is copper jacket spinning around lead core during flight

[ntsqd]

Slow down gentleman. The bullet is getting it's speed from the gasses released by the powder burning. Because it is ahead of the high level of temperature of the burning powder for milliseconds it gains more heat from speed through the atmosphere than it does in the barrel. Non-bonded bullets jackets can become separated from the lead at high twist rates and excessive speed.

I'm not sure about that last sentence. The issue that I have with it is WHEN the core becomes separated. It has been witnessed to have happened after the bullet is at rest thousands maybe literally millions of times, but all that tells us is that it did happen and not when it happened. So did it happen in flight or did it happen as the bullet was coming to a stop?

I contend, as I first posted, that the jacket distortion due to the rifling is a serious impediment to the core rotating relative to the jacket while in flight. That jacket distortion will make the interior surface of the jacket not round, and that will make the exterior surface of the core also not round. That will make it much more difficult for them to rotate relative to each other while in flight.

As I also previously said, a thick jacketed bullet will distort it's interior surface lass than a thin jacketed bullet. However, it is the thin jacketed bullet that it likely to be going faster. There is the issue of jacket spring-back. I would expect the spring-back to create a tiny air gap between it and the core in a non-bonded core bullet. I would also expect a thin jacket to spring-back more than a thick jacket. This spring-back would be a good reason for core loss in a non-bonded bullet at the terminal end of the bullet's flight as the core is now loose in the jacket but that does not mean that it was necessarily free to rotate relative to the jacket.

 

[Radman]

Something I've always wondered about high power rifles shooting copper jacketed bullets with a lead core. If you had a 12 inch twist on a barrel shooting 3000fps, then at the muzzle the bullet is spinning at 3000 revs per second, times 60 which is 180,000rpm. If its a 9 inch twist then its 240,000rpm. That means this heavy mass goes from 0 to 240,000rpm in about 2 milliseconds. That's incredible moment of inertia. Now, Lead melts at 621 degrees F and is a heavy mass, yet the copper jacket is being spun around the lead which is probably either liquid or close to it, very soft. The question to be answered is does the lead core reach melting point but I think its possible that these copper jackets are spinning around the lead core which is moving at a slower rotation. Does it cool down and match speed with the copper jacket in the next 1.5 seconds before it hits the target? I doubt it. It might even be getting hotter from the air friction. Maybe they put some baffling in the empty jacket to keep this from happening even if the lead is liquid, I don't know.

You certainly made an interesting & thought-provoking post, I'll give you that! I have truly enjoyed all of the interesting & varied opinions & feedback. It's good that so many can share facts, thoughts & opinions without folks getting angry or childish. jmo

 

[bluedog69]

Here is what a bullet manufacturer/designer/developer/tester stated

"With "normal" bullets, us manufacturers get nervous when RPM reach the 300,000 range, and double the failure probability when dealing with thin jacketed varmint bullets."

This was in response to a question about a 1:9 twist at 3300 being too fast. Thinking that was about 264,000 RPM.

 

[Professor Doolittle]

Slow down gentleman. The bullet is getting it's speed from the gasses released by the powder burning. Because it is ahead of the high level of temperature of the burning powder for milliseconds it gains more heat from speed through the atmosphere than it does in the barrel. Non-bonded bullets jackets can become separated from the lead at high twist rates and excessive speed.

