OK,
I’m going to ask a stupid question. What does bullet lube do? I’ll bet most of you answered that bullet lube lubricates the passage of the
bullet down a rifled bore, to eliminate galling of a soft metal as it traverses
a hard metal cutting edge. Well, yeah, I suppose that’s true enough, but that’s
not all it does, nor is it necessarily even the most important job that it does.
Let’s assume for the moment that lubrication is the sum total of its job -- is
the lube on a given bullet lubricating the passage of the bullet that carries
it, or the bullet that follows after it? Another way that I’ve had this question
posed to me was, should the lube groove (s) be on the front of the bullet (where
they could lube the passage of that bullet), or towards the rear of the bullet
(where they could leave a healthy lube film for the next bullet in line)?
Part of the problem with this line of reasoning is that it assumes that the lube
is delivered to the bore by simple bullet/barrel contact and smearing, and hence
the lube can only lube that which is behind the reservoir (lube groove). Looking
at things in this manner results in a fairly simplistic, almost static picture
(hard surface, soft surface, slippery stuff in between), and the firing of a
revolver shot is a very dynamic process. What else does bullet lube do?
Or perhaps more accurately, what else is done to the bullet lube?
Let’s just set the record straight, lube is not simply smeared from the lube
grooves onto the bore, nor is lubrication the sole function of bullet lube.
There were a couple of excellent articles published a few years back in The Cast
Bullet on lube pumping mechanisms. In a nutshell, the conclusions were that
bullet lube was pumped to the bore surface by 3 different mechanisms --
compression, linear acceleration and radial acceleration. In compression, the
force applied to the base of the bullet causes the compression of the bullet’s
core underneath the lube groove, resulting in expansion of the core diameter and
shrinkage of the lube groove width. Both of these factors results in the
reduction of the volume of the lube groove itself, and hence compress the lube
and force it to the bullet/barrel interface. There is solid physical evidence
supporting this mechanism (especially in rifles). The linear acceleration
mechanism is pretty straightforward -- the inertia of the lube at rest causes it
to be forced towards the rear of the lube groove as the bullet is accelerated
forward by the burning powder. When the lube encounters the beveled (or radiused)
rear face of the lube groove, it is once again forced to the barrel surface. In
the third lube pumping mechanism, radial acceleration, as the bullet begins to
spin faster and faster as it progresses down the barrel, at some point
sufficient radial acceleration (think "centrifugal force") is generated to
overcome the viscosity of the lube and it gets flung off of the lube groove
surface and outward onto the barrel. All three of these mechanisms come into
play when any cast bullet is fired, although the magnitude of each will vary
significantly with the application (e.g. .38 target wadcutter vs. .30-06 or
.45-70 hunting load), and will be dependant on velocity, pressure, alloy
hardness, bullet diameter, etc. Indeed, the magnitude of each will vary for any
given shot, depending on where the bullet is in the barrel -- linear
acceleration will be dominant early in the shot, compression will take over as
pressure peaks and radial acceleration will become more significant as the
velocity increases.
Delineation of these mechanisms provides a significant level of understanding in
terms of cast bullet shooting and design, as well as bullet lube formulation.
However, these mechanisms still have the bullet serving as nothing more than a
brute-force paintbrush, slapping on a fresh coat of grease of the bore for the
next bullet in line. This is all well and good, but it is an incomplete description
of the process. I believe that there is another mechanism operating, one that
accentuates a second and perhaps even more important role that bullet lube
serves.
Back in the 50s and 60s, some very knowledgeable Handloader's performed extensive
tests to understand what made the best bullet lube and why. One of the more
notable efforts in this area was the work done by E. H. Harrison of the NRA
Technical Staff. These results were originally published in the American
Rifleman, and were subsequently reprinted in "Cast Bullets" by E. H. Harrison,
and available through the NRA (buy this book if you don’t already have it!). The
most important property of the lube formulation was found not to be the inherent
lubricity of the mix, but rather its flow properties (we will return to this
shortly). It is interesting to note that Mr. Harrison was singing the praises of
moly loaded bullet lubes back in the 1950s. It seems "the wheel" has been
rediscovered…
Why
are flow properties important? Most barrel tolerances are generally good to less
than .001", where can the lube flow to? As the bullet undergoes the
violence of being engraved by force, if there is any slippage or
variation in groove/land width, this will result in there being a gap between
the trailing edge of the land and the groove engraved in the bullet’s face. Gas
molecules are very, very small things, and at the temperatures and pressures of
burning powder they‘re buzzing like an angry swarm of hornets.
