From: Bart Bobbitt <bartb@hpfcmgw.fc.hp.com>
Subject: Calculating Rifle Barrel Life

I finally completed my research on a way to calculate how many rounds
a rifle barrel can be expected to deliver its accuracy level.  By that,
I mean the barrel can be expected to have an average group size for
so many rounds before that average group size starts to get larger.

First, the rule-of-thumb formula I derived will produce a barrel accuracy
life of about 3000 rounds.

Second, if a lot of rapid fire (one shot every 5 to 10 seconds) is done,
the accuracy life will be less.

Third, if full-auto or very fast fire (a few shots per second, or one shot
every second) is done, accuracy life will be much less.

Fourth, the accuracy levels are for ranges through 600 yards.  Once the
barrel life calculated limit is reached, groups will probably start to
get bigger at the longer ranges before they are noticeably bigger at the
shorter ranges.

My formula, or rule-of-thumb process, is:

   1. Calculate the bore area in square millimeters.

   2. Use one grain of powder for each square millimeter.  This is what
      I call the reference, or base powder charge.

      Example:  .30 caliber bore = 45.6 square millimeters.
                Base powder charge for .30 caliber is 45.6 grains.

      A .30 cal. cartridge that burns 45.6 grains of powder should give a
      barrel life of about 3000 rounds of good accuracy.

   3. If a larger cartridge is used and it burns more powder, the
      accuracy life in rounds for that bore size is reduced.  The amount
      of reduction is determined by

        a. Divide the increased charge by the base charge, then square
           the answer.

        b. Divide that answer into 3000.

      Example:  .28 caliber bore has a base charge of 38.5 grains.
                Cartridge burns 57.8 grains of powder.
                (57.8/38.5) squared is 2.25.
                3000 divided by 2.25 is 1,333 rounds.

If anyone can shoot a hole through this theory, I welcome that shot.  This
is more or less an emperical process based on accurate barrel life in
several calibers as reported to me by lots of folks.  All I did was study
the data and determine what math would give a best-fit formula.

And if someone has a better method, I'd like to know what it is.  My
formulas may not be the best.

From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Subject: Re: Throat Erosion??
Organization: Hewlett-Packard Fort Collins Site

william E Davidsen (davidsen@yeti.crd.ge.com) wrote:

:   I have a friend who shoots 1000yd matches, and I'm pretty sure he
: quoted his barrel life as 200 rds. He's given me a few nice top name
: barrels, but they are no longer accurate and need to be bored to a
: larger caliber to restore accuracy.

:   Therefore I would conclude that the useful accurate life depends on
: how well you and the gun shoot as a starting point.

Well, I've not heard of anyone getting that short of life out of a barrel.
I knew a couple of folks who used the 6.5 X .300 Weatherby for a 1000-yd
rifle and they got about 500 rounds of accurate life.

But I agree with your comment about how well someone shoots as well as
the gun shoots is a big factor in deciding when to retire a barrel.  If
someone doesn't notice his system (person, rifle and ammo) accuracy has
got worse, they probably won't think their barrel has worn out even
though it really has.

For example, a benchrester gets a new rifle barrel and it shoots no
worse than .250 inches in good conditions at 100 yards; many groups go
under .150 inch.  When that barrel starts shooting groups in the .300 to
400 inch size at 100 yards, he gets a new barrel.  With the .22 and 6mm
PPC cartridges these folks use, this drop off in accuracy typically starts
at about 3000 or so rounds.  Should an average varmit hunter's rifle
start shooting groups a tenth of an inch larger at some point in its
barrel life, he probably won't notice it and continue to use that barrel
for many hundred more rounds, perhaps a thousand or more.  His acceptable
level of accuracy may not be as stringent as the benchresters.

My comments regarding barrel life are relative to when accuracy starts to
degrade.  Whether or not the shooter notices it, or even cares about it,
is an entirely different matter.

BB

From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Subject: Re: Barrel wear on M-16 or Beretta 92F
Organization: Hewlett-Packard Fort Collins Site

Richard Cower (cower@CSLI.Stanford.EDU) wrote:

: Does anyone know how many rounds can be fired thru the Colt M-16
: before the barrel is worn out?