Pressure = temperature in a gas. The pressure can't ever lag or lead the heat because they are the same thing, the kinetic energy of the gas molecules. PV=nRT, if you know pressure you know temperature, if you know temperature you know pressure.
But you bring up an interesting question, how long does it take for 99% of the gunpowder to be burned and where is the bullet located when that point is reached. I would guess the bullet is traveling down the barrel at that point, not yet out. The visible muzzle blast is just decompression after the bullet is out and takes time, but you do see little sparks sometimes that makes me wonder how much gunpowder is still burning or if those sparks are something else such as primer fragments.
As far as saying it gains more heat in the atmosphere than the barrel I doubt that. My understanding is the bullet gets compressed or deformed into the barrel. That alone will raise the heat. If you could crush a bullet down with a press and instantly feel it it would be very hot. Also copper is one of the best thermal conductors, so before the bullet even starts moving heat is being transferred from inside the cartridge through the copper into the lead. Its being heated while being crammed into the barrel. Then, on top of that, the lands and grooves are forcing an extreme rotational acceleration from the skin of the bullet inward radially which will cause deformation and pressure that also heats up the bullet. I guarantee you the bullet is hotter at the muzzle than at the target.
Once the bullet leaves the barrel air friction will add heat to the nosecone, lets call it, but also subtract heat from the rest of the bullet by carrying away kinetic energy, meaning the air atoms go away hotter because they touched the bullet. The net total is going to be cooling. I could be wrong if the cooling capacity doesn't have enough time before the bullet lands. It depends on time of flight and which effect is dominant, cooling or heating.

 

[DMP25-06]

I think that we need some input from Bryan Litz of Berger Bullets , or other notable bullet designer/engineers of cup and core constructed bullets , as to what their findings and theories might be .

 

[emp1953]

Pressure = temperature in a gas.

I don't think the function is linear, I believe it's exponential. Also since the process of going from a solid (the powder), to a gas, ignition and high pressure, to out the barrel pressure zero takes place in milliseconds, I think quantifying it would be extremely difficult. I agree with DMP25-06 that bullet manufacturers somewhere have a handle on what is going on here.

 

[bluedog69]

My guess is that it doesn't liquify or spin around the lead.
When you shoot a deer at 30 yards and recover a mushroomed bullet that went
length wise through the deer.

If it were liquid when it left the barrel would it hold together at all when it went
through 3 feet of flesh and bones?

 

[Professor Doolittle]

I think that we need some input from Bryan Litz of Berger Bullets , or other notable bullet designer/engineers of cup and core constructed bullets , as to what their findings and theories might be .

I agree.

 

[bluedog69]

I think that we need some input from Bryan Litz of Berger Bullets , or other notable bullet designer/engineers of cup and core constructed bullets , as to what their findings and theories might be .

I don't think the function is linear, I believe it's exponential. Also since the process of going from a solid (the powder), to a gas, ignition and high pressure, to out the barrel pressure zero takes place in milliseconds, I think quantifying it would be extremely difficult. I agree with DMP25-06 that bullet manufacturers somewhere have a handle on what is going on here.

Here is what one of the gentlemen who is a ballastician in a bullet company quoted to me years ago


"With "normal" bullets, us manufacturers get nervous when RPM reach the 300,000 range, and double the failure probability when dealing with thin jacketed varmint bullets."



So bullet manufacturers start to get nervous about vaporization of varmint bullets at 300,000 rpm was my take on what he said.

 

[del2les]

As an longtime bullet swager and past licensed ammo maker, the lead "core" does not melt, and if the lead core has been swaged properly in an appropriate thickness jacket, usually, it will not separate in flight. Some Thin jacketed varmint bullets may and have jacket separation in flight due to high centrifugal forces, rifling engraving, rough bores, etc, but usually, this warning will be applied by the manufacturer to not use beyond a certain twist rate and/or velocity. This has nothing to do with the actual core/jacket bond or a core melting.

In flight projectile heating is real, but there have been no known or observed instances of that temperature reaching the point of melting the lead core. Having tested and observed numerous projectiles after being fired into water, gel and other traps, we have never seen any indications of core melting. At longer ranges and via radar BC testing, there have been some indicators that a plastic or lead "tip" may actually deform from resistance and heat and reduce the original BC of the projectile, and this was the reason behind Hornady's change from AMAX tip to the ELD tip.

During the lead core swaging process, the pressures applied to the core enlarges the core and jacket until it matches the ID size of the die, and once pressure is removed, the jacket ever slightly springs back giving an extremely tight grasp on the core material. Only a poorly swaged bullet will allow for any slippage between the two materials within modern shooting velocities and twist rates.

