Cast Bullet Blow Up Or Bulge

[joeb33050]

CAST BULLET BLOW UP OR BULGE
Do cast bullets "blow up" because they are spinning/rotating due to the rifling? Do cast bullets change shape or "bulge" because of the spinning?

Calculations and explanations can be found on the book site, http://sports.groups.yahoo.com/group/CB-BOOK/
in "FILES", in "ERRATA". There are two EXCEL workbooks called "CAST BULLET BLOW UP" and "BARREL HEAT" dealing with this project..

Conclusions
Cast bullets can and rarely do blow up at very high RPMs/velocity.
Cast bullets change shape, bulge, at high RPMs/velocity.
Cast bullets are hot as they exit the muzzle, and may/do get hotter going through the air, for some distance-this varies with the load.
Blow up or bulging of cast bullets occurs in a certain RPM area, and is independent of velocity or twist rate and varies slightly with caliber. Plotting the curves of "Centripetal Force = Tensile Strength", and working with the arithmetic shows this to be true. For instance, if a certain alloy bullet will blow up at 400 degrees F, then:
.224 bullets blow up at 126,400 RPM
.308 bullets blow up at 114,320 RPM
.375 bullets blow up at 106,400 RPM
.457 bullets blow up at 98,360 RPM

This range, from 98,360 to 126,400, is relatively slight. See the chart "C.F. VS. T.S. (RPM)" on the " CAST BULLET BLOW UP" workbook.
This fact, that cast bullet blow up or bulge is primarily related to RPM, was far from obvious or expected in the beginning-but is obvious now.
I don't know if this has anything to do with Larry Gibson's "RPM Threshold" theory, but suspect that it does.

The Model
I start with a cylinder of a certain alloy, diameter = caliber and length also = 1 caliber.
(For example, for a .308" caliber the cylinder diameter would be .308" and cylinder length would also be .308".)
This cylinder may be considered to consist of a tube with .025" wall, diameter = caliber, length = 1 caliber, and a small cylinder with diameter of caliber-.050" and length of 1 caliber.
(For the .308" example, the tube O.D. = .308", I.D. = .258", length = .308"; the small cylinder O.D. of .258", length 308".)
We're interested in figuring out when the tube would "blow up" and leave the small cylinder.
Centripetal force = CF = mv^2/r where CF is in pounds, m = mass in slugs = pounds weight/g with g = 32.15, v = fps, and r = the radius of the small cylinder.
(In the .308" example, r = .258"/2 = .129".)
CF is easily calculated for any caliber/cylinder diameter, tube dimensions, tube length velocity and twist.
Lead has a tensile strength of ~ 1920 pounds per square inch. (BHN 4 * 480 = 1920 pounds per square inch tensile strength)
I think that when the CF = the tensile strength of the inside area of the tube, that the bullet will blow up

Bullet Heat

Cast bullet blow up has to do with the temperature of the bullet. As the temperature increases the tensile strength decreases, and the velocity/rpm at which centripetal force equals tensile strength decreases-making blow up more possible/likely.
I found one site talking about measuring the temperature of a bullet as it left the muzzle;
http://www.rangerats.org/bullet.html
where they found the temperature of a 5.56mm NATO bullet leaving the muzzle was 267 degrees C = 513 degrees F.
The author mentions that "The bullets cooled as they traveled away from the gun."

"Garandsrus" on CAST BOOLITS was kind enough to send me a copy of the article "Comet Tails" by Gardner Johnson, from the April 1998 PRECISION SHOOTING. The article includes a table of Tensile Strength of lead in temperature.
I used this table to construct a graph of Tensile Strength(lb/in^2) vs. Temperature (Degrees F) for lead, WW and Lino.