The barrel should maintain its initial accuracy level for about 3000
rounds when shots are fired no faster than about 1 per minute.  Then
its accuracy will start getting worse, although it may not be noticed.
For plinking or hunting use, it could last for about 5000 rounds; it
depends on what your accuracy requirements are.

If a lot of rapid firing is done, like shooting at a rate of 10 or
more rounds per minute, and doing this a lot, the barrel life will be
shorter by as much as 50 percent.

BB

From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Subject: Re: Barrel wear on M-16 or Beretta 92F
Organization: Hewlett-Packard Fort Collins Site

Aaron Good (ucla-cs!netcon.smc.edu!agood@CS.UCLA.EDU) wrote about my
comments on .223 barrel life:

: I have trouble with these numbers, they seem awful low, are you sure ?
: IMHO that many rounds would be fired just in basic training. Are you
: talking about match grade barrels or something ?.

I'm talking about any barrel.  Match and sporter barrels wear at the
same rate; they're made from about the same materials, only the uniformity
of the bore/groove/twist dimensions are different.

As the user gets to decide what the `real' barrel life is, each user's
number may be different.  But the point at which the initial accuracy
level starts to open up is the basis for my numbers.  Some folks will
accept the groups getting a lot bigger than others after this point
is reached.  I've seen some .308 Win. barrels that have had more than
15,000 rounds through 'em.  For the first 3000 rounds, they would shoot
better than half MOA through 200 yards.  At 15,000 rounds, they shot
3 MOA groups through 200 yards; for some folks, that may be acceptable.

BB

From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Subject: Re: Barrel wear on M-16 or Beretta 92F
Organization: Hewlett-Packard Fort Collins Site

Richard Cower (cower@CSLI.Stanford.EDU) wrote:

: Does anyone know how many rounds can be fired thru the ........
: Beretta 92F before the barrel is worn out? How about the Glock
: 17 or 19?

My barrel wear-out numbers were for rifles; not handguns.  Please don't
mix or confuse the two; handgun barrels last much, much longer; they
burn much less powder.

My only real knowledge about a handgun barrel's life is based on a test
the US Navy did in the late 1960s.  An accurized M1911-A1 in .45 ACP,
a softball (wadcutter) competition pistol, was built and then put in
a machine rest for its final tuning and initial accuracy level.  It
started out with 20-shot 2.5-inch groups at 50 yards.  About every 5000
rounds, it was put back into the machine rest and retested.  At each of
these tests through 25,000 rounds, it still shot the same size groups.
At 30,000 rounds, the groups had opened up to about 3.5 inches; at 35,000
rounds, the groups were about 4.5 or so inches.  The ammo used was
Remington 185-gr. jacketed wadcutter match ammo as loaded by Remington.

The top pistol shooters on the team had been wondering just how many
rounds a softball gun would last before it needed rebarreling.  They
knew the hardball match pistols (using full-power 230-gr. loads) went
for about 15,000 rounds before needing a new barrel.  By this, I think
it means that the initial accuracy level of the barrel was good for
about 15,000 rounds and it started to open up after that.  Again, this
is where accuracy starts to drop off, but for purposes other than pistol
competition, the user gets to decide when the barrel is worn out.

Interestingly enough, both handguns and rifles for the .22 rimfire round
last about the same number of rounds.  That's expected, as the both use
the same amount of powder.  These rimfire barrels go for about 5,000 to
10,000 rounds before accuracy starts to drop off.  As their barrels are
typically made from steel that's somewhat softer; they wear out faster.
I know there are some .22LR rimfires around with 50,000+ round through
them that their owner's think their accuracy is just fine; and that's
just fine, too.

From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Subject: Re: Barrel wear on M-16 or Beretta 92F
Organization: Hewlett-Packard Fort Collins Site

Quoc Tuan Pham (quoctp@cs.toronto.edu) wrote:

: For the Anschutz rifle, it takes about 500
: rounds to "break in" the barrel (so it becomes smooth and does not lead
: up easily). Serious shooters usually shoot 15,000-20,000 rounds a year
: for training and some of them are very fussy about the accuracy. So
: probably these barrels don't wear out that fast as said in other
: postings for center-fire barrels.

In talking with some of the folks at the Olympic training center and other
top smallbore competitors in the USA, your rounds-per-year numbers are
about what smallbore barrels used to get for top accuracy.  But that was
before RWS and Ely changed their priming compound formula to be more
`environmentally correct.'  For about the last 5 years, barrel life of
a smallbore match rifle got cut about in half.  Seems the new priming
compound causes more (faster?) throat erosion.

