.

After reading all the comments, I want to conclude that maybe the BC number stated isn't quite correct on one of the bullets...different caliber, weight bullets with basically same BC fired at same time, side by side, on two different targets, both guns good no wind zero, both twist rates to stablize the bullets at same velocity...velocity and draft not the same at the target.

 

[WSnyder]

After reading all the comments, I want to conclude that maybe the BC number stated isn't quite correct on one of the bullets...different caliber, weight bullets with basically same BC fired at same time, side by side, on two different targets, both guns good no wind zero, both twist rates to stablize the bullets at same velocity...velocity and draft not the same at the target.

If you've ever taken advantage of AB's custom drag model done with Doppler radar which will measure the real bullet drag out of your rifle the BC that gets spit out is generally different than what it's BC is in the catalog or on the box. So yeah, two different bullets with the same theoretical BC shot at the same velocity might give different results. Same for why your particular bullet in your rifle might not exactly line up in your ballistics app. The old article in Precision Shooting "Why BC is BS" had some great info. I (or someone) should see if I (they) can find it in the digital archives.

To add: Ever notice manufacturers (Berger in particular) will update and change BC's now and then? Dies wear out, procedures change, different production runs, etc... can all make differences in the bullet that is made along with BC changes. So how can you take a stated BC and hang your hat on that exact number?

 

[Bottom Fodder]

My mother told me if she kicked in my butt with her lite weight pointed shoe it would hurt more than if my dad kicked me with his heaver flat toes shoe and she was right. But that was 60 years ago things may have changed

 

[TylerFromMS]

Yes this topic has been kicked around many times. The point is that in real live testing(in most basic terms of what we do as F-Class shooters) seems to show different results sometimes as to what ballistic programs project, or what we expect according to the math.

After reading all the comments, I want to conclude that maybe the BC number stated isn't quite correct on one of the bullets...different caliber, weight bullets with basically same BC fired at same time, side by side, on two different targets, both guns good no wind zero, both twist rates to stablize the bullets at same velocity...velocity and draft not the same at the target.

I can help you out with finding the "true BC". It's sort of like guessing how old you are. Grab a couple of bullets and call me.

 

[Texas Solo]

My BC's are too high, my ES & SD's are to low.

 

[Tommie]

Be truthful..... you did this to kick off the winter pissing match contest didn't you.

Usually we wait until Thanksgiving, but since Home Depot is already busting out the fake Christmas trees I guess it’s officially cabin fever season.

 

[xr650rRider]

I guess it didn't sink in that a published BC is an average value, the actual BC changes throughout a projectiles flight as it's velocity decays. You can calculate a BC given a muzzle velocity and a downrange velocity. It's an iterative calculation that back calculates what BC would it take to equal the 2 velocity readings. So for your experiment, if you used 2 bullets that had exactly the same muzzle velocity and same downrange velocity, same environmental conditions, same drag functions from the lookup table at same Mach numbers. Then yes, they would have same wind drift, drop, etc.

 

[Tangent]

I'm concluding from this the reason the F class shooters are keen on 180gr 7mm bullets is:

A combination of higher BC than anything smaller and less recoil than anything larger, easier to handle accurately.

Yes I know a lot use 300 WSM too but I think the same reasons stand.

If a lighter weight bullet had an equivalent BC/accuracy they'd be shooting it.

 

[Alex Wheeler]

I will never be convinced that BC and velocity tell the whole story for windage. Drops, yes mostly. On top of that, tune can change windage. No numbers can predict that.

 

[cp255]

All a ballistic coefficient really does is scale the retardation (deceleration) curve of the standard reference projectile (G1, G7, etc.). The reference projectile tables do not just give a drag coefficient—they give you a standardized deceleration vs. velocity for a projectile with a specific mass, diameter, and shape.

BC is a single number that wraps up your bullet’s mass, diameter, cross-sectional area, and form factor relative to that standard. That’s why scaling the reference retardation curve works: BC implicitly bundles all those geometric and mass terms into one multiplier.

For example (numbers made up): if the standard G7 projectile decelerates at 1,000 ft/s² at 3,000 fps, then a bullet with a BC of 0.5 would decelerate at roughly 2,000 ft/s² at that same speed. A BC of 1.0 would match the reference exactly. This is why two bullets with the same BC and the same muzzle velocity will produce essentially identical trajectories in a pure BC-based point-mass model.

Of course, this is only an approximation. Real bullets don’t follow the G7 retardation curve perfectly. Different shapes match the G7 profile better or worse in different velocity bands, so two bullets with the same BC will usually be close but not identical, especially in the transonic region.

Wind drift is handled by computing drag based on the air-relative velocity vector (bullet velocity minus wind). The sideways component of the aerodynamic force naturally produces wind drift.

This simplified point-mass model does not include second-order aerodynamic effects such as a bullet’s lateral force coefficient (“sail effect”), spin drift, or aerodynamic jump. Those require additional aerodynamic terms or a true 6-DOF simulation.

 

[Tesoro]

I will never be convinced that BC and velocity tell the whole story for windage. Drops, yes mostly. On top of that, tune can change windage. No numbers can predict that.

I believe the ballistics computations are based on pure physics without the influence of barrel harmonics, which is your job to normalize!

 

[Steve Donlon]

We have to keep in mind that the lighter bullet with the “same” BC as a heavier bullet is not just going to be a scaled down version of the heavier bullet, it’s going to be far “longer and sleeker looking” if the BC is the same and the materials are similar.

If the lighter bullet was simply a scaled down version, then it would have a FAR lower BC even though its proportions are the same. So, mass already gets its due credit in the calculation.

Here’s a good example. The top bullet is the DTAC 115 .243 and the bottom bullet is the 95 SMK .224. Both are Sierra-made. Tap on the picture and it will expand fully.

Clearly, the 95 grain bullet is longer, pointier, heavier “for caliber” - and nearly as long in absolute value.

Yet the bottom bullet has a lower BC, not because the form isn’t more aggressive, it definitely is, but because it’s giving up 20 grains of weight. It’s arguably harder to make longer and heavier for caliber bullets agg, as well, (BR certainly doesn’t seek out extreme shapes for group size) so, there is still no free lunch in the choice, and when the different shapes are considered, that the paths match each other if BC is the same makes good sense.

does it have anything to do with "proportional speed" ? Like moving in the water. At some point , weight matters more. Unless the ratio is to small in a bullet.

 

[xr650rRider]

I believe the ballistics computations are based on pure physics without the influence of barrel harmonics, which is your job to normalize!

Exterior ballistics are just that, outside of the barrel.

 

[Tesoro]

Exterior ballistics are just that, outside of the barrel.

Vs interior ballistics? what calculator do I download for that?!

 

[xr650rRider]

Vs interior ballistics? what calculator do I download for that?!

Quickload, Gordon's reloading tool