is copper jacket spinning around lead core during flight

[Professor Doolittle]

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.

 

[MagnumManiac]

This a known phenomenon with light for calibre bullets spun at very high speeds. It is called 'stripping' the rifling, the jacket and core become separated as you describe and spin independently of each other.
It is often the reason that bullets are ripped apart by fast twist rifling.
Best I ever saw of it was with a 300WM firing Speer 125g TNT bullets at about 3500fps….a large puff of blue about 20-25yrds from the muzzle and the target was peppered with tiny bits of lead, no jacket material was found that far out. It was witnessed by half a dozen people at my range….we all had a chuckle over it.

Cheers.

 

[bozoben]

That's a thought that I don't think my brain would have ever thought up. Interesting.

 

[cohunt]

If the plastic tips don't melt ( or do they) then would the lead core melt too? Wouldn't a liquid be spun out by centripetal forces? Does the bullet really reach those temperatures that melt the lead? If your lead core melted, then all lead bullets ( jacket or no jacket) would melt--- right? So we are now all shooting liquid lead bullets? Possibly overthinking the physics aspect of ot all?

I'm sure this will bring up lots of interesting comments and opinions

 

[QuietTexan]

It seems like there would be a convective transfer issue where the lead core is not evenly heated and would not uniformly liquify. There would be a boundary layer formed between the jacket and solid portion of the core, and as small as bullets are a shear flow layer would form and the relatively high viscosity of lead at its melting point would resist the shearing motion, so the non-uniformly liquified lead core would still rotate along with the jacket. This might be why bonded jacketed bullets perform better than straight cup and core bullets at higher impact velocities/ faster twist barrels, because the core and jacket have a chemical bond that has to be overcome, not simply a convective heat transfer from one metal to another.

I'm not convinced the core actually melts, I don't know how to calculate it but I'm sure some engineer could do the math on how much heat the core absorbs by convection over the relatively short time window of the bullet's flight. Think of the RPMs of a 36gn Varmint Grenade going 4000FPS out of a 12-twist 22-250. Those bullets hold together, and they function by having a sintered metal core that fragments when it hits the target. I'm sure the melting point of the copper-tin powder is higher than lead, but there's also significantly less mass available to absorb heat from the shot. If the core was melting it wouldn't explosively fragment the way that we see them do in ballistic gel.

 

[Straight Shooter]

^^^^^ Holy Moly.

 

[Ken61]

Might be hard on a suppressor

 

[shootallday]

Perhaps if you could catch a bullet and slice it open if the core did melt the grain structure of the lead would be twisted. I don't think anything happens tho.

 

[Professor Doolittle]

It seems like there would be a convective transfer issue where the lead core is not evenly heated and would not uniformly liquify. There would be a boundary layer formed between the jacket and solid portion of the core, and as small as bullets are a shear flow layer would form and the relatively high viscosity of lead at its melting point would resist the shearing motion, so the non-uniformly liquified lead core would still rotate along with the jacket. This might be why bonded jacketed bullets perform better than straight cup and core bullets at higher impact velocities/ faster twist barrels, because the core and jacket have a chemical bond that has to be overcome, not simply a convective heat transfer from one metal to another.

I'm not convinced the core actually melts, I don't know how to calculate it but I'm sure some engineer could do the math on how much heat the core absorbs by convection over the relatively short time window of the bullet's flight. Think of the RPMs of a 36gn Varmint Grenade going 4000FPS out of a 12-twist 22-250. Those bullets hold together, and they function by having a sintered metal core that fragments when it hits the target. I'm sure the melting point of the copper-tin powder is higher than lead, but there's also significantly less mass available to absorb heat from the shot. If the core was melting it wouldn't explosively fragment the way that we see them do in ballistic gel.

I agree its the boundary between the copper and lead that is most likely to either melt or become extremely soft, enough to not hold against high moment of inertia, or competing moments of inertia, one lead and one copper. Now the copper is an excellent thermal conductor for the same reason its an excellent electrical conductor, it freely releases its surface electrons which carry thermal energy. That's why the copper feels colder than the brass or steel. The high heat in the casing as it starts to push the bullet is instantly transferred through the copper into the lead. Unless the lead is an alloy I find it hard to believe it won't melt or at least become very soft at least at the tail end. I think what you're saying is the tail end might melt but that doesn't mean the whole core melts enough to spin free of the jacket. Sound's plausible. If it does spin free it must have a destabilizing effect if it syncs up again in flight.

 

[MagnumManiac]

I do not believe the core melts per se'.
I think it is more the fact that jackets are lubricated prior to the lead slug being inserted…the heat causes this to get even slipperier and the core is able to slip inside the jacket. Have sectioned many bullets, the core slides upwards feeding the mushroom as they expand, so cup and core bullets are free to move independently of each other.

Cheers.

 

[Professor Doolittle]

Perhaps if you could catch a bullet and slice it open if the core did melt the grain structure of the lead would be twisted. I don't think anything happens tho.

Good idea.

 

[DUSTY NOGGIN]

no way the lead melts

 

[QuietTexan]

I agree its the boundary between the copper and lead that is most likely to either melt or become extremely soft, enough to not hold against high moment of inertia, or competing moments of inertia, one lead and one copper.

The heat transfer coefficient of lead is approximately 10x that of copper, would that create a situation where the jacket is significantly hotter than the core in the very short time frame we're dealing with because the core can't absorb the heat any faster? Maybe the boundary layer is melted or softens enough to slip but still keeping to core moving within some high percentage of the jackets rotation. I was generally thinking longitudinally about the core and jacket down the bearing surface with essentially a cylindrical shear boundary around the entire circumference of the core, but your point about the base of the core heating differently than the nose would also come into play.

The core is probably something like a 90-94% lead, 5% tin, 1-4% antimony alloy. Jacket is probably 90-95% copper and 5-10% zinc.

Unless the lead is an alloy I find it hard to believe it won't melt or at least become very soft at least at the tail end. I think what you're saying is the tail end might melt but that doesn't mean the whole core melts enough to spin free of the jacket. Sound's plausible. If it does spin free it must have a destabilizing effect if it syncs up again in flight.

I think is what also Magnum is getting at. Something happens to the lead core, maybe even isolated to the back portion of the core, melting or softening or slipping or something, that allows the core to move independently from the jacket at impact as is proven by flow from the core moving forward as the jacket is decelerating upon impact.

Maybe the core/jacket boundary up along the ogive isn't heated as much by air friction as the base is by the powder charge, and while the base of the core could be melting/softening it's held in place until the meplat opens and allows the solid front of the core to keep moving forward as the jacket sheds?

 

[Grey-Bearded-Geezer]

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.

Now...that is an interesting question.
IF you are deeply concerned about the theoretical possibility of
lead core melting in flight...
Copper Solid Bullets do exist.