|
Preface --
This is the third of a series of articles in which we are tapping
the vast resources of Boots Obermeyer's mind as well as his
experiences in order to bring numerous "gems of information" to
the attention of the readers of The Accurate Rifle. The first article
dealt with the muzzle crown -- an important component for an
accurate rifle, while the second article dealt with 5R rifling
-- a rifling that Obermeyer made popular with the military and
civilian shooter (1, 2)
In the two previous articles, I indicated
that Boots was willing to answer questions of general interest
as well as to consider suggested topics that could be covered
in future articles. Topics and questions should be emailed to
me at the cheyenne@cheyennetactical.com address. The previous
two articles generated some very good questions and suggested
topics. Unfortunately, we are unable to address all suggestions,
but for this article, Boots selected a topic that was provided
by several readers: Barrel Life.
Our last article that appeared in The Accurate
Rifle dealt with 5R rifling, a type of rifling associated
with Boots Obermeyer. Since the article's release, several
readers brought to my attention a report by Virginia Hart Ezell
published in the National Defense magazine (3). The
US Navy SEALs recently purchased 300 Knight SR25 sniper rifles
with Obermeyer's 5R barrels. The SEAL's new version of the
model SR25 called the Mk 11 Mod 0 is slightly different than
what Knight has produced during the past. The previous 24-inch
barrel is now a 20-inch
Obermeyer barrel -- a better length for combat
conditions according to the SEALs. In order to maintain Knight's
guaranteed 1 MOA accuracy with this shorter barrel, additional
changes were made; i.e., changes in the firing pin, ejector,
extractor and buffer.
Today, rifle accuracy is not solely dependent
upon the quality of a rifle barrel, but in the past it was the
primary factor. Several generations ago competition shooters placed
such enormous faith in rifle barrel makers of the day; e.g.,
Pope, Scholck, Schoyen etc., for their extremely accurate
barrels that the names of these men are still known today. From
that period to the present, rifle barrel accuracy continues to
rank as the utmost importance for the target rifle used by competition
shooters, while rifles used by the military and hunters can accomplish
their tasks effectively without such a high degree of accuracy.
Prominent exceptions are rifles used for sniping and varmint shooting.
The success of a rifle barrel's ability to place
one projectile after another consistently in the same spot is dependent
upon several factors; e.g., the configuration and composition
of a projectile mated to a quality cartridge case, the type of
bedding of an action to the stock, etc. Assuming a rifle
possesses a highly accurate barrel attached to a high quality action,
is properly bedded, and is firing cartridges designed for accuracy,
it should be extremely accurate providing the shooter does his
part. Exceptions to this statement exist and are feared by individuals
who spend large sums of money to construct what they hope will
be an extremely accurate rifle. Fortunately, these exceptions are
not common and usually are due to one or more factors unknown to
the rifle maker and shooter. In some cases, time and money is spent
to uncover and
correct this factor, while in other cases the
rifle is trashed or sold to an individual requiring less accuracy.
A second rifle is then constructed with extreme care and trepidation.
The adage that a good thing does not last forever
is unfortunately true of superior accuracy from what one might
consider to be a perfect rifle barrel. For those shooters who record
shots fired, accuracy may diminish around 2500 rounds, while in
other cases it may diminish around 4000 rounds. Odds are that the
diminished accuracy is related to a change in the rifle barrel
-- sometimes referred as the barrel's life is over. The barrel's
life is not really over as it can still stabilize a projectile
to hit the target; however, its life is over for the competition
shooter because groups are not as tight as those when the barrel
was in its prime. So when the change is detected, the competition
shooter probably assumes the barrel is "shot-out."
How does the competition shooter know when the
barrel has passed it prime? The obvious answer is when the barrel
no longer provides tight groups. This may appear suddenly or it
may appear gradually over a period of time. Some competition shooters
don't wait for the barrel to pass its prime but instead, change
barrels after a given number of shots.
