Question about taking B.C. to extremes.

[samsmix]

Okay, we have our 6.5mm 140s, and 100gr pills for the .224 Valkyrie. Heavy for caliber bullets offer penetration and retained energy all out of proportion to cartridge, and trajectory out of proportion to recoil and muzzle velocity, when compared to what we were doing 20-30 years ago.

Why not have a 200gr 6.5mm? A .30 cal. 300gr? Could a magnum case push it fast enough to be useful, without the barrel wear of blistering speed?

Where do we reach the point of diminishing returns?

[HiBC]

I don't mean this as a smart alec answer,but your question answers itself.

The answer is "The point of diminishing returns"

Wildcatters and gun cranks push the envelope till they find it .

Then a new piece of brass or a new line of bullets or a new powder comes out and the games begin again.

You can compare the 7-08 with the 7mm STW and consider diminishing returns.

The individual decides what he will trade for what return.

[condor bravo]

And not being able to stabilize the longer bullets at a safe pressure to obtain the necessary velocity would enter the picture.

[Jim Watson]

There is an L/D ratio that cannot be spin stabilized.
I figure a 300 gr lead and copper .30 would be pushing it.
There were some very heavy bullets made with tungsten rear cores with lead nose cores for expansion. Velocity wasn't great but penetration was deep.

[std7mag]

I believe you would lose some velocity just from the faster twist rate, possibly causing pressures to rise.

[TXAZ]

How much pressure can your barrel handle? 50-60k like many rifles?
Push that up to a million PSI and you can do whatever you want. Just make sure you can afford a barrel that can handle that pressure. (Which you cant)

[hounddawg]

In one word - physics. You would have a slow tumbling bullet with your idea. You want a lot of weight, poor ballistics, and a kick like a mule you can always go with a .45-70

on smaller calibers look up the Miller Rule and the go play with the Berger twist rate calculator

[Unclenick]

Some of the above answers need a bit of modification. Length of a bullet is the most significant factor in how much twist you need. Mass is the next most significant factor, which is why the old Greenhill formula in its second most complex iteration has a square root of specific gravity ratio appended to it. When you add mass without changing the size and shape you decrease required rifling pitch (twist rate) and increase BC in proportion to the increase in mass (increased net density of jacket and core). For two identical shape bullets with the same jackets and core volume, but one with a lead core and the other with a gold or a tungsten core (gold and tungsten are the same density; 19.30 grams/cc while lead is just 11.34 grams/cc), the one with the gold or tungsten core would be both heavier and have a higher BC and require slower spin to be stable at a given velocity than its lead core counterpart. That is because gold and tungsten are 1.7 times more dense than lead, so they weigh that much more than lead an equal in an equal volume. It may seem backward that the heavier bullet needs less spin, but all the charts saying a heavier bullet needs more spin are assuming the bullet got heavier by being longer, not by changing density, and it is actually that added length that causes it to require a shorter rifling pitch.

How much energy from the powder goes into the spin? With conventional rifling, not a lot. I've used my CAD software to model a bullet and find its moment of inertia about its spin axis, and from that used the formula for kinetic energy of rotation to see how much is in the spinning bullet. It then subtracted that amount from the muzzle energy as if it were directly lost from it and then I worked backward to the resulting new muzzle velocity at that lower muzzle energy. I found the difference was on the order of one foot per second. This was a while ago and I don't recall the bullet or the muzzle and spin velocities involved, but the bottom line is that it was less than shot-to-shot velocity standard deviation.

With very extreme rotation rates the above might change, but what are the limits of rotation? The British found long ago that you could not make a gun work with twist faster than about 6 calibers. Faster, and the bullets just strip in the rifling or blow the gun up. But that's very fast. It's a little faster than a two-inch twist in 30 caliber guns. Most guns have much less than that, so I don't think we have any practical need to be concerned with this limit today.

Another limit is that too much rotational acceleration will cause core stripping in a cup and core bullet. This is where the rotational acceleration at the pressure peak is so high that inertia pulls the core loose from the jacket and slips inside it. So the jacket gets full rotational velocity (angular velocity in physics) but the heavier core is turning more slowly when the two exit the muzzle. With the rifling behind them, the two equilibrate their angular momentum and you wind up with a bullet unbalanced by the gap developed between the jacket and core and spinning at a lower rate than intended. Harold Vaughn measured this happening in a 270 Winchester when a light bullet was driven too fast in it. Scattergun accuracy.