A cast bullet is heated up by:
the powder gas as it travels through the barrel (I don't know how to estimate this.)
the friction of the bullet rubbing on the barrel
the air friction after the bullet leaves the muzzle

Barrel/bullet friction
How much does friction heat up the bullet and barrel?
I tapped a 31141 bullet into the bore of my 30/30 bench gun, then tapped and pushed it back and forth in the barrel. It never loosened up; it always had a lot of stiction = static friction. I had planned to use a scale on the rod to get a notion of the force required to move the bullet, but that didn't work. A better setup is required for good readings.
So I did the arithmetic, and made a spreadsheet that would answer the question of how much the barrel and bullet might heat up at various amounts of force.
If it takes 20 pounds of force to move a 170 grain bullet through the 2' barrel, there's 40 foot pounds of work done. (20 pounds is in the ball park, it ain't 2 pounds and it ain't 100 pounds.) This work turns into heat that goes into the bullet and the barrel. Let's say the barrel weighs 6 pounds.
If all the heat goes into the bullet, (and it doesn't), then this work raises the bullet temperature 1221 degrees C.
If all the heat goes into the barrel, (and it doesn't), then this work raises the barrel temperature 1.28 degrees C.
Somewhere between these two is the point of equal increased temperature of both the barrel and the bullet. This point is where the barrel and the bullet both increase temperature by 1.275 degrees C.
It seems to me that the tendency would be toward equal temperature increases; toward the 1.275 degree C increase for both bullet and barrel.
All if I did the arithmetic correctly.
The workbook is up on the book site; I'm hoping that somebody who knows about this stuff will review my arithmetic and point out the mistakes.

Bullet/air friction
Energy loss

As a bullet loses velocity and energy as it goes through the air; the energy has to go to heating up the bullet and moving (and heating) the air. We know how much energy the bullet has, and loses. How much energy does it take to heat up a bullet?
The Lyman Cast Bullet Handbook, 3rd edition, tells us that the 311291 at 2500 fps loses 377 foot-pounds of energy in the first 50 yards of travel. The same bullet starting at 1400 fps loses 107 foot-pounds of energy in the first 50 yards.
If the calculations are correct, it takes 12.8 foot-pounds of energy to raise the temperature of a 170 grain lead bullet 500 degrees Fahrenheit.
The bullet loses 377(mv = 2500 fps) or 107(mv = 1400 fps) foot pounds of energy in the first 50 yards of travel, and it would take 12.8 foot pounds of energy to raise the temperature 500 degrees Fahrenheit.
Doesn't mean that the temperture does go up, just that the energy loss is there, sufficient and available.

A skin temperature estimating formula

The article "Comet Tails" by Gardner Johnson is from the April 1998 PRECISION SHOOTING.
While I have some problems with the article, Johnson explains that there is a formula for predicting skin temperature of high speed aircraft.
Fiddling with the table of temperatures and velocities in the article, I find the formula to be: Delta T =:Temperature rise above ambient (Degrees F) = V^2(fps)/13310. Means that skin temperature equals velocity squared, divided by 13,310.
Increases to T are:
V(fps) Delta T
500 19
1000 75
1500 169
2000 300
2500 469

In the example of lead/1.5% antimony with a melting point of 600 degrees F, and an ambient temperature of 75 degrees F, the skin of a lead alloy bullet would become liquid at 2646 fps.
The calculated skin temperature falls as bullet speed falls; and that's pretty fast. For example, a .202 BC bullet with 2700 fps mv has skin temperature of 548 degrees F above ambient at 50 yards; and 179 degrees F above ambient at 300 yards when v has fallen to 1545 fps.
It seems unlikely that the center of the bullet reaches anything like the skin temperature.

[44man]

Math always eludes me, never that good with it because I have trouble remembering phone numbers and names, let alone formulas I never used in 70 years.
But this is very interesting and enlightening, good job! :drinks:
Too bad the experiment of measuring heat can't be carried out further by using actual lead boolits and a variety of lubes to see what effect the lube has on final boolit heat.
I have often wondered if heat prevention from friction by using certain lubes is what allows some to shoot cast faster then others.
The next question is what happens when a lube such as Alox burns in the bore and smokes at the muzzle? Does the creation of smoke cool the boolit or does the fouling layed down creat more friction with the next shots.
Boy, what a pickle we get ourselves into trying to shoot bare lead.
On top of that, all the boolits we put into the air contributes to global warming! :mrgreen:

[Bass Ackward]

Joe,

Very good.

There are a whole bunch of conclusions we can draw from this.

I find that peak accuracy and final velocity or RPM levels depend on friction. And there are many variables there as well. And it also alters it differently for every different barrel too. Each barrel (bore condition) reacts differently from enlarging bore diameter to actually lengthening and even curving the bore if the steel has stress in it.