A mistaken view is that loss of barrel life is
due solely to the abrasive action of the projectile with the barrel's
rifling. This is partially true; however, the main culprit is bore
erosion followed by bore wear. Bore wear and bore erosion are not
synonymous. Bore wear results from the removal of metal from the
bore's surface by mechanical abrasion. Bore erosion results from
the removal of metal from the bore's surface by hot propellant
gases. Abrasive wear is gradual and usually even to the surface
of the bore. On the other hand, erosion can be rapid, particularly
when the bore is exposed to continuous high temperatures. Erosion
is the major cause of barrel wear with the greatest focus directed
toward the area where the rifling begins (Figure 1). The hot metal
from the bore's surface reacts with the propellant gases to produce
a brittle state that results in metal pieces being removed during
subsequent firings. In addition to bore wear and erosion, corrosion
should be considered as a contributing factor even though it plays
a minor role with non-corrosive propellants and primers.
Specifically, what is the cause or causes for
a barrel to become less accurate after a given number of rounds?
The answers to this question are found within the military literature
for here barrel life is critical, especially with full automatic
weapons. The literature is consistent in that the major culprit
to loss of barrel accuracy is due to thermal stress or heat (4-8).
What is thermal stress and how does it affect
accuracy? Thermal stress is due to the production of very hot propellant
gases as well as to the friction created when the projectile first
engraves into the lands of the rifling. Approximately 30% of the
energy available in the propellant is transferred as heat by both
convection and radiation to the barrel within milliseconds after
powder ignition. With ignition of the first cartridge, the barrel
chamber is shielded from the newly created heat by the cartridge
case. So the duration of the heat build up is very short with one
firing and is restricted to the bore surface. With continuous ignitions,
heat builds up as it is transferred from the chamber to the rest
of the barrel. Rifles chambered for a cartridge with muzzle velocities
that excel 3000 fps can burn out after a few hundred rounds if
shot consecutively without cooling. Heat build up can be contained,
with time interruptions between ignitions -- allowing the barrel
to cool down between firings.
Thermal stress of barrels can be divided into
two categories: 1) uninterrupted multiple firings over a short
period of time without cooling the barrel, and 2) few firings over
a long period of time with cooling the barrel between each firing.
The first category is characteristic of machine guns (as well as
over enthusiastic prairie dog shooting), while the second category
is characteristic of rifles used for sniper, competition and less
enthusiastic varmint shooting.
Either multiple or few firings result in a transfer
of heat into the interior of the barrel wall. To appreciate the
dynamics of the events, let's focus on multiple firings without
cooling. This scenario reveals an increased barrel wall temperature,
which is transversed three-dimensionally within the steel. The
maximum temperature of the barrel's internal surface depends upon
the type propellant being used, barrel composition and duration
of firings. With long durations, the elevation of barrel temperature
results in an increase of bore diameters. Simultaneously, a change
in the depth of the land engraving to the projectile's surface
occurs, which results in the projectile leaving the muzzle with
an undesirable yaw. Is there any way to overcome or to reduce these
changes? Current methods used to dissipate heat from barrels include
graphite coating the barrel, fluting the barrel, liquid holding
jackets (e.g., water liquid nitrogen etc.), increased
barrel diameters etc.
Unlike machine gun barrels, the literature focusing
on research using proper controls is scant with regard to life
of competition rifle barrels. Some literature exists, but most
is directed toward sniper rifle barrels that are goal-driven; e.g.,
SEALs 190gr SMK in .300 WM, are difficult to obtain and are in
some cases conducted without proper controls. Other literature
exists which are published by others -- including competition shooters,
but are usually restricted to the shooter's experiences with one
or two rifle barrels. Two notable exceptions are Kevin Thomas'
articles; the first deals with the perceived value of cryogenic
rifle barrels versus non-cryogenic rifle barrels, while
the second deals with the perceived effects of molybdenum disulfide-coated
projectiles versus non-coated projectiles (9, 10). However,
some valuable information has emerged, which over a period of time,
has been confirmed by shooters other than those who first reported
it. For example, rifle barrels chambered for cartridges with large
diameter projectiles will last longer than those chambered for
smaller diameter projectiles. Rifle barrels chambered for cartridges
of a given diameter but with less powder will last longer than
those with more powder. There is some evidence that stainless steel
rifle barrels will last longer than chromemoly rifle barrels. Even
though temperature usually falls rapidly between non-rapid shots,
with time the bore surface will develop small cracks.
