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Lead fouling on cleaning patch

 Thought Experiment on Bore Leading



 

There are many types of leading. Here, we are going to concentrate on the leading of the bore and perform a thought experiment that may make it more visually apparent what is going on.

Imagine a generic .357 Magnum barrel. In this case a single shot because we don't want to be concerned with the cylinder and gap issues. We will "select" a Thompson Contender 12 inch .357 Magnum barrel.

For the load we will use something completely generic, a 158 grains semi-wadcutter with a load of 12 gr. 2400 a velocity of around 1200fps ( 800 mph ) and a chamber pressure of about 38,000 psi.

Most .357 Magnums have bore diameters of about .357 inch.  In real life, they vary from about .355 to .358. My actual Thompson contender barrel slugs .3562 groove diameter with a bore diameter of .34995. Since we are doing a thought experiment, I think our test barrel will have a groove diameter of .3600 and a bore diameter of .3540. This will give us grooves with a depth of .003, the same depth as my Thompson Contender.

Now what happens when this thing is fired. It is really yard to imagine. This is where our thought experiment begins. Imagine we could magnify this thing 100 times. Make it large enough we could see it.

Our barrel would now have a groove diameter of 36 inches and a bore diameter of 35.4 inches. The grooves would be .30 deep, that is really shallow when you put it in perspective. It would be 100 feet long.

Our  158 grain SWC would have a diameter  of 36.1 Inches ( equivalent to .001 over grove diameter ), it would be 75 inches long and if we could weigh it, it would weigh approximately 22,000 pounds.

Well, I can't imagine really doing anything with that so how else could we describe it?

Imagine a 36 inch concrete storm drain. It is perfectly straight and 100 feet long. Now imagine a section from the trunk of a large pine tree. It is 36.1 inches in diameter. The diameter of the storm drain is 35.4 inches on the top of the lands. That means 0.70" of wood is going to have to be moved, removed, or rearranged.

Now imagine the flood is carrying the tree trunk toward the storm drain and it just happens to be perfectly aligned with the center line of the storm drain. When the trunk enters the storm drain, it is already moving at around 400 mph and is being pushed by a force of over 38,000,000 pounds. To put this into perspective, the first stage of the space shuttle produces a thrust of 1,225,704  pounds. As it passes through the drain, it will accelerate to around 800 miles per hour.

If you are still here, imagine that the drain is strong enough to hold all this force and mass this together and not shatter, crack, or expand. When the tree trunk enters the drain, approximately three fourths inch of wood is going to be displaced. I say displaced because I don't know where it is going.

You have seen what happens to a race car when it skids along a concrete barrier at only 150 miles per hour. Our event is going to be massively more violent than that.

That .70" of material that is going to be displaced, calculates out to about 5,900 cubic inches of wood. To be fair, only about a third of the length of a real bullet is bearing surface so let's reduce that to 2,000 cubic inches of wood to be displaced. That is about 35 quarts or over 8 gallons of "sawdust".

Where did it go? If you have ever missed a nail with a hammer and hit the board, you know that wood is easily compressed. It is reasonable to assume that most of the material was pressed into the surface of the log but at the same time, a significant amount was "sanded" off and captured by the pores in the walls of the concrete tube. At 800 miles per hour, the friction would be so great that I would expect some of it to vaporize, but wait, wood doesn't melt, you can grind it finer and finer but there is no oxygen in the space between the wood and the cement. This would argue that when it leaves the tube, a large amount of very hot wood dust would explode into flames the instant it comes into contact with the atmosphere.

Since wood is not elastic, once compressed, it stays compressed so the log exiting the drain would be slightly smaller in diameter than when it entered.

Now imagine a repeat of the same experiment except the tree trunk is only 35 inches in diameter. that means it has a clearance of 0.20" all the way around. It should pass through easily but we know it will not align exactly and pass through without touching, It will bounce back and forth off the walls at speeds approaching 800 miles per hour. Remember what happens when a race car bounces off the wall at only 150 miles per hour? This trim seems to me to be even more violent than the former example.

How can we apply this to the real bullet in the real barrel? Actually pretty well. The numbers are significantly smaller, for example the force on the base of the bullet is only 3,800 pounds. ( Since we are dealing with the area of the base of the bullet, it is not 100 times less but 100x100 times less.)

That is still plenty of force to deform the bullet but here things begin to change. Lead is easily deformed but it is not compressible. when the lands compress the lead back into the body of the bullet, the bullet has to change in some other dimension. What happens is that It grows slightly longer. You can check this for yourself with a sizing die and a good micrometer. The 158 gr .357 SWC will grow by about half a thousandths when sizing from .359 to .357.