Interestingly enough, there's a bunch of new Anschutz barrels stored in
closets by 'smiths catering to the smallbore competitors.  Seems there is
two reasons for this: Hart barrels shoot more accurately and they last
longer for accuracy.  One smallbore 'smith told me a few years ago that
the Anschutz factory barrels were a tad softer than a Hart and that's why
they didn't last as long.

BB

From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Subject: Re: Barrel wear on M-16 or Beretta 92F
Organization: Hewlett-Packard Fort Collins Site

Clark Towle Gunsmith (towle@vssteg.enet.dec.com) wrote:

:  Can't say I've ever seen a Beretta 92F barrel get worn out but an M16 barrel
: in the 1/7 twist can go south in as little as 2,000 rounds depending on the
: rate of fire it is subjected to. Dumping several 30 round magazines in a row
: without allowing the barrel time to cool off creates a premature throat
: erosion condition which only gets worse with time.

Amen to that!  Several years ago, one of the military rifle teams got a
bunch of .300 magnums rebarreled with match-grade tubes.  One of the things
going around at the time was that these barrels needed to be broken in by
shooting them.  Seems that gave a case of ammo to someone to take out to
the pistol range and shoot several rounds through each barrel.  Well, the
well-meaning person picked up a rifle, chambered a round pointed it into
the backstop and pulled the trigger.  Then lowered the rifle immediately,
chambered another round and fired it.  This process was repeated for about
50 or 60 rounds per barrel; all shots for each rifle being fired in about
4 minutes.  Three of these rifles were used in a match the following
weekend, but without any great success.  The folks who shot them said they
all just didn't shoot as well as expected, and perhaps the ammo wasn't too
good as it was a new, just loaded lot.  The following week these rifles
were taken out to the local range and tested with the remnants of the last
lot of known good ammo; they shot about like a barrel that had a thousand
rounds through it.  The other three rifles showed the same thing.  Then
someone remembered the break-in process and suggested the person who broke
in the rifles be questioned.  Well, after the questioning, everybody knew
why the rifles didn't shoot well; they had been burned out through break in.
The match barrels used weren't as hard as a machine gun barrel; that's why
a 60-round rapid fire string wore them out for competitive purposes.

BB

From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Subject: Re: Barrel Life During Combat Conditions
Organization: Hewlett-Packard Fort Collins Site

Bill Vojak (vojak@icebucket.stortek.com) wrote:

: This made me wonder; In actual COMBAT situations, how often does the military
: change the barrels.  It would seem that a soldier in combat could put lots
: of rounds through a barrel in a VERY brief period of time.  Does the military
: just say "oh well, so what if the 100 yard groups are now 5"", or do they have
: some standard for replacement.  I would think that during Vietnam some of
: those rifles had 10s of thousands of rounds put through them over the years,
: many while the rifle was in a burst or full auto mode.

Our armed forces do have a way of determining when a barrel should be
replaced.  But even then, when that time is reached, it should be somewhat
convenient for the person who has a worn out barrel.

A throat erosion gage is used.  It has a tapered, hardened steel plug on
the end that goes into the throat.  Taper is typically .010-in. per inch.
for 30 caliber arms, such as the 7.62mm NATO, the gage will go into the
throat so far as to have the number 0 on its body next to a reference
point in the receiver.  If the gage goes in to where the number 5 is next
to the reference mark, that means the bore diameter has opened up about
005-in. at the origin of the rifling.  Usually, this means the barrel
is about half way worn out.  If the gage goes all the way in to where the
`reject' point on the gage is next to the reference mark, that means the
barrel should be replaced.

Military armourers who work on match grade service rifles usually rebarrel
when the gage reads 5.  That's the point where accuracy starts to drop off
from the level it had as a new barrel.

In talking with military folks who rebarreled a lot of service rifles,
the number of rounds varied quite a bit amongst barrels for a given
number read off the throat erosion gage.  Sometimes, if the barrel was
used for a lot of rapid fire, the barrel might reach the `reject' point
in about 3000 rounds.  If mostly slow fire was done, the barrel might go
for close to 10,000 rounds.

