#### kac1345

##### Well-Known Member

C/ 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 3000 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 3000 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 3000 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 3000. 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. 3000 divided by 3.6 is 833 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. 3000 divided by 0.7656 equals 3918 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 3000 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, ½ 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).

So According to this Fig Kirby your 7mm Allen Mag is as follows

7mm. 41 sq. mm, 118 grains

118/41 = 2.88 squared = 8.29 Then Didvide that bye 3000 = 362 rounds

Now just for [censored] and giggles lets figure the 122 grains that you say you can gain with it re-throated

7mm. 41 sq. mm, 122 grains

122/41 = 2.98 squared = 8.88/3000 = 337 Rounds Whew unless this formula is really jacked up ..That is a bbl SMOKER /ubbthreads/images/graemlins/shocked.gif Now before I get jumped...I know Kirby has his own figures on these Carts..His figs are for a sporter weight rifle use to 700 yards on deer size game..I believe his figures are about right + or - a few rounds

and as Kirby stated powder choice will play a big role in throat erosion Kiby I must say has takin all thse factors into consideration.. and designed these carts for optimum bbl life...Man I still want one Kirby..that 7mm Allen Mag has reall peaked my interest...AGAIN LOL!