There is hopefully a layer of lubricant between the lead and the steel surfaces. That would mean that the lead should not be scraped off the bullet by the steel barrel, right?  No. Take a piece of wet or dry silicon carbide abrasive paper. Imagine you are going to polish a piece of metal. What is the first thing you do? You apply a lubricant to the paper or to the metal to "carry away the cuttings".  The lubricant will help reduce lead shaving if it is there in adequate quantity but it will not totally prevent it. The key is the right lubricant in the right amount in the right place at the right time. Whew, no wonder it fails so often.

What are factors that could cause the bullet to leave lead in the bore.
( Remember, this is a single shot, we are not talking about revolver problems.)

  1. Insufficient lubricant
  2. Inadequate lubricant
  3. Hard lubricant that does not flow
  4. Soft lubricant that gets "blown away"
  5. Bullets too small that "rattle down the barrel" bouncing off the sharp edges of the lands
  6. Bullets too small that allow gas to leak by.
  7. Bullets with irregular bases that allow powder gases to leak past the base up on the side of the bullet.
  8. Rough bore that scrubs lead off the side of the bullet.
  9. Rough spots or dings at the muzzle.
  10. Rings, constrictions where s part of the bore is smaller than the rest.
  11. Powder that is too fast for the application
  12. Loads so hot the bullet structure is compromised.
  13. Failure to obturate ( deform the base to seal the gases in.
  14. Bullet metal too hard to obturate.
  15. Powder charge too light to obturate.

 

Cover lube star and lead star here.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Photo instructions:  Expand images with the control key plus the mouse wheel or maximize them by click on them. They will open in a new window at the maximum resolution. Control + Wheel Forward zooms in. Control + Wheel back zooms out.

 

 

This close-up shows the mouth of the cartridge case crimped into the crimping groove, cannelure, of the bullet. The question is how much is enough and how much is too much. Notice the crimp on the outside of the cartridge mouth. It looks like the mouth of the case was squeezed in or compressed into the bullet.

 Jacketed Bullet Crimp
Left: Dog Town 55gr .224 Spitzer. ( Midway USA house brand )
Right: Remington factory loaded .223 55 gr soft point removed from the featured cartridge.
.223 bullet cannelure groove 
Here is a closer look. A groove was cut so you could see exactly what is going on in that bullet cannelure groove.    
 Here is a close-up of the inside of the mouth of the cartridge case. That crimp ring is really small.  
An extreme close-up of the inside of the case mouth showing the crimp ring after the cutting tool marks have been polished away. This view shows a decisive thickening of the case neck into the area where the bullet cannelure would be when the bullet is in the case.   
   
   
   
 

Chapter 2 ( Military 5.56x45mm NATO TW 73)

 
Case weight 94.2gr
Bullet Wt 55.1gr
Bullet style fmjbt
Powder 26.5 gr ball
Bullet:
Diameter shank .2221 - .2227
Cannelure .2216 - .2222
Nose .2223 - .2227
 5.56x45mm bullet

Here is a close-up of the crimp. The most noticeable thing about this crimp is that it is virtually invisible. In fact, it appears to be, and actually is, raised instead of crimped inwards.

The crimp is very effective. It took over 50 blows with an Inertia type bullet puller to get it out. 60 hard blows to move the bullet far enough that the crimp ring was outside the mouth. Then about ten more lighter blows to completely expel it.

 
 Bullet Crimp

Here is a close-up view of the cannelure. It looks identical to the commercial version. It is compressed into the jacket material. It is rounded, not square. I am sure that is intentional because metal tends to break at square corners.

 

The bullet appears to have a bulge at the base. I am convinced this is an optical illusion.
I cannot feel it, I cannot measure it, and I cannot photograph it.

 

 Jacketed Bullet Cannelure
 

Here is the case mouth crimp with the bullet removed. The surprising thing is that it is very small. I don't know how it held on to that bullet as tightly as it did. The crimp appears to consist of a series of very small beads extending both into and out of the case mouth.

It appears they were created by compressing the mouth metal between the beads with some sort of crimping tool that works along the axis of the cartridge case instead of squeezing the mouth of the case inward.

 Rifle Cartridge Case Mouth
Here is the cutaway view. You have to look close and expand the image to see it but the case mouth does project into the cannelure.   5.56x445mm crimp
Here is a close-up of the case wall thickness at the mouth of the case.  I can see no hint of a real crimp here but there is a tiny protrusion into the cannelure groove - See arrow.  Compare this to the Commercial version directly below.  Cartridge Case Wall   
An extreme close-up of the inside of the case mouth showing the crimp ring after the cutting tool marks have been polished away. This view shows a decisive thickening of the case neck into the area where the bullet cannelure would be when the bullet is in the case.  This is the close up of the commercial .223 in chapter one above. It is reproduced here so you can compare the two crimps more easily.  
   
   
Can you rely on the cannelure ring to set your overall cartridge length?

You be the judge. 
.223 cannelure groves  
 


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
 

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