There are instances where a shooter may feel that
his rifle barrel is shot out, but this is not the case. Readers
may recall in our first article (1), that Boots pointed out that
this so-called shot-out rifle barrel used for competition shooting
could be in many times brought back to life with a re-crowning.
This assumes that erosion to the throat region is not extensive.
A good approach to learn about the practical aspects
of a rifle barrel's life is to tap the knowledge of an experienced
rifle barrel maker. Such an individual over a period of time would
be able to identify developing trends and would be able to convey
them to the dedicated shooter. Boots Obermeyer more than qualifies
for this role.
John: Boots, over the years as you have
made thousands of rifle barrels, the topic of rifle barrel life
must have arisen a number of times in a number of different ways.
Can you provide the reader with an overview of your experiences
with rifle barrel life?
Boots: My experience with barrel life
comes from two areas. The first is with pressure barrels, while
the second is with competition rifle barrels. When the Amron operated
in Waukesha, Wisconsin, I made their test barrels, which allowed
me to observe problems as they developed. A major problem was erosion
loss of the piston holes located in the chamber. There would be
some leakage to the rear of the gas port because the thickness
of the cartridge case wall at that point didn't allow it to seal
perfectly. Often the test barrel could be set back an inch and
be used again simply by adding a correction for the velocity loss
due to shortening of the barrel. I noted that as the bore became
eroded the velocity spread became a problem. In the .243 Win. caliber
at approximately 2000 rounds, the velocity spread widened greatly
even though the pressure remained constant. Most of the ammo that
was being tested was made for a large company that sold under their
own brand name. They demanded tests for each lot of ammo to insure
that it had reasonably low velocity spread. Simply applying a correction
factor wasn't good enough for them.
The other area is competitive high power rifle
shooting. John, as you know, not only do I make barrels for sport
shooting; I'm a competitive shooter as well. I believe that this
gives me special insight to a number of problems that might be
missed by others. I've concluded that barrel life has a time volume
relationship.
John: What is a time volume relationship?
Boots: Contrary to popular opinion, rapid
fire does not eat up barrels that are used for target shooting.
Instead in long-range matches, it's the use of heavy bullets that
is the culprit. For example, one shoots lighter bullets for standing
and sitting rapid fire at 200-yard targets and rapid fire prone
at 300-yard targets. Then one switches to heavier bullets with
a stiffer powder charge for shooting at 600 yards. With the .308
Win., I have found that barrel life could be extended to 6000 rounds
using a barrel that win matches, if 75% of the firings were with
cartridges using the lighter bullets. Conversely, during the mid-1980s,
we increased the twist and shots 220gr bullets for long range.
Barrel wear was noticeably accelerated. Then in the 1990s the small
calibers became popular and again barrel life was reduced.
John: Did you notice any typical characteristics
that alerted you that the rifle barrel was going bad?
Boots: The problem became evident with
the increase in the vertical shots causing lost points. This is
usually referred to as stringing. With the 220gr bullets, we found
that if the barrel was set back at about 3500 rounds, one could
then get another 3000 plus rounds out of it. Curiously, the barrel
shot better than when it was new. I believe that this might have
something to do with the taper worn on the lands. With the 6.5
x 08 caliber, we found that the ideal set back time was in the
2500- to 3000-round fired range when used with a mixture of bullet
weights just like the .308's. This indicates to me that erosion
can be viewed as a heat engine. With the heavier bullet -- like
the 220gr bullets in the 30 caliber, there is an expansion of the
width of the top of the pressure curve over what one would see
with lighter bullets. Also keep in mind that slower burning powders
are used for the heavier bullets, while faster burning powders
are used for the lighter ones such as the 150gr and 168gr bullets
in the 30 caliber for 200- and 300-yard targets. In addition, usually
lighter loads are used to insure easy function of the rifle. This
is why throats are eaten out because the volume for the gas to
expand is reduced.
