I got curious about barrel burners and how people talk about overbore guns, so I got Wikipedia open and graphed case capacity vs bullet diameter, then threw in a correction factor to bring 270 win into liking a 22" barrel.  Coincidentally, 45acp ended up liking a 5" barrel.  I have no way to post the picture as I did a screen copy to my email.   Can I email it to one of you guys so you can post the link here?  I know people in the precicion rifle forum talk a lot about calibers for long range and barrel length so it seems relevant.

Figured out my photoshop password.  Here it is.  Several small problems, but it gets the point across.



This would be better if I had case capacity under the bullet instead of total case capacity.  I also had a more complicated correction formula in there, not just a factor, to get 45acp to 5" and 270win to 22".  The second 7-08 is wrong.  That data point showed more case capacity than 308, so bad info.  

The 300RUM bullet is capacity under bullet where as just 300RUM is total case capacity like all other data.  This is the only one where I had both.

Ratio = case capacity / (dia x dia)

Then correction formula makes everything look neat and in a format of barrel length.

Tagged.

[email protected]

make sure to include a link to this thread.

Well, looks like he got it posted.

here's a another copy anyway since I went to all the trouble.




There's obviously some data not shown there and there is at least one duplication of a case with a different value so I'm curious if there's a bullet weight or burn rate or energy density factor of something we're not privy to in this version.

However, If you want to get an idea of this sort of thing I think I have an idea.

This version is purely empty total case capacity / (dia x dia), then enter that into a complicated correction formula to make the data correspond to the barrel lengths I picked for 270 and 45ACP.  I wanted to include the pressure either as MAP or CIP or SAAMI pressure multiplied by the ratio.  That would make the pistols even lower as well as 30-30, but would have little effect on the center and right of the chart as most of these are 50,000 psi and above anyway.  from 60 to 50,000 psi would only make a 17% difference, but that is still a difference.  

Duplicate 7-08 cause I found two different case capcities online.  I trust the one on the left as the one on the right had more case capacity than its parent cartridge 308 which cant be right.

Duplicate 6.8 cause of the variance in different manufactures cases

300 rum bul - case capacity under bullet for someones particular load.
300 rum - complete empty case capacity

223 and 556 cause of the variance in available data on google and Wikipedia.  They are in the same ballpark so who cares

In any rate, most the duplicates are in the same ball park. I could just pick one to delete with little difference.

I wanted to do this as case capacity under bullet times either average pressure, MAP, or SAAMI pressure or something.  However, it took more than one lunch break to put this together.  I could find total case capacity easy enough, but it gets tricky to find capacity under bullet as that varies by load.  It can be done easy with excel just by knowing bullet length, case length, and OAL with a simple formula assuming flat based bullets and subtracting out the bullet volume.....but ....THAT WOULD TAKE FOREVER to collect all that data.  Adding the pressure would be easy as far as the formula, but take A LOT OF SCREEN TIME to find and enter.  

Capacity under bullet only made 1" difference for 300 RUM.  For 300 black and the pistols, it would make a HUGE difference.  300 subsonic would go lower too if pressure was a part of the equation.

My point of view is this showed me what I want to know and put it in a redneck friendly format I can understand (barrel length).  The raw data was a meaningless, unitless number.  I like what the correction factor does as far as appearance and relevance.

As far as missing data, sure I didn't get every caliber and could have put in 30 carbine, 380,, 338 lapua 375 whizbang or any of the PPU or PPCs, If you want to complete the data, I can send it to you with the equations ready to rock.  I have put enough time into it to learn what I wanted to know.

You can definitely pick out "classes" of calibers.  At right you have the known barrel burners.  257WBY, 26 Nos, and 7 RUM.  These are known to prefer very long barrels, but most manufacteres top out at 26".  Still, these top the charts even without considering pressure.

50bmg, 264WM, 300RUM,and 28Nos.  The heavy magnums.  Also demand over 24". I was surprised to see 50bmg here, but it does have a massive bore to counteract the "overboreness" factor.

7Mag, 25-06, 300WBY, and 300 mag.  Guess what, most manufacturers sell these in .......... 24" barrels.

270 and 243, the classic "sporters" sold with 22" barrls, though varmit rigs let the 243 stretch out.

Then the 338win to 375 H&H category.  From small to large these are all in the same class.  Theoretically you could put a 243 bullet in a 6.8 case and start this list off, then a 6.5 creedmore (308 based), then a 30-06 (30-06 native), 338WM (magnum based), and finally full blown 375 H&H for the collarbone breakers.  The 7-08 shown does not belong. That data point is an error.

correct 7-08 kinda floats in between here

next group starts with 5.56, then would probably have the 6.5 Grendel had I put it in here, then go up to 308 (308 native case), 338-06 (30-06 case).  It was interesting to see our US military calibers 556 and 308 fall in the 18" barrel range as the first ARs were 20" and many hunting AR10's are 18" or 20" rather than sporter 22" or 24" that ammo is rated at.

Below this, 6.8 falls around its happy 16" and things go downhill.  300 black would be lower than 12" if I was using case capacity UNDER bullet becuae it is nearly a straight wall cartridge.  The pistol cartrides would go off the charts low if pressure was included plus they would lose a higher proportion of case capacity to the back of the bullet than all the rifle cartridges.  

Basically, I say everything from 6.8 on to the right is good comparison between calibers.  I can tweak the formula to make ANY cartridge fall at ANY barre length you want.  You just have to pick something on the left and something on the right and accept where things fall in the middle.  I picked 45acp at 5" and 270win at 22".

The data is UNITLESS, so I can do what I want with it and make it show what I want.  I could make it be portions of a marshmellow and it would still have some cryptic meaning as far as comparison of one cartridge to another.  I just picked barrel LENGTH as barrel LIFE would be too subjective and foreign to most people.  My brain can follow barrel length easy enough  Besides, my favorites all fall where the industry sells them at.

I was there is 1992....

rec.guns ›
Calculating Rifle Barrel Life
1 post by 1 author  



Bart Bobbitt

11/4/92


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.

Interesting formula. With my 6.5 Creedmoor loads it's saying between 1800 - 2150 rounds...but I'm pretty sure guys are getting more life than that from 6.5 CMs.

Cool none the less and thanks for sharing