Rigidity of Octagonal Barrels??

19 November 2003, 04:51

Anybody care to venture a guess at this one:

When dealing with fluted barrels, I believe the current gospel has it, that the rigidity of the fluted barrel is the same as an unfluted barrel of the same diameter. Presto: by fluting you save weight, but don't sacrifice rigidity by going to a slimmer barrel contour. (This quite apart from possible advantages of heat dissipation found in fluted barrels).

Question: does the same principle apply to octagonal barrels?? E.g. a round barrel of muzzle diameter of .625" mills up at .577" across the flats as octagonal (all according to Lilja's Contour Page). Does the octagonal barrel still have the same rigidity as the .625" diameter original??

- mike

[ 11-18-2003, 19:57: Message edited by: mho ]

19 November 2003, 06:11

wallyw

Barrels of the same weight are as stiff. Flute or put flats on a barrel and it looses stiffness.

Very little is gained in terms of cooling ability. They do look good [Wink]

19 November 2003, 06:35

Stu C

Mike,
Can't answer your questions but you might like to look at Mark Stratton's web page http://www.gunmaker.net
I take it Oct barrels are one of his specialties.

I've also emailed you a scanned article that he wrote in "Gunmaker" journal.

19 November 2003, 08:59

m1carbine

mho, You seem to be a little confused on the stiffness of fluted barrels. A fluted barrel is not as stiff as the barrel of the same external demensions. It is stiffer then a barrel of the same weight. Mass is mass, if you remove it you make the barrel weaker, there is now way around it. An octagonal barrel should be just a little weaker then a round barrel it was milled out of. But it would be stronger then a round barrel of the same size. Clear as mud?
Ray [Wink]

19 November 2003, 09:49

Iron Buck

I disagree. Mass is not stiffness. A steel "I" beam gains its stiffness and strength from it's shape. It is stronger than a "chunk" of steel the same size/mass of a rectangular shape.The ridges of an octagonal or fluted barrel have the same effect. They cause a stiffer barrel than that of the original outside diameter. They are stiffer and with less mass. This is the advantage of having barrels made this way.

[ 11-19-2003, 01:02: Message edited by: Iron Buck ]

19 November 2003, 09:59

mete

Iron Buck is right. If I remember the technical term is 'polar moment of inertia' . Stiffness is often confused with other things.

19 November 2003, 11:31

<Marc>

Iron Buck is wrong! A fluted barrel will be stiffer than a round barrel of the same weight. A fluted barrel will be less stiff than a round barrel of the same diameter.

Take your stiff I-beam and make the sections thicker, but keep the same height and width. I think you will agree the thicker beam is stiffer.

19 November 2003, 12:06

m1carbine

Iron buck, Your comparing two different things. Compairing an I-beam to a round bar is not related to this problem. If you were to flute an I-beam it would not be a strong as the un fluted I-beam, but would be stronger then one of the same weight. As for barrels a 1 inch untapered barrel will be stiffer then a 1 inch barrel with flutes. The 1 inch fluted barrel will be stiffer then a 3/4 inch barrel that weighs the same as the fluted 1 inch barrel.
Ray

19 November 2003, 12:09

Iron Buck

I am sorry but you guys are wrong. take 2 barrels of the same diameter, one fluted and one non fluted. The fluted barrel will be stiffer. It is because of the creases of the flutes adding stiffness to the shape.

19 November 2003, 13:00

m1carbine

Iron buck, What you are proposing has been done and it proved that the fluted barrel isn't as stiff as an unfluted barrel of the same diameter.
Ray

19 November 2003, 13:16

m1carbine

Iron buck, here are some links you sould look at. foulton armoury , Score High Gunsmithing.
Ray

19 November 2003, 13:26

jeffeosso

number 1 - FLuting is 99.9% of the time a matter of preference... is DOES look cool, on the right gun....

people mix this issue up all the time...

FOR THE WEIGHT AND length, a fluted barrel is stiffer... as the same weight and length solid barrel is a rather easily bent soda straw,

for the length and diameter, the unfluted barrel is stiffer.... and heavier...

so, remember, if the outside diameter of a barrel is say a 1" straight taper, and the length is 24", the FLUTED barrel will weigh less, and the unfluted will be stiffer....

and then again, the same barrel, at 18" will be MORE rigid....

and, any frickin way you look at it, the fluted barrel has more SURFACE AREA to dissipate heat....

which is why weatherby uses fluted barrels on it's ultralights, and nothing else....

and why remington seems to like them on all their "sniper" guns.... who ever heard of a sniper laying down a field of fire that would make the marginal increased heat signature significant?

