Bullet BC theory- Have you proven it?

[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.

 

[FNSafari]

Boy am I glad I shoot point blank benchrest!!

 

[davidjoe]

This is from the deck of the Missouri, yesterday, the shape chosen for the 1.89 million grain, 27 mile range 16 inch round. Toured the ship yesterday. It was amazing to think that 2,200 sailors and the entirety of the vessel exists for the singular purpose of delivering these rounds. These bullets were why we had the ship. We love our guns and gear, but they exist just to deliver the bullet. It all begins and ends with the bullet.

 

[charlie b]

Was visiting Watervliet Arsenal back when I was still in the Army. Toured the barrel mfg area to back up some research I was doing on barrel steel.

At the time the big building that made all the 16in guns was there. A HUGE lathe with cutting bits that were 10"x10"x5ft. Back in WWI they had to knock out a side of the building and put on an extension to handle the longer barrels. They were using that section as long as the big guns were in service. They don't fire that many rounds before the barrels need to be worked on.

 

[J_idaho]

Ok so if I’m understanding this correctly. Aside from energy BC is the only thing that matters? I always wondered if BC included spin drift but I guess BC takes care of everything.

 

[jelenko]

For spin drift, along with mv, the twist rate needs to be specified.

 

[Joe Salty]

Guys like I've always said Barrels and Bullets! If the Barrel doesn't like the bullet, have fun trying to get it shooting.

Joe Salty