John: What is the situation with calibers
smaller than 30? I understand that special problems can be encountered
with them.
Boots: The smaller calibers, such as the
6.5mm, work the same way but an additional factor has to be considered.
The expansion ratio of volume of the bore behind the bullet is
reduced a great deal with the reduction in bullet diameter. This
is the reason why smaller calibers have a reduced life when compared
to larger calibers. A better way to consider this is to calculate
the volume of the cartridge case and then determine how far the
bullet must move to equal this volume. For example, take a cartridge
about the size of a .308 Win. with a case volume of 0.220 in3.
A 28-inch target barrel in .308 Win has 26 inches of bore that
the bullet will pass through. The volume of the bore is 1.937 in3.
This means the case volume can expand 8.8 times or once for every
2.95 inches of distance the bullet travels. Now consider a smaller
caliber such as a .243 Win., the bore is now 1.206 in3.
This means the volume expands 5.5 times as the bullet moves down
the barrel or once for every 4.7 inches of forward motion. This
is why throats are eaten out because the volume for the gas generated
by the powder combustion expands at a slower rate. As a result,
this widens the time period for the high-pressure gas to eat out
the throat area (Figure 2).
The throat becomes rough from the gas cutting
into it and this is what causes the vertical stringing, which are
not under the control of the shooter. As I have said many times,
a well-used throat of a stainless steel barrel looks like a dried
up puddle. Occasionally, a large flake will pop out. Accuracy is
quickly lost from such a barrel as I have noticed for a number
of years. This appears to be due to when the key flake pops out,
it drags on the bullet, which causes a large velocity change. This
flake is the random error that just caught up to you and gave you
a poor shot at that range. When this happens, most people don't
know what it is. I call these "drag errors" that are
seen at 600 yards before they are seen at the shorter ranges. This
is due to the time of bullet flight where small velocity changes
are seen until the acceleration of gravity or the fall rate catches
up with the shooter. This is why short-range groups can be misleading,
which can result in a shooter selecting poor loads because vertical
stringing isn't detected at 100 yards.
The fall rate at 1000 yards is about 54 ft/sec,
which means small time changes translates into a lot of inches.
On the other hand, I have seen rifles with well-worn barrels shoot
well and others with little use suddenly become a problem. During
the past, I have had barrels of my own die suddenly with little
use -- then with a set back of the barrel and re-chambering, the
throat area is improved and they will shoot well for quite some
time. The bottom line is generally that hot loads with heavy bullets
will accelerate these problems.
John: Boots, what is your experience with
chromemoly rifle barrels?
Boots: With chromemoly barrels, the wear
appears like gravel in a steam bed with structures that appear
as fine lines that are somewhat rounded. My experience with moly-coated
bullets is that the moly allows the barrel to be used much longer
than when moly is not used. The fine lines may hold some of the
moly. What appears to be happening is that the moly does not reduce
the wear but instead, it simply allows the bullet to function over
it. So when using moly, one has to rethink the logic behind the
loading. One must load to maintain the desired velocity, but currently
my experience is that a close contact of the bullet to the lands
may be a negative rather than a positive factor. John as an interesting
side note, in 1997 I made a chromemoly barrel in 6.5 x 08 that
I used only with moly coated bullets. It went almost 7000 rounds
with 30 scores of 200 at 600 yards. At 6200 rounds during September
of 1999 at Eau Claire, I fired a 200-17X with iron sights. The
bullet was jumping a 1/4-inch before coming in contact with the
rifling.