If nothing else, one might be able to say that the barrel would alter friction as heat increases. Friction alters vibration, so as temperature increases, barrel vibration pattern also changes. Explains why I do better with heavy bedding pressure to control vibration for cast. Which is the exact opposite for jacketed. Free floated for cast and you experience RPMs at WAY LOWER levels as there is nothing to help control the change in the oscillation pattern.

This is why I can get better accuracy up at higher RPMs as long as I fire slowly enough NOT to heat my barrel. Which is easier for me since I am interested in hunting or 5 shots groups only. Someone stuck to a timed event or a higher round content invariably has a lower RPM limit or the need for a harder bullet.

So in a way you answer your own question about why guys don't shoot 10 or 20 shot groups for statistics, unless they have the patience of Job. Each shot alters temperature and the friction pattern each slug sees so the barrel throws it in a different direction as it exits.

As bore diameter increased, RPM levels lowered. Might just explain why rifling height is critical for high velocity lead. So what you found might be worded to say, "The RPM ceiling for each bore diameter is really a function of the percentage of bullet diameter controlled by the rifling height needed for the required twist rate to stabilize it.

It also says to me that bullet damage or more importantly bullet change is actually occurring in the bore if changing conditions affects RPM level.

And it also basically supports my argument that if you look solely at RPM levels you are ignoring those variables that do allow you to go higher. RPM numbers really provide a statistical failure level for that load combination that trivializes many important variables that do make a difference.

So at the end, the lube function question we need to ask, friction? Or seal?

[Junior1942]

I'm only certain about what I see with my own eyes. At circa 120,000 RPMs, my cast bullet groups become patterns, and the bullet holes--the ones on target, that is--have commas.

[45 2.1]

What a fella does determines a lot more than what he thinks is going to happen. There is absolutely no substitue for trying it yourself. Your choices and methodology determine what results your going to get. Correct alloy, lube and load balancing determine the differences we get. I get a lot higher velocity and better results due to the choices i've made. What you get is determined by the choices you've made. Several people have gotten what i've done by making differnet choices than the regular "i'm doing what everybody else is".

[looseprojectile]

In my experience with high pressure hydraulics and air I have always wondered how a rifle barrel can withstand 20 to 60 thousand pounds pressure. Normally what you have to work with in hydraulics is limited to a couple or three thousand pounds of pressure in aircraft actuating systems. Three thousand pounds is a lot for air cylinders.
Think of air over oil recoil systems in naval guns. These systems build up heat fast because of friction and compression. Now we come to the piston and cylinder of the gun. Can't have too much lube or you can ring the bore. Can't have too little lube or you get leading. I have always seen the solution as being somewhere in the middle. Pretty harsh enviroment.

[runfiverun]

i believe lube is for friction, your boolit should be sealing the bore.

it is also notable that as the area of the boolit enlarges the level of rpms lowers.
more of a percentage of the boolit exposed toheat? combined with a larger rotatng mass.
and the larger centripedal force? should equal an easier failure. especially if it absorbs more heat [ can hold more as the mass is larger]

a boolit with a harder exterior should hold up better to the forces exerted.

[Billwnr]

I'm only certain about what I see with my own eyes. At circa 120,000 RPMs, my cast bullet groups become patterns, and the bullet holes--the ones on target, that is--have commas.

Junior, what combo are you shooting?

I calculated out what 120,000 rpm is and it is a bullet at 2000 fps shot out of a 12 inch twist barrel. The benchrest cba (heavy class) shooters regularly hit that milestone and the better shooters average around half inch groups at 100 yards.

I put a new barrel on my rifle last month and did load development last week with a chronograph. I found my lower accuracy node at 1900 fps and ran out of bullets before I found the upper node. My barrel has a 12" twist and I was considering using the 11 inch twist.

My bullet is a 311679 weighing 197 grains and .309 diameter.

[pdawg_shooter]

So if I wrap my bullets in a paper patch to insulate it from heat caused by friction, than I can stand way more RPM than a non patched bullet. Seams to me as easy and cheap it is to paper patch that is the only way to go.

[Tiger]

Joeb

Lots of work but I think rpm blow up figures are off. I know this because myself and friends have shot even soft alloy at crazy high velocity and rpm from very fast twist .224 caliber barrels and not blow bullet up.