<wow, I didn't read the fulton's site till after i posted>
jeffe

[ 11-19-2003, 04:28: Message edited by: jeffeosso ]

19 November 2003, 18:27

45LCshooter

Stiffness of any cylinder is related to its diameter. Fluting takes a stiffer barrel (a barrel of greater diameter) and attempts to make it lighter. It will be stronger than barrels of the same length and weight.

Now for a pointless exercise: for a given barrel length and weight, a .458 bore barrel is much stiffer than a .270 while a .270 of the same diameter would only be negligibly stiffer than a .458 because the metal on the inside of a cylinder contributes less to stiffness than the metal at the circumference.

[ok, jeffeosso and I did the same thing, that is a good web page. As far as cooling goes, that is purely a matter of surface area - with fluted barrels, I wonder if the added surface area makes up for the loss of mass....]

[ 11-19-2003, 09:40: Message edited by: 45LCshooter ]

20 November 2003, 01:11

Arock

So getting back to mho's original question, are octagonal barrels similar to fluted barrels as pertains to weight/diameter/stiffness/rigidity?

I've pondered if those Winchester rifles of the 1870's were on to the same principle as fluting but using available technology to achieve it.

Or maybe not.

[Big Grin]

20 November 2003, 02:44

Bill M

mho,

Here's my two cents....

To perform stiffness analysis of a barrel, you use the equations for a simple beam. Assuming equal material strengths, it comes down to the moment of inertia. Moment of inertia is area times distance (from an axis or a plane). Larger moment of inertia is why a 10 pound/foot I-beam is stiffer than 10 pound/foot round or square bar stock. Same "area", but distance from center is increased. Also why an I-beam carries more load when load is applied in the web direction ("I") than flange direction ("H").

The octagon barrel you describe would be stiffer than a round barrel .577" in diameter, but less stiff than a round barrel .625" in diameter.

Fluting a barrel reduces the moment of inertia as compared to an identical un-fluted barrel, and is less stiff. The number one reason for fluting, IMO, is to reduce weight.

For more on barrel stiffness, Dan Lilja has an excellent article on his website:
http://www.riflebarrels.com/articles/barrel_making/rigidity_benchrest_rifles.htm

Regards, Bill

20 November 2003, 04:22

mho

quote:
Originally posted by Bill M:

The octagon barrel you describe would be stiffer than a round barrel .577" in diameter, but less stiff than a round barrel .625" in diameter.

For more on barrel stiffness, Dan Lilja has an excellent article on his website:
http://www.riflebarrels.com/articles/barrel_making/rigidity_benchrest_rifles.htm

Regards, Bill

Thanks to all for providing feedback. I can't say that I have a full understanding about what the effects of milling an octagonal barrel will be in terms of barrel rigidity, but the statement above probably pretty much sums it up - even if it does not quantify the effect - which I could hardly expect, given the complexity of the calculations involved (see Lilja article quoted by Bill). That is fair enough.

The following statement from the Lilja article deals with the effect on barrel rigidity by fluting:

quote:
From Lilja barrel stiffness article

Fluting a barrel removes weight, up to one pound or so depending on flute size. It also lowers a barrel's moment of inertia value but not by very much. Some have the mistaken idea that fluting alone increases the stiffness of a barrel. This is not true. The fluted barrel of a given weight and length will be stiffer than an unfluted barrel of the same weight. The fluted barrel will not be stiffer than the same taper and length barrel that is not fluted though.

The Lilja article explains the reasons for the above, but I'll leave that to the article.

I think that the long of the short of this matter for me is: I was probably hoping for too much in terms of barrel stiffness in an octagonal barrel. I probably will loose some rigidity when compared to the original round barrel size needed to mill the octagonal. How low I'm willing to go in (octagonal) contour will depend very much on barrel length, and (to a lesser degree) on bore diameter. Unfortunately, by switching to an octagonal shape, I seem to have lost the "rules of thumb" that I have seen used for round barrels: either: "make sure barrel diameter is .30" bigger than bore" or to be on the safe side: "make sure barrel diameter is .36" bigger than bore" (either of these rules of thumb to be applied to sporter barrels, only - probably). Obviously, it is possible to get thinner barrels to shoot well, length of barrel greatly influences rigidity, as does bedding. But the long and the short remains: it is easier to get a "thick" barrel to shoot than a "thin" one.

Again, thanks to all who contributed. I guess I still have something to think about... [Wink]

- mike

[ 11-19-2003, 19:24: Message edited by: mho ]

20 November 2003, 04:32

Bill M

mho,

Now I think I understand what you are getting at.

Minimum "safe" barrel muzzle dimensions for .XXX" bore diameter. Whatever you calculate for your minimum round barrel diameter would have to be your minimum "across the flats" dimension for the octagonal barrel, for equal strength. I forget what the formula is, though.

