Bullet BC theory- Have you proven it?

[cp255]

6DOF would be the holy grail. They are all point mass solvers for intervals along a projectiles motion. So for some interval, maybe every .01 ft, you solve the equations of motion for a body. Dump them into an array and graph them. 6DOF would have every input acting on a body and you would think it would have to be perfect. Problem is, you don't necessarily have inputs for yaw, pitch and roll. We're solving a simple equation with 3DOF and it gets you close, it doesn't absolutely model anything. So use more computing power...............or fire another shot. Never designed for you to punch in some numbers and make a first round hit on the bullseye. Unless your bullseye is the size of a town. Think of the wind drift side, you input a single vector that you have determined some magnitude and direction. You ever watch smoke? How many variables in wind do you think there are between the muzzle and target. Nothing but a WAG (wild-ass-guess). If you use instruments along the way, then it's SWAG.
To see the guys at the Tackdriver shoot sub 0.25 moa groups @ 330 yards. is bordering on sorcery and magic. Lots of experience.

I think the issue here

Maybe, Im no expert on that. But the thing Im talking about seems very hard to predict. Different stability based on tune thats changing through out the day or barrels life. And not all barrels can achieve the same level of stability. Thats why theres hummers. Seems like an impossible thing to predict. Outside of the BR world most will deny this is even true.

I think another big issue is that even if two rifles fire the same bullet, from the same lot, at the same muzzle velocity, you’re still going to get differences downrange. A bullet engraved by 4-groove rifling isn’t going to behave exactly the same as one engraved by 6-groove rifling. Tiny variations in launch dynamics—engraving forces, bore diameter, muzzle crown uniformity, barrel harmonics, how the muzzle blast pushes the bullet for the first few inches, and variations in the bullets themselves—all add up.

Even in a vacuum chamber you still couldn’t perfectly model this, because many of these variations happen during launch, not during flight. Barrel whip, the exact instant of muzzle exit, the alignment of the bullet as it leaves the bore, and how the muzzle blast imparts its last bit of momentum are all unmeasurable on a shot-to-shot basis.

Once the error in your external ballistics model drops below the magnitude of those physical variations, further refining the model doesn’t help, because other sources of scatter overwhelm it. When you have multiple independent sources of variation, their uncertainties combine in a way where reducing just one of them barely affects the total. That’s why chasing tiny refinements in the model can’t compensate for much larger real-world variations.

Unless you’re shooting in a perfectly controlled environment—with zero wind and perfectly repeatable launch conditions—the atmosphere and the rifle itself introduce far more variability than any subtle improvement in the drag model can overcome. More exact models demand more exact inputs, and most of those inputs simply can’t be measured in the field (or in the lab.)

Even inter-planetary spacecraft, which actually do fly in a near vacuum, still require periodic course corrections. Tiny forces like solar radiation pressure, thermal asymmetries, and minute gas venting accumulate over time. If their trajectories drift despite near-perfect conditions, it’s easy to see why bullets drifting through a turbulent atmosphere are even less predictable.

 

[Krogen]

Good points cp255. I've seen point-of-impact variations when fiddling with a tuner. Not just group sizes but POI. This is with the same load, fired in succession, in consistent conditions while fiddling with tuner settings. The same occured with my old and disused Magnetospeed. The mass of the "bayonet" on the muzzle can change point of impact as well as group size.. So trajectory/bullet drop can vary, just by changing the mass at the end of the tube. No external ballistics program can account for that!

 

[Tangent]

High speed film of bullets exiting a barrel is very interesting. The difference between suppressed, unsuppressed and muzzle brakes is very clear.

I would also have though bullet "damage" due to rifling could be a factor too.

 

[xr650rRider]

Good points cp255. I've seen point-of-impact variations when fiddling with a tuner. Not just group sizes but POI. This is with the same load, fired in succession, in consistent conditions while fiddling with tuner settings. The same occured with my old and disused Magnetospeed. The mass of the "bayonet" on the muzzle can change point of impact as well as group size.. So trajectory/bullet drop can vary, just by changing the mass at the end of the tube. No external ballistics program can account for that!

The trajectory/bullet drop, at least due to gravity, will be the same. But what your doing is changing the zero point where the bullet is leaving. Change that and the impact point changes. If your introducing yaw, roll or pitch, then your introducing effects that aren't part of the 3DOF model. Even Hornady's 4DOF model I suspect is nothing more than 3DOF with Brian Litz's spin drift and maybe Coriolis approximations added. Not really 4DOF.