For solid bullets, more can be done with fast twists. I don't know where the limit will lie. One thing that has changed over the last 50 years is the average bullet manufacturer is doing a better job of making bullets with axially symmetric mass. That is, if you make your cartridge perfectly concentric so the bullet doesn't tilt any in the bore, you can spin them faster than you used to be able to do before groups open up due to the lateral center of mass drift or wobble in flight. This has increased the bullet lengths we can use because the more rapid spin required doesn't any longer impose as low a limit as it once did on bullet length.

I expect those limits to keep growing as bullet makers get still more precise in what they do, so I think you can expect heavier and longer projectiles to continue being introduced as time goes by. The 6.5 Creedmoor has a longer throat than the 260 Remington so it can accommodate extra long bullets. When bullets get even longer you will see new chamberings with still longer freebores.

So, where is the point of diminishing return? That is something you have to work out based on practical considerations. Every time you double the BC, you reduce bullet drop by a smaller factor as the time of flight gets closer to the vacuum time of flight. But wind drift cuts at least in half or it can be more, depending on where in the drag curve the average velocity lies. The changing slope of the drag curve with velocity makes this difference non-linear. So you just have to run a ballistic calculator with the possible new values and see how much difference it makes to how you shoot. Diminishing wind drift is certainly desirable as it reduces the accuracy requirement for your wind reading skills, one of the tricky parts of long range shooting.

[5whiskey]

Unclenick for the win again with a super detailed explanation. The question is nuanced, IMO. Every other poster before me hit on good points. Diminishing returns, velocity needed to stabilize (versus allowable pressure to reach said velocity), sheer recoil... all are considerations.

OP interesting question. I've studied and shot with some folks from Applied Ballistics. They publish free articles and for-purchase books that go into great detail on such matters. It's where I read one of my favorite quotes that pertains to your question.

Muzzle velocity is a depreciating asset, not unlike a new car, but BC, like diamonds, is forever
-German Salazar

[samsmix]

To those who answered with facts and formulas, thank you. I am neither a reloader nor a machinist, but your answers have given me a better understanding.

It would seem that, barring a new Barrel material, we have just about reached the Pinnacle of cartridge development. Flatter trajectories will require a new scientific breakthrough in order to progress much past the current state of the art.

[Unclenick]

Well, keep in mind that BC only improves the flatness of fire as compared to a lower BC bullet fired with the same muzzle velocity. If you got the higher BC by increasing the weight of the bullet, then the muzzle velocity of a maximum load in the same gun will be lower, so the arc of the trajectory may be just as larger or larger than the lower BC bullet exhibits. It just won't be deflected by wind as much on the way to the target and the higher SD from the extra weight may enable it to penetrate better.

I can think of a contrary example. The Sierra and Lapua 155 grain .308 Palma bullets have about the same BC as the 175 grain SMK does. So, you could get a flatter trajectory out of the 155 than out of the 175 because you can drive it faster. But mostly, you pick bullets based on the fact they shoot well in your gun and what kinds of targets you shoot at and in what conditions. That will tell you what aspects of performance you need to be paying attention to.

Also, don't get too wrapped up in flat trajectory. Remember, the bullet of a zeroed rifle traces a path that first climbs to the apogee of the trajectory from the location of the muzzle and then starts to fall below that. The fact it traverses the same vertical distance, first up, then down, using up its total drop number, keeps the point-blank range from being decreased by a flatter shooting bullet as much as many expect it to. Run some ballistics programs with different rounds to get an idea how much difference you can make.

[Dano4734]

Uncle nick knows more about this stuff than anybody alive I believe

[Sevens]

I wouldn't go quite that far with regards to unclenick as I wouldn't think that to be fair at all to many great minds that don't post here in our forums.

However!
When it comes to being able to explain things in a way we can (usually) understand, keep it interesting (when too much technical detail clouds things) and -- it MUST be said -- is willing to continue performing this service to these forums on and on with no end? Just consider the time he spends sharing this wisdom and experience.

unclenick, as a forum contributor, has an irrationally high "ballistic coefficient." And no diminishing returns either!

[TXAZ]

As far as "diminishing returns" consider this for ballistic coefficients for 3 rounds fired at about 3,000ft/sec muzzle velocity.

Diameter BC Max Range
.223 .2 1 mile
.50 .9 7 miles
16" 15 20 miles

If you want to shoot flatter or further with similar shaped projectiles, the math say use the biggest caliber you can find.