Junior I think maybe your rifle barrel and target not exactly in 90 degree plane to one another. I too have holes with coma when my target is leaning.

The gascheck in my mind is to stop gas cutting. The other qualities are by product such scrap of the fouling or lead from bore.

Yes a paper patch bullet will shoot higher velocities and maybe easy way for unaccomplished cast shooter to get higher velocity with cast bullets.

I and friends have gotten high velocity and rpm to from different rifling type and height. I see some deep rifling shoot as good as rifling not so deep. I think here choice of bullet very important along with alloy mixture.

Soon I will be doing shooting with 7x57 Mauser and new 6.5 Lapua. I need find way to post picture when I get them.

Ralf

[joeb33050]

So if I wrap my bullets in a paper patch to insulate it from heat caused by friction, than I can stand way more RPM than a non patched bullet. Seams to me as easy and cheap it is to paper patch that is the only way to go.

I think first that there is little heat added to the bullet by bullet/barrel friction. The workbook supports this opinion.
I think secondly that paper patching was the original solution to swaged bullet leading. Swaging was less costly than casting, swaging =ed soft bullets and no grease grooves. Leading was stopped by paper patching. A by-product was that grease groove swaged bullets have higher BCs than cast grease groove bullets, another that swaged bullets didn't have the ?holes? that cast bullets may have.
Some BP and LR shooters use swaged paper patched bullets, and it works and makes sense to them.
Nobody has used paper patched bullets in serious CB competition in my memory. For paper shooting, I consider paper patching a solution in search of a problem. Hunting? Maybe.
joe b.

[Bass Ackward]

I think first that there is little heat added to the bullet by bullet/barrel friction. The workbook supports this opinion.joe b.


Joe,

One of your conclusions: <<Cast bullets are hot as they exit the muzzle, and may/do get hotter going through the air, for some distance-this varies with the load.>>

We know that lead has a very poor heat transfer rate. That's why it leads. And also why it takes so long to heat treat in an oven. So if it exits the barrel hot, then it had to be heated by something other than the gas it was leaving behind. That almost HAS to come from friction or passing in the warm barrel. But warm ain't hot, so we are back to friction.

So which is it? Hot or not?

[joeb33050]

Joe,

One of your conclusions: <<Cast bullets are hot as they exit the muzzle, and may/do get hotter going through the air, for some distance-this varies with the load.>>

We know that lead has a very poor heat transfer rate. That's why it leads. And also why it takes so long to heat treat in an oven. So if it exits the barrel hot, then it had to be heated by something other than the gas it was leaving behind. That almost HAS to come from friction or passing in the warm barrel. But warm ain't hot, so we are back to friction.

So which is it? Hot or not?

I don't know what lead's heat transfer rate is.
I don't know that "that's why it leads.".
Lead has a low specific heat, the amount of heat required to raise a mass of mkaterial by a degree. Lead s.h. = .031, carbon steel = .12 both kcal/kg degree C. Means it takes LITTLE heat to raise lead temp.
I believe that it's the powder gas heating the bullet.
The WORK done in pushing a room temp bullet through a room temp barrel is not enough to heat the bullet much at all. You can test this. Rifle in vise, start the bullet in the rifling, push it through with a long rod, measure FORCE with a scale, bathroom? after the bullet gets going. I have no vice, please try this.
The air/bullet heats the bullet after the muzzle, I think, because all anecdotes tell of blow up "down range", not at the muzzle.
The friction in the bbl just ain't enough.
If I did the arithmetic correctly.
joe b.

[beagle]

Bullets are "warm" coming from the muzzle of a gun but not "hot". This heat dissipates as they go down range. I believe this is a combination of heat from the initial powder ignition, propellent gasses blown by the bullet and some from friction as it goes down the barrel. Based on the short time the bullet is exposed to these forces, it can't get very hot.

I've watched 30mm projectiles on the Apache through the infrared sight at high magnification and they are initially fairly "warm" and easy to pick up in flight. The "trace" dims as it gets closer to impact and eventually fades just before impact and these are not tracer rounds....just heat. Also the heat signature is nowhere as bright as the charcoal brickets we used as aiming reference points or old tank hulks that have been sitting in the sun all day and retain heat.

Now, this is considering a steel projectile with copper driving band which is started way hotter than our cast loads and the friction produced in the barrel is way more than with lubed lead. We're also considering a very slow burning ball propellant here that is probably burning all the way up the barrel.