 

[dellet]

I had a conversation recently concerning spin decay down range. Of course Brian Litz and Modern advancements in long range shooting came of the shelf to find the chapter that would certainly provide the formula to calculate this.

Surprise, surprise surprise. In a nutshell, too many variables. Number of grooves, depth, length……. So the decision was made to use a super high tech bullet spin calculating device. Basically three sheets of foam, a wooden box and a sharpie. Shoot a bullet through the box, calculate the degrees of rotation over a given distance. You have the spin rate.

In theory two bullets with the same BC, same velocity hit the target at the same time. The elephant in the room is if they enter the chamber with the same BC, will they still have the same BC after leaving the muzzle. This in essence was what caused the testing of spin decay to be done manually.

It’s been a long time since that book was published, maybe more testing has been done. Maybe a barrel maker has teamed up with a bullet maker to design two different bullets that will exit two different barrels with the same BC. Or was that article in the bulletin the day my Wi-Fi was down and I missed it?

 

[davidjoe]

The question becomes a bit reductive because if the bullets didn’t strike at the same time, they didn’t actually have the same BC, after all. It’s not that the theory of BC is fallable or flawed, rather the bullets were not assessed correctly.

I know our bullets do not closely resemble the G1 projectile, but I’m not convinced that is disqualifying. If the 1 pound flat base bullet drops and drifts a known measure in certain conditions, and my bullet performs 78% as efficiently as it does at the relevant trap, and your bullet performs 66% as efficiently, then it is really only functioning as a ruler, right?

Won’t any “relative” idiosyncrasies the big bullet potentially has apply equally to all the little boattail bullets that are being compared to each other?

 

[DaveTooley]

Something to ponder. This is about as granular as you can get.

 

[Steve Donlon]

Weight overcomes shape in drift and drop. The shape of a Lapua 155 grain 30 cal and a 16 inch naval cannon round is virtually identical. The muzzle velocity is also very similar, and density if anything favors the small arm, yet the naval gun’s range is on the order of 10X the .308.

But BC calculations capture this. We intuitively know that every dust particle we can see floating around the room is of course denser than its volume of air, and that if we enlarged that same substance by 100X in every direction, it would not float around nearly endlessly before settling down.

I’m not certain exactly why air resistance is not scalable to a given shape and density, without reading up more, but it’s definitely not. Liquids can usually be viewed as merely denser gas in this context.

—————
Edit: on reflection, as to the naval round and .308 of similar velocity and shape, the naval round’s frontal area is on the order of 2,800 times larger, but it weighs on the order of 120,000 times more than the .308. This no doubt accounts for the range disparity.

And mass increases by the cube and drag by the square. A lighter bullet will slow down faster at some point.

 

[antunRC]

Something to ponder. This is about as granular as you can get.

Do you have the same PDF (graphs) for 180gr or 184 Hybrid Berger bullet?

 

[DaveTooley]

Do you have the same PDF (graphs) for 180gr or 184 Hybrid Berger bullet?

Sorry no I don't. All my work was military related. Picatinny was looking for a bullet for the 300 Norma Mag at the time as well as other lesser calibers in use.

 

[RaySendero]

All a ballistic coefficient really does is scale the retardation (deceleration) curve of the standard reference projectile (G1, G7, etc.).

Reading all was half way thru and cp255 started with this sentence.
He was discussing something unrelated to my thought
YET my mind "clamped" on G1,G7 ballistic coefficients !
So this maybe a silly question, but;

Can similar bullets have the same G1 yet a different G7
and vise-versa
Can similar bullets have the same G7 yet a different G1
???

 

[cp255]

Reading all was half way thru and cp255 started with this sentence.
He was discussing something unrelated to my thought
YET my mind "clamped" on G1,G7 ballistic coefficients !
So this maybe a silly question, but;

Can similar bullets have the same G1 yet a different G7
and vise-versa
Can similar bullets have the same G7 yet a different G1
???

Sure it can, BC is just a curve-fit to a reference projectile, so a bullet doesn’t have one universal BC, it has whatever value best scales the G1 curve and whatever value best scales the G7 curve. Since the G1 and G7 standards have different shapes, two similar bullets can match one curve about the same (giving the same G1, for example) while matching the other curve differently (giving different G7s), and the reverse is also true. In other words, BC isn’t an intrinsic property of the bullet, it’s just how well the bullet’s real drag happens to fit each reference model, and those fits don’t have to line up across G1 and G7.