Machine guns are noted for wearing out barrels quite fast.  Most machine
guns are built so barrels can be changed very quickly; makes sense when
they're in a combat situation.

As arms used for service/combat applications don't need to be as accurate
as a competition target arm, they typically last a few more times as many
rounds.  Besides, the skill level most service folks have, and the
environment they're in, doens't require half MOA accuracy for the life
of the barrel.

I doubt if `tens of thousands' of rounds per service barrel is even
closely approached.  With the extreme amounts of throat erosion that many
rounds would cause, the reduced pressure levels would probably not be
enough to cycle a semiauto or full auto action.  In talking with Vets
from WWII, Korea, Vietnam, few fired more than 4000 to 5000 rounds in
combat with semiautos.  But the machine gunners did; and also replaced
a lot of barrels.  With the machine gunners, they said they could tell
when the barrel needed to be replaced; tracers started going in all
directions.  They didn't need a throat erosion gage; just a few minutes
to replace the barrel.

BB

From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Subject: Re: Barrels and rapid fire
Organization: Hewlett-Packard Fort Collins Site

Rapid fire does wear out barrels much faster than shooting at a slower
pace.  The typical rifle in .308 Win. will keep its original accuracy
for about 3500 rounds when fired not faster than about 1 shot per minute.
If 30% of the rounds fired are rapid fire, that is one shot every 4 to 5
seconds, the barrel life will be about 3000 rounds.  If 60% of the shots
are rapid fire at the same rate, barrel life will be about 2500 rounds.
And if 90% are rapid fire, expect only about 2000 rounds of original
accuracy.  If the powder charge is larger for this same bore size, the
barrel life will be shorter; for example a .300 Win. Mag. will last about
1100 rounds of original accuracy when shot no faster than once per minute.
But rapid fire will wear one out in only a few hundred shots.

Increasing the rapid fire rate to one shot every 1 to 2 seconds will
reduce the above shorter barrel life increments by about 50%.  In other
words, the 30-60-90 percent of rapid fire cuts the accuracy life to
2750, 2000 and 1250 rounds, respectively.

These numbers are for type 416R stainless steel barrels, the type most
used for competitive shooting which is what my numbers are based on.
Machine gun barrels are typically much harder (but not as accurate) as
barrels used for sporting purposes.  They'll last much longer; perhaps
three times longer.  But replacement barrels are usually nearby for those
combat-environment machine guns and they can be replaced in a very short
time.

Some years ago, one of the military rifle team armorer's sent 5 brand new
30-.338 magnum target rifles out with someone to `break them in.'  That
meant they should be shot, then cleaned, then shot and cleaned again for
about 20 shots.  That's at a rate of about one shot every 5 minutes or
thereabouts.  Well, seems the `shooter' was on the pistol team and he just
didn't understand all of this.  He shot each one about 30 times at a rate
of one shot every 5 seconds; just about as fast as he could shoot, reload
and shoot again.  All five of those rifles were taken to a 1000 yard match
the following weekend.  None of them shot accurately.  Expecting them to
produce groups at 1000 yards of about 7 to 8 inches, the best of them did
only about 1.5-foot groups; the worst about 3-foot groups.  Needless to say,
the managing armorer was angry enough to chew those ruined barrels into a
pile of stainless steel wool.

BB

From: bartb@hpfcla.fc.hp.com (Bart Bobbitt)
Subject: Re: Competition small bore [.22].
Organization: Hewlett-Packard Fort Collins Site

Geoff Miller (ghm@sserve.cc.adfa.oz.au) wrote:

:     ...a well-maintained smallbore target rifle is almost impossible to
:     wear out (unless you're doing as much practice as an international-
:     class shot).

Prior to about 1988, one could get 20,000 rounds of accurate life from
a .22 LR target barrel.  But RWS R50 and Eley Tenex (Lapua Dominator,
too, I think) changed their priming compound to a more environmentally
compatible one.  In doing this, the increased amount or addition of
one component in the priming mixture is now more abrasive to the barrel
steel.  Nowadays, one gets about 10,000 rounds of accurate barrel life.
That means one can shoot about thirty 3200-point matches with some
sighters included and no practice shots at all.  Then, you need to either
set the barrel back a couple of inches or replace it if you want the same
level of accuracy in your mouse gun to be maintained.