Regards, Bill

20 November 2003, 06:17

Arock

quote:
Originally posted by Bill M:
Also why an I-beam carries more load when load is applied in the web direction ("I") than flange direction ("H").
[/QB]

So is an octagonal barrel in the web orientation with a rib at TDC or a flat at TDC?

[Confused]

20 November 2003, 06:46

Bill M

quote:
Originally posted by Arock:
So is an octagonal barrel in the web orientation with a rib at TDC or a flat at TDC?

Apples and oranges - but short answer "rib" (or corner), due to higher moment of inertia.

I was alluding to the fact that an I-beam is stronger when loads are applied perpendicular to the flanges (web direction), because the moment of inertia of an I-beam about the x-axis is greater than the moment of inertia about the Y-axis.

[ 11-19-2003, 21:54: Message edited by: Bill M ]

20 November 2003, 07:03

Arock

Thanks Bill M that's what I couldn't figure out. I'm not an ME in real life nor do I play one on TV although I have stayed in a few Holiday Inns. [Big Grin]

Looking at a recent production Winchester M70 octagonal barrel it has a flat at TDC. Oh well.

[Roll Eyes]

Where do lines of stress accumulate in this type of structure?

Thank you.

[ 11-19-2003, 22:08: Message edited by: Arock ]

20 November 2003, 07:08

Bill M

Arock,

It really doesn't matter which way the flats are oriented on a rifle barrel, because the "loads" are not applied in a particular x/y axis direction, as they would be in a (gravity) loaded I-beam. For all pratical purposes, the Octagonal barrel could be considered a cylinder.

20 November 2003, 07:09

Arock

Ah! Lowly grasshopper says thank you.

[Wink]

20 November 2003, 07:32

Bill M

Arock,

Just to clarify, you may have "got me" on that rib/flat question [Wink]

....

I went with my first impression, which still could hold true, but I would have to calculate the moment of inertia of that octagon about an axis through corners vs. through the flats, to be sure. Whichever is greater would be the "web" direction, and after further thought, it is going to be very close.

When you fire a gun, the barrel whips around in what could be best described as a "sine" curve, but in two axis, not one. Whip could be side-side, up-down, or, more likely, in a combination....

We know exactly how we are loading that I-beam.

What we are doing when we "tune" a load to a particular barrel, is getting the bullet to exit the muzzle when the barrel is pointed in basically the same place for each shot, at the "peak" of the sine wave, where the curve is relatively flat.

Sorry, but I do not have time right now to calculate the moment of inertia of that octagon through the flats/corners, so I rescind my previous answer - I abstain [Big Grin]

!

Regards, Bill

20 November 2003, 07:41

edi malinaric

Darn it BillM - why did you dodge?

Anyway just to unpuzzle everyone. An octagon has the same Moment of Inertia across the flats as across the points - just one of those happystances.

cheers edi

20 November 2003, 07:43

Bill M

Thanks Edi - I didn't want to do the math!

Bill

20 November 2003, 07:45

Arock

Thank you again Bill. It's really the middle of a workday for me too. [Cool]

I asked about stress accumulation not thinking about the obvious internal stresses of firing the projectile through the barrel. [Embarrassed]

My thoughts were centered on gravity effects on lines of stress in the barrel. I'll take your word whether GRAVITATIONAL stress is better distributed with rib or flat at TDC.

[Razz]

20 November 2003, 07:47

Arock

quote:
Originally posted by edi malinaric:
Darn it BillM - why did you dodge?

Anyway just to unpuzzle everyone. An octagon has the same Moment of Inertia across the flats as across the points - just one of those happystances.

cheers edi

Edi thank you. Your typing skills are superior to my thought processes. [Wink]

BTW is the tablecloth on the mountain today?

[Big Grin]

20 November 2003, 20:38

edi malinaric

Hello Arock - from my balcony

Swells running at 2 - 3 m building.

Gannets diving for breakfast. A couple of seals making passage to Robben Eiland. No whales in sight - or sound!

Coastal low - calm - low cloud up to 1500'- max temp 22C.

Forecast for later in the day - light northwesterly flows increasing in strength with 40% chance of light rain on Saturday and Sunday.

Damn - I was hoping to take my G43 down to the range.

Don't you wish that you were here?

cheers edi

21 November 2003, 00:50

Arock

Yes I do wish I was there.

[Cool]

Best,
Arock

21 November 2003, 01:00

D Humbarger

Where is Saeed!? This calls for a test to settle this issue like the one he did with the barrel crown. In this case use two barrels the same length & weight, same material, one fluted the other unfluted. The proper measuring equipment & determine the force required to bend the barrels.
This would be very interesting. I will contribute $20.00 toward the purchase of the barrels. Of course the barrel will be junk after the test but this is in the name on science! [Big Grin]

What say ye?

[ 11-20-2003, 16:00: Message edited by: D Humbarger ]