I've also watched mini-guns fire under NVGs from the side and slightly behind and you can see individual bullet paths and can clearly pick out the tracer rounds as they have a subdued burn before igniting and will show up hotter than regular ball rounds which produce a very dim trace.

Based on these two experiences, I don't beleive bullets get that hot. They're probably at their hottest at the muzzle and quickly lose heat as they go down range...especially at our cast velocities.

Just my experiences and opinion./beagle

[Tiger]

Joeb

I arrive at conclusion you don't know much about friction. All I can say is your are wrong about the powder burning is the major source of bullet heating. Come on Joe look to wax bullets. Does the powder melt them? Look to paper patch bullet does the paper burn or is it even charred? To both questions No. For one there is not enough time for powder burning to act upon the bullet. Why is barrel more hot towards muzzle? I think one reason is bullet is going very fast by the time it get to that end...more friction. In end I do not think bullets are coming out of barrel hot enough to do damage to them.

Ralf

[joeb33050]

Beagle;
The only cite I've found for a measurement of bullet heat is in the first post, where the author found the 5.56 Nato bullet to be at 513 degrees F at the muzzle. The only cite. I'm looking for others, and welcome the reference. Until then I'll stick with the sources and believe that the bullets are hot at the muzzle, and get hotter if v is great enough.
My question is"Can/do cast bullets blow up or bulge in flight?" My answer is "Yes, but at quite high RPMs/velocities, higher than common."
joe b.






Bullets are "warm" coming from the muzzle of a gun but not "hot". This heat dissipates as they go down range. I believe this is a combination of heat from the initial powder ignition, propellent gasses blown by the bullet and some from friction as it goes down the barrel. Based on the short time the bullet is exposed to these forces, it can't get very hot.

I've watched 30mm projectiles on the Apache through the infrared sight at high magnification and they are initially fairly "warm" and easy to pick up in flight. The "trace" dims as it gets closer to impact and eventually fades just before impact and these are not tracer rounds....just heat. Also the heat signature is nowhere as bright as the charcoal brickets we used as aiming reference points or old tank hulks that have been sitting in the sun all day and retain heat.

Now, this is considering a steel projectile with copper driving band which is started way hotter than our cast loads and the friction produced in the barrel is way more than with lubed lead. We're also considering a very slow burning ball propellant here that is probably burning all the way up the barrel.

I've also watched mini-guns fire under NVGs from the side and slightly behind and you can see individual bullet paths and can clearly pick out the tracer rounds as they have a subdued burn before igniting and will show up hotter than regular ball rounds which produce a very dim trace.

Based on these two experiences, I don't beleive bullets get that hot. They're probably at their hottest at the muzzle and quickly lose heat as they go down range...especially at our cast velocities.

Just my experiences and opinion./beagle

[Tiger]

Joeb

I see you wish to ignore me. Maybe I challenge you too much hey. Anyways here are too ideas for you think about:

And heating need not be evenly distributed. Consider a bullet. A pistol bullet flying below the speed of sound pushes air out of its way and encounters resistance, which heats it up. A rifle bullet is supersonic, and in addition also compresses air in its front, creating much greater heating. But the heating is not evenly distributed, the tip gets much hotter than the rest (in either case). It is not just "what speed", but what part of the object, too.

QUESTION:
I'd like to pass on this student's question from my 8th grade Science class. We had been talking about how meteors heat up from friction while traveling through the earth's atomosphere. The question was : "Do bullets heat up while traveling through air?"

ANSWER:
By a bullet I presume you mean a rifle bullet. Here the exploding gunpower and rapid acceleration down the barrel cause the bullet to emerge already very hot. When it becomes a free projectile the bullet will encounter air fraction which will "heat it" but, since it is already hot, it will also be radiating away energy, that is it will also be cooling. I suspect that the cooling from radiation will be greater than heating from friction and the bullet will cool down as it travels. If, however, you have something like a BB gun which has no explosion it seems possible that heating from the air friction would prevail; however, the temperature increase would probably be immeasurably small.


Notice Joe in second statement where it say ....rapid acceleration down the barrel. This is friction heat Joe.

By way second statement come from physicist website where many physicist try answer many complex questions.

Ralf