I've heard the M52 will be back in production, but with hammer-forged
barrels.  'Twill be interesting to see how well they shoot.  The original
M52D cut-rifled barrels were pretty good; about one-third of them shot as
well as a Hart barrel.  But they needed to be set back and rechambered
with a quality match reamer; factory chambers were a tad big and irregular.
If the reintroduced M52s are well made, this may be a boone to the .22 LR
competition folks getting started in the smallbore discipline.

BB

From: bartb@fc.hp.com (Bart Bobbitt)
Subject: Re: ACCURACY?
Organization: Hewlett-Packard Fort Collins Site

David Post (post@hpfcso.fc.hp.com) wrote:

: And there was the discussion of copper jacket versus steel jacket ... the
: conclusion of which seemed to be that the jacket metal did not matter.  You
: see, I am having trouble seeing why the different metals have similar wear
: properties.

My use of soft-steel jacketed bullets (Norma) and other match bullets
shows equal barrel life with both, for the same caliber.  Steel jacket
material Norma uses has about the same hardness and other properties as
gilding metal used in most bullet jacket material.

: But now we are talking about barrel hardness affecting useful barrel life.
: While it seems reasonable, I'm having trouble rationalizing it with the
: other discussions.

Stainless steel used for rifle barrels varies somewhat in its metalurgy and
hardening processes, as well as machinability.  With softer stainless steel,
button rifling can make much more uniform groove diameters; especially when
there's a few non-homogenous places in the barrel blank.  Oft times, no
bore lapping is needed to get bore/groove dimensions to within .0001-in.
or less of desired dimensions.  This means the barrel costs less for the
customer; a very common thing with some barrels.  Harder stainless steels
require some lapping of the button or cut rifled bore to get very uniform
bore and groove dimensions.  That's more work for the barrel maker, so he
wants paid for his extra labor.  That's fine, as the harder barrels (Hart,
Krieger, Medesha and Obermeyer) last longer.  When you divide the cost of
having a barrel installed by the number of shots of accurate life, the
harder stainless steel barrels that last longer also cost less per shot for
their life.

Another cause of shorter barrel life is rifling groove depth.  The better
barrels in 30 caliber, for example, have a groove depth of 4 to 6 thousandths
of an inch.  For a given barrel metallurgy, barrels with grooves this deep
will have an accuracy life about 40 to 60 percent more than one with grooves
only 3 thousandths of an inch deep.  Shallow grooves plus softer steel means
a really short accurate barrel life.

: And what is it about the above calibers that actually causes accelerated
: wear and how does that relate to the mentioned discussions?  It is not
: intuitive to me, especially after comparing published max load velocities
: for 165-170 gr bullets.  ( Hodgdon lists ~3200, ~2800 and ~2600
: respectively).

I've used the following rule of thumb for several years to track the
accurate life of barrels shoot everything from PPC cases to very large
magnum cases in 30 caliber.  It's proved very accurate, at least within
10% of what folks get.  My data is bases on what competitive shooters get
for accurate barrel life.  But hunters, plinkers, and other rifle shooters
will get the same results; the barrel doesn't know the shooting conditions
it's being used in.  Whether or not the shooter-rifle-sights-ammo system
can tell when accuracy starts to get worse by a quarter or third MOA is
subject material for someone elses keyboard time.

   Rule 1.  A centerfire cartridge whose maximum safe powder charge weight
            in grains is equal to the bore's cross sectional area in square
            millimeters will have about 3300 rounds of its initial accuracy
            maintained when fired shots are no faster than about one minute
            apart.  Firing more rapid will shorten the barrel's accurate life.
            This is at bore capacity for that bore's diameter.  Harder
            steels with groove depth 4 - 6 thousandths are used in this
            emperical extragavanza.  Softer steel barrels or shallower
            groove depth will reduce barrel life by 20 to 50 percent.
            Bullets used are about .0005-in. greater in diameter than
            the barrel's groove diameter.

            For example:

                      Bore Diameter ->  .22   .24   .26   .28   .30
            Bore Capacity in grains ->  24.3  28.3  33.2  38.5  45.6

   Rule 2.  Increase bore capacity powder charge by:

                100%, barrel life shortens 75%

                 50%, barrel life shortens 67%

                 25%, barrel life shortens 33%



BB