Even a gap between the trailing edge of the
land and the engraved groove of the bullet of only .001" will leave enough
room for over 50,000 of these gas molecules to line up "shoulder to
shoulder" and still not bump into the outer boundaries of the gap.
The point of bringing all this up is to show how easy gas leakage is through
this sort of defect channel, even though at first glance it appears to be quite
small. In addition, there are similar (somewhat smaller) channels on the grooves
and lands, left over from the machining processes that gave rise to the rifling,
and these defects also contribute to potential gas leakage. Gas pressure rises
much faster than the bullet is accelerated, so therefore as the bullet’s
surface is ravaged by the lands and gas leaks past the base band, the lube
reservoir becomes pressurized, with the gases entering from the rear and pushing
forward. This rapid pressurization forces the lube to flow into the defect
channels in the engraved driving band in front of the lube groove, sealing off
the gas flow and limiting the damage due to gas cutting. If the cast bullet is
appropriately sized, then this controlled injection forms a floating pool of
lubricant that follows the bullet down the barrel, lubricating the bullet/barrel
interface and sealing the high-pressure gases. Kind of a ballistic stop-leak, if
you will.
This is why some of the new hard lubes perform their best at higher pressures.
Gas leakage into the lube groove melts the lube, and the liquid lube then gets
forced into the microscopic defect channels ahead of the groove. Some of the
commercial hard lubes work just fine at 800 fps and 1300 fps, but at
intermediate velocities or say 1000 fps, they lose some of their shine. At the
lower velocities/pressures there are few demands placed on the lube, and these
can be addressed by simple frictional smearing of the lube displaced from the
lube groove by the land. As the pressures/velocities rise into the intermediate
range (+P level, 20,000 psi, 1000 fps) however, the mechanisms outlined above
can’t pump the hard lube to the bullet/barrel interface fast enough to keep up
with the lubrication/sealing demands of the system, resulting in leading and
poor accuracy. As pressures/velocities climb into the magnum level (35,000 psi,
1300+ fps), enough hot gases are injected into the lube groove to melt some or
all of the hard lube, allowing all of the lube pumping mechanisms outlined above
to come into play, resulting in effective lubrication. These high-pressure gases
also cause the molten hard lube to be injected into the defect channels in the
forward driving bands, thereby sealing off gas cutting. Lube pumping and
high-pressure injection cannot take place efficiently until a hard lube melts.
For a soft lube, it’s not necessary to melt the lube for this injection to
happen, the soft lubes are capable of flowing from the start, which is why they
lubricate cast bullet revolver loads effectively across the entire range of
velocities from 600-1500 fps. The commercial hard lubes are well-suited for
magnum revolver and rifle cast bullet velocities.
Undersized cast bullets leave a gap between the bullet and barrel, leaving them
unable to restrict this pressure-induced lube flow. As a result, the lube very
quickly gets blown out of the barrel in front of bullet, leaving the bullet
"naked", un-lubricated and unprotected. This phenomenon is especially problematic
with the hard lubes; once molten, the low viscosity liquid lube gets blown out
rapidly if the bullet is undersized.
Concerning the flow properties vs. lubricity issues cited above, E. H. Harrison
explored the use of molybdenum disulfide (aka "moly") as a bullet lube back in
the 1950s. He found that dry moly was inadequate as a bullet lubricant for
.30-06 loads at 2000 fps, but that when it was incorporated into a more
traditional grease/wax lube formulation, that it worked quite nicely indeed. By
incorporating moly into a soft lube, the desirable flow properties of the lube
are maintained, as is the ability to leave behind a moly coating on the barrel.
This moly coating serves to protect the bore from oxidation, in addition to
serving as a lubricant, preventing adhesion of leading deposits. More recently,
a lot of work has been done looking at hard-cast bullets dry coated with moly,
and this has been found to work nicely for routine handgun velocities in the
800-1000 fps range. These observations reinforce the conclusion that simple
lubrication is sufficient at lower velocities, but as pressures and velocities
climb, the role of bullet lube is also that of a fluid gasket to seal the
bullet/barrel interface.
In
summary, bullet lube is pumped from the lube groove to the barrel surface by
compression, linear acceleration and radial acceleration. In addition, lube is
injected forward during the firing process, as the result of high-pressure gas
leakage into the lube groove. This injection process forms a floating fluid
gasket around the bullet, and serves to limit gas cutting and is a kind of
ballistic stop-leak. Hard lubes must first melt before they can be pumped or
injected by any of these mechanisms. By incorporating moly into the mix, the lube
delivered to the barrel surface can serve to prevent adhesion of future leading
deposits by passivating the steel surface.
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