I have just been informed of some new information
that has some relation to our discussion. The appearance of pits
in new hammer-forged barrels has been reported recently. What was
reported to me is that the pits come from the process used for
high volume barrel production. The steel blank used has a larger
diameter than the finished barrel and is about 8 inches long. The
steel is heated to red-hot and then a hole about the size of the
chamber is poked through it. Next the mandrill is inserted and
then followed by the hammering of the blank. The blank not only
is reduced in diameter but is also expanded in length. With high
volume production, hammering of the contour of the outer diameter
is done at the same time as the hammering of the rifling. Since
the blank is red hot, a scale like substance forms on the inner
surface. This results in impurities being hammered into the surface
of the bore. After a number of rounds shot through the barrel,
the impurities pop out and appear as pits.
John: It appears that big cartridge cases
along with heavy projectiles are not an ideal combination. We know
that large propellant charges speed up barrel erosion. The .300
WM with the 190gr SMK is a good example. It appears that it advantageous
to find the correct combination. What are the advantages for competition
shooting?
Boots: For a moment, let's look at light
bullets and recoil. I made up a small table (Figure 3) to show
some interesting relationships. The table gives some comparisons
of recoil and is based on the Franchu program. If a 14 lb .308
target rifle with a sling can easily change 1 MOA from standing
to sitting position, think what is likely to happen with a lightweight
magnum like the .300 RUM with four times the recoil energy and
about two and half times faster motion on the recoil. I don't know
how much error the heavy recoil produces but let's say it is four
times greater than the .308. That means the error is four MOA and
that would mean twelve inches of error is likely at 300 yards without
allowing for the group size or shooter error. Now again, I don't
know if the error works in these proportions, it could be less
but it also might be greater. We would need to do some form of
testing in order to check it out.
Different types of powders accelerate the bullet
at different rates. In my experience, I found RL15 has less barrel
time than 4895 with 168gr bullets in my .308 Win. match rifle.
This resulted in 1 MOA lower point of impact at 200 yards with
the 4895 load. Checking the velocity I found the RL15 load was
15 ft/sec faster which showed that the RL15 had less barrel time.
Since less time means less error from sling and position changes,
it raised my rapid-fire scores. Another thing to think about is
the angle of the recoil. A sporting rifle usually has more butt
drop than a target rifle. So when I used a 1 MOA change for a match
rifle from standing to sitting, it may be more for a sporting rifle.
John: Boots, let's examine barrel life
in relation to the method of making a rifle barrel. For example,
hammer-forged versus button versus cut rifling. Can
any generalization be made?
Boots: Over the year I have noticed button
barrels often have a short life when compared to cut barrels. I
believe the main reason for this is that some button-rifled barrels
have shallow rifling due to the mechanics of the process. Cut barrels
are normally cut to full depth -- remember it is the lands that
drive the bullet. Also cold forming puts stress into the steel.
In stainless barrels I have noticed wear in the throat is much
faster when compared to chromemoly barrels. I recall one 40X with
the accuracy gone after 2500 rounds, in which the throat area had
many large chips. The erosion was worst than what I was seeing
in cut rifle barrels with 5000 rounds through them. In the 1980s
I went to extra deep rifling for the .308 Win. barrels with a .298
bore diameter. These barrels worked to 7000 rounds of normal loads
without a set back. When we went to 220gr bullets, the throat wore
much faster resulting in the barrel being set back at 3500 rounds.
Then barrel worked well through 6000 rounds plus, but because the
wear on the lands with the 220gr bullets was quite noticeable,
the bore became shallow and the performance went down.
John: What about hammer-forged rifle barrels?
One of our readers, Bernd Kaercher from Germany indicated that
he is very interested in the differences that result from the various
rifling methods. He stated in an email, "No doubt the hammer
forged barrels will last the longest of all of the methods, but
they have disadvantages when compared to the other methods. On
the other hand, cut rifling does not yield as hard a surface as
hammer forged and button barrels. So most people expect a softer
surface from a cut-rifled barrel. Is this more theoretical or is
there a difference in real life?"
Boots: Bernd is mistaken about hammered
barrels, as is the general public. The cold hammered and button
rifling have a reduced barrel life when contrasted to cut rifling.
I don't believe any hard-core scientific tests have been conducted
but instead, there are experiences gained over the years by several
barrel makers. Many years ago I examined a number of hammer forged
barrels that had a short life. One of the common traits was a pitted
surface at the muzzle. It appeared to be spalling off of the metal
surface as the gases blew by as the projectile exited the bore.
