Have you read this?
http://www.google.com/groups?selm=b...u&oe=UTF-8&output=gplainC/ Does anyone have the complete (barrel life) equation?
Yes, I do. I originated it on rec.guns years ago. And yes, it's
empirical. Over many years of competitive shooting, I learned from
all the top-scoring shooters their loads and rounds of accurate barrel
life. The key thing here is "accurate" barrel life by their
standards. In highpower competition, accurate barrel life ends when
shots missing where called by about 1/3rd more than when the barrel
was new and in its prime. In other words, when the barrel's grouping
ability is about 33% bigger.
The most common cartridge used until the mid 1960s was the .30-06 with
powder charges about 48 grains of powder. Top shooters would
typically rebarrel at about 3000 rounds; most would not enter the
Nationals with more than 1500 rounds through their current barrel.
Note that this number is based on shots at a rate of one per minute
for 10 to 20 minutes. For rapid fire at one shot every 6 seconds for
one minute, each of these rapid-fire shots is worth two of the
slow-fire ones. If lots of rapid fire is done, then naturally the
barrel life is shorter. I started looking at some fact about this
cartridge I could equate to its barrel life. It just happens than the
cross sectional area of a 30 caliber bore is about 48 square
millimeters (calculated with bullet diameter). It would seem that a
powder charge equaling 1 grain for each square millimeter of bore
cross sectional area means 3000 rounds of accurate barrel life.
Then I looked at other competition cartridges' barrel life as reported
by top scoring competitors:
.222 Rem.; 24 sq. mm, 21 grains, 4000 rounds.
.22 PPC; 24 sq. mm, 24 grains, 3000 rounds.
6mm PPC; 30 sq. mm, 28 grains, 3500 rounds.
.243 Win.; 30 sq. mm, 38 grains, 1600 rounds.
6.5x55 Swede; 35 sq. mm, 42 grains, 2000 rounds.
.264 Win. Mag.; 35 sq. mm, 72 grains, 600 rounds.
7mm-08; 41 sq. mm, 41 grains, 3000 rounds.
7mm Rem. Mag.; 41 sq. mm, 61 grains, 700 rounds.
.308 Win.; 41 sq. mm, 43 grains, 3500 rounds.
.30-.338 Mag.; 41 sq. mm, 65 grains, 1400 rounds.
.300 Win. Mag.; 41 sq. mm, 72 grains, 1000 rounds.
.300 Wby. Mag.; 41 sq. mm, 91 grains, 800 rounds.
This was a most interesting discovery. Cartridges using powder
charges equaling bore area (bore capacity?) got about 3500 rounds of
barrel life. Those using charges twice bore capacity got about
one-fourth of that or around 750 rounds. And those using less got
more than 3500 rounds. I've wondered where the term "over bore
capacity" came from; maybe this is why.
I did some trial-and-error math routines to come up with formulas to
plot a curve that fairly well tracked these numbers. Such empirical
processes have been used since the 1920s to calculate non-linear
ballistic cams used in military mechanical analog computers for aiming
large caliber guns with great accuracy results. So, I figured it
would work for this situation, too.
My formula is:
1. Calculate the bore area in square millimeters using bullet
diameter.
2. Use one grain of powder for each square millimeter. This is what I
call the reference, or bore capacity powder charge. Example: .30
caliber bore, .308-in. (7.82 mm)bullet diameter = 48 square
millimeters. Bore capacity powder charge for .30 caliber is then 48
grains. A .30 cal. cartridge that burns 48 grains of powder (.30-06)
gives a barrel life of about 3500 rounds of best accuracy.
3. If a larger 30 caliber 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 bore capacity, then square the
answer.
b. Divide that answer into 3500. Example: A .300 Wby Mag. has a bore
capacity of 48 grains. This cartridge burns 91 grains of powder.
(91/48) squared is 3.6. 3500 divided by 3.6 is 972 rounds. Three
competitive shooters told me their .300 Wby. rifles gave them between
800 and 900 rounds.
This also seems to work when using powder charges less than bore
capacity. The .222 Remington was the winning cartridge in benchrest
matches until the PPC family came along. It burned about 21 grains of
powder in a barrel whose bore capacity is about 24 grains. So,
(21/24) squared is 0.7656. 3500 divided by 0.7656 equals 4571 rounds.
Top benchresters in the �50s and �60s rebarreled their triple-deuce
stool guns at about 4000 rounds.
But this empirical formula has limits. The old .22 rimfire long rifle
burning 1 grain of powder in a bore whose capacity is 24 grains would
calculate to give a barrel life of some 1,728,000 rounds. But the top
international shooters I've talked to rebarrel their $4000 Anschutz
free rifles at about 25,000 rounds. And then there are thousands of
folks who claim a .22 rimfire barrel will never wear out.
And bore material and manufacturing processes can make a difference,
too. Most top-quality 30 caliber barrels get about 3500 accurate
rounds for the .308 Win. There's one make that gets about half that;
but they've been used to win the Nationals and other big highpower
matches more than once.
The intended firearm use and accuracy expectations comes to play, too.
Take the M1 and M14 service rifles. Their typical combat barrel life
established by arsenals is about 10,000 rounds with service ammo.
It's accuracy requirements was (as I remember) about 3.5 inches at 100
yards. Compare this to top competitive highpower shooters
requirements of no more than 1/4th inch at 100 yards. That's what it
takes in highpower to stay under 1/3rd MOA at 200 yards, � MOA at 600
yards and 3/4ths MOA at 1000 yards.
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 plus a
couple dozen barrels I've worn out. 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.
And if your experience differs with this data, that's fine. Make your
own barrel life determinations based on what you observe using your
own accuracy standards. I don't expect everyone to have the same
standards, but it helps when they're realistic for the shooting
discipline. We all get to decide the most we want to miss where we
call the shot on the targets of our choice; animal (game & varmints),
mineral (metallic silhouette) or vegetable (paper targets).