While I understand one's reasoning behind the question, it isn't anything to worry about.

As an FYI, I have swaged very light for caliber bullets and fired them at 5,250fps, and I have never experienced separation nor indications of melting.

 

[Professor Doolittle]

I don't think the function is linear, I believe it's exponential. Also since the process of going from a solid (the powder), to a gas, ignition and high pressure, to out the barrel pressure zero takes place in milliseconds, I think quantifying it would be extremely difficult. I agree with DMP25-06 that bullet manufacturers somewhere have a handle on what is going on here.

For a given gas composition it should be linear but I think what you mean is the Ideal Gas Law only holds up under limited conditions which may not apply at the high pressures in the chamber. But its still true that pressure cannot lead or lag temperature. Its not intuitive but the heat of a gas and its pressure are the same consideration.

 

[Professor Doolittle]

I think that we need some input from Bryan Litz of Berger Bullets , or other notable bullet designer/engineers of cup and core constructed bullets , as to what their findings and theories might be .

They would know vastly more. I'm probably going to buy their applied ballistics software package, I think its like $200. What I really love from the demo I saw is it shows you the probability of a hit by distance, meaning the odds of hitting a bullseye at various distances depending on all the variables involved, including wind.

 

[ntsqd]

We can assume that the leading surface area of the jacket is being heated by the air as it moves thru it, and we can assume that the trailing surface area of the jacket is being cooled by the air blowing off of it and taking some heat with it. Given that thermal transmission to the core occurs at a boundary of dissimilar metals I would expect this transmission to be far less efficient than the thermal conductance of the jacket itself. Meaning that the jacket is transmitting more heat from front to rear than it is transmitting to the lead core. Leaving far less thermal energy to melt that core than that gained from moving thru the air. For the lead to melt from just air resistance the heat created by the bullet moving thru the air would have to be great enough that after what was lost to convection cooling would still be high enough that after the inefficient thermal transfer to the lead that the lead would rapidly rise in temperature and melt. I see that as a very unlikely. Perhaps is the bullet's time of flight were measured in tens of minutes it would have time to partially melt the lead core.

We do know that the friction of moving thru a bore, if fast enough, along with the thermal energy from the burning powder needed to move it that fast, can melt lead. It may not make it a liquid, but it is getting close. As soon as we put a jacket on that bullet we've essentially put a thermal insulator around the lead. And we've reduced the friction if we chose the correct material for the jacket, which reduces the total thermal energy generated in the first place.

Been a while since Chem 1A, let's see if I remember this correctly. PV = nRT: Pressure x Volume = n (# of mols of the gas) x R (Ideal Gas Law Constant) x Temperature. For more general, ambient conditions it can be used as PV=T if we accept that there will be inaccuracies if we explore the outer edges or are dealing with some weird gas (say gassified granite rock for instance). Set V = 1 or 10 or a Brazillion. and hold it constant. Since the volume is held constant it becomes clear that as the temperature goes up so does the pressure.
The fundamental problem with this is that we've simplified things too much and PV = T doesn't come close to matching what is really happening. I will suggest that PV = nRT does, but that the values for some of those variables will be mind boggling and may be the result of some truly ugly calculations (that n variable alone in this use could easily be a nightmare!). But it is useful to simplify things to start out with and then add the complications as we get comfortable.

 

[Lenny Foffa]

I do not. Know the. Science behind this but my. Early 1979. 270. Weatherby mag load development when. I. Went. Above 68 grains of 4350 pushing a. 130 Grn Sierra. Boat tail to faster than 3400 FPS I would see lead splatter around the . Bullet t hole on the paper target ! Since the bullet had an exposed lead tip, I assumed lead was melting inside the. Jacket and even the tip was melting , causeibg lead splatter on the. Target ! Excellent. Topic and. Response guys !!