I have lapped and re-crowned a number of these barrels. I recall
one barrel that increased groove diameter by 0.001+" -- much
to my surprise, a quick lap resulted in easy removal of the surface
of the bore. This could not be done with cut barrels. Here one
has to work hard just to lap off a couple of tenths of surface.
Clearly, this is a clear indication of a difference the surface
hardness of the lands between the two types of rifling. The reconditioned
forged barrels are good for hunting but not target shooting.
John: In the introduction to this article,
I outlined some results from testing on machine gun barrels, as
the literature is rich in this area. The literature is meager with
rifle barrel testing. Is it possible to make extrapolations from
the machine gun data to rifles or are there pit falls?
Boots: It is helpful to note that rapid
fire in a machine gun is not the same as a target rifle. Machine
guns have rates of fire measured in the hundreds of rounds per
minute. A high power shooter or varmint hunter in rapid-fire shoots
a shot every four or five seconds. While the barrel gets hot to
the hand, the heat on the bore surface has a period of time to
be absorbed into the mass steel below it. On a barrel like the
20mm with burst fire, the surface has a very small time to spread
the heat. These barrels are often rifled with a gain twist. One
obvious reason is to reduce the thrust on the lands as this point
for rotation is also damaging to the lands so the gain twist is
an important factor in increasing the barrel life. I have made
some of these barrels in the past and currently have some orders
for 20mm and 23mm test barrels for the lab at Aberdeen1 A
23mm round exiting at a twist rate of 1/18 would have an angle
on the land of about nine degrees. That would mean if it is a constant
twist and fired rapidly there will not be only the thrust to drive
over a projectiles driving bands or it jacket -- whatever is the
case, but also considerable side thrust on a very hot surface.
Even though these are very large bores, the cartridge cases are
also extremely large so a lot of powder is being burned. The gain
twist barrel starts at zero or a very slow rate and then the twist
rate picks up as the throat area is left behind.
John: Boots, I want to thank you for some
new insights in rifle barrel life.
For Additional Information
Obermeyer Rifled Barrels
23122 60th Street
Bristol, Wisconsin 53104
Voice: 262-843-3537
FAX: 262-843-2129
References
1. Taylor, J. D. 2000. Boots Obermeyer on Extreme
Rifle Accuracy -- I. Barrel Crowns. The Accurate Rifle, Vol. 3,
No. 10, pp. 7-12.
2. Taylor, J. D. 2000. Boots Obermeyer on Extreme
Rifle Accuracy -- II. 5R Rifling Profile. The Accurate Rifle, Vol.
3, No. 12, 15-24.
3. Hart Ezell, V. 2000. Navy SEALS Choose Knight's
SR25 Sniper Rifle. National Defense, Vol. LXXXV, No. 563, p. 38.
4. Allsops, D., Popelnsk , L., Balla, J., Cech,
V., Prochzka, S., and Rosichz, J. 1997. Military Small Arms
-- Design Principles and Operating Methods. Brassy, London.
5. Moss, G. M., Leeming, D. W. and C. L. Farrar.
1995. Military Ballistics. Brassy, London
6. Ahmad, I. 1988. The Problem of Gun Barrel Erosion:
An Overview. In: Gun Propulsion Technology. Progress in Astronautics
and Aeronautics (Stiefel, ed.). Vol. 109, pp 311-356.
7. Marchant Smith, C. J. and P. R. Haslam. 1982. Small
Arms & Cannons. Brassy, London
8. Anonymous. 1981. Oeklidon Pocket-Book. Oerlikon-B hrle
AG, Zurich.
9. Thomas, K. 1999. Moly, Son of Cryo Test. Precision Shooting,
Vol. 46, No. 9, pp. 8-15.
10. Thomas, K. 1998. Meanwhile, 17224 Rounds Later. Precision
Shooting, Vol. 6, No. 5, pp. 8-21.
|
