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
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
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.)
- Insufficient lubricant
- Inadequate lubricant
- Hard lubricant that does not flow
- Soft lubricant that gets "blown away"
- Bullets too small that "rattle down the barrel"
bouncing off the sharp edges of the lands
- Bullets too small that allow gas to leak by.
- Bullets with irregular bases that allow powder gases
to leak past the base up on the side of the bullet.
- Rough bore that scrubs lead off the side of the
- Rough spots or dings at the muzzle.
- Rings, constrictions where s part of the bore is
smaller than the rest.
- Powder that is too fast for the application
- Loads so hot the bullet structure is
- Failure to obturate ( deform the base to seal
the gases in.
- Bullet metal too hard to obturate.
- Powder charge too light to obturate.
Cover lube star and lead star here.
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