Rifle barrel Fluting?

[VKC]

I understand that fluting a barrel ideally decreases the weight of a rifle without sacrificing much stiffness, and can cool the barrel faster. However, fluting a barrel in different ways may effect accuracy. Fluting also effects barrel vibrational harmonics and frequency.

What are your thoughts on straight vs. spiral fluting? Even vs. odd number of flutes? Grinding vs. milling for fluting? Any other different types or methods of fluting worth mentioning? Anyone care to comment on physics of barrel fluting?

 

[GunjunkieM24]

Re: Rifle barrel Fluting?

Not a Physicist but I have had all of the above and have never lost accuracy on any.

I have had straight 6 and 8 flute barrels ( deep and shallow ), 6,8 and 10 flute spirals and never had an issue. I usually shoot the rifles after first assembly for testing and function checks. I then disassemble them and send them off for engraving and fluting....never seen a loss in accuracy...none.

All of my barrels have been either fluted by Rock Creek, Kreiger, Shilen or Kampfeld Customs. All do great work and all used a mill.

Hope this helps...

 

[Mosdet]

Re: Rifle barrel Fluting?

I read once that the proper way to flute a barrel is to do it prior to boring it. This makes a lot of sence to me as a non-technical person. However, in reality I have no idea

 

[Richard Clark]

Re: Rifle barrel Fluting?

I have heard that fluting does not actually help in cooling at all. I read a really convincing argument, can't find it though.

I don't really like the look of most flute jobs and consequently have never bought one. I think I would only get the flutes on a a .338LM so I could comfortably stalk something - from 1000m away

 

[Hateca]

Re: Rifle barrel Fluting?

Most of what you have read regarding fluting is conjecture and or speculation.

Bottom line you want fluting be sure the barrel maker will cover their barrel if fluted, some don't.

You want fluting get it because it looks cool, other then that you are back to conjecture and or speculation.

 

[RobertB]

Re: Rifle barrel Fluting?

How much weight does it really save?

 

[bcw1284]

Re: Rifle barrel Fluting?

That depends on the barrel contour, and the size/depth/quantity of flutes.

 

[RobertB]

Re: Rifle barrel Fluting?

I mean best case senario. How much weight? can't be much I wouldn't think

 

[Hateca]

Re: Rifle barrel Fluting?

You'll save more weight by cutting the barrel shorter plus it will balance better.

 

[bohem]

Re: Rifle barrel Fluting?

Here's some thoughts that I have on fluting a barrel, and the cooling ones are not necessarily positive.

1) HateCA nailed pretty much all of it. I'll try to address some finer details, but he's right on the money with the advice. Some of the heat interactions are simplified as I'm trying not to lose people with it, but it's somewhat boring and technical.

Fluting for reduced cooling times is an interesting thought. Yes, the surface area goes up, but I'm not convinced this is a good idea... just a heads up.

In quiescent (stagnant) air around the barrel you'll get a slightly induced flow from the hot air rising of the barrel and cool air circulating in to replace it. This is a very low speed flow and is not a dominant term in the heat flow calculations.

It would be a dominant flow if it was ducted properly, but it's completely free form, so it's essentially "in the noise"

In a windy environment you've now placed a hot object into a free stream to increase the heat flow from the barrel, however, the Rayleign and Nussault numbers we're dealing with, plus the shape of the barrels flutes (specifically the round cross section and sharp edge on the flutes) turns the flow into turbulent. The heat flow in turbulent boundary layers is much less than a laminar boundary layer. We're talking 25% rates.

So the fluting tripped the boundary layer and cut the heat flux by a factor of 4. Do the flutes increase the surface area by a factor of 4 or more?

since Q = h*dT*A

Q = heat flux (Watt)
h = heat flux coefficient ( Watt/m^2)
dT = Temp difference between 2 materials
A = surface area

h is calculated based on the Rayleigh number, in turn the Nussault number, and both of those are affected by the Reynolds number, which tells you about the laminar vs. transitional vs. turbulent boundary layer.

So back to the turbulent vs. laminar balance and how the surface area plays in above.

Turbulent flow with 50% more surface area is going to be a wash or in some cases, a hinderence to cooling.

Keep in mind as well that the best rifle barrels are ones that shoot in a stress free state. IE, the barrel doesn't twist or warp as the barrel heats/cools.

If you heat a barrel up and then stick it in an air stream like th wind you will cool the upwind side much faster than the downwind side. The heat induced stress from having a temp difference from the upwind to downwind side of the barrel can now affect the shots.

Now lets talk about even vs. odd numbers of flutes. Symmetry in load path is key for this game. There's a reason why our bolt rifles have evenly spaced locking lugs, the AR has 8 lugs that appear to lock, but in reality 1 of those belongs to the extractor and it's cut short so it doesn't engage, the opposite lug from it is also cut short for to disengage it. It locks only 6 of the lugs. This aids in the repeatability of the load path.

In regards to that reason, I won't take an odd number of flutes in a barrel UNLESS they're spaced evenly. A 5 flute barrel must have flutes spaced every 72 degrees on center. A 7 flute is 360/7 degrees, etc.

If they're spaced properly there should be no difference between a 3 flute, 4 flute, or a 14 flute barrel.

Grinding vs. milling: I have no experience here for fluting a barrel so I'll refrain comment.

Spiral vs. Straight flutes: Again, no immediate experience, I'll refrain comment.

I have some ideas on both the grinding question and the spiral question but they're simply conjecture and I don't want to throw them in here without data to back it.

Fluting before vs. after boring. I don't think it matters as long as the barrel is properly stress relieved AFTER the boring and fluting are both done. Straight bores are the idea, but we all know that the barrel is a banana or an S or a double S, the goal is to make sure that whatever shape it likes to be in is

1) Not bound by internal stresses induced from processing
2) Stable
3) Pretty close to straight so that you're not fighting the scope trying to get the rifle dialed and shooting.

We've seen that a heavy barrel and a light sporter barrel can both shoot great over and over again, but they must be properly stress relieved and processed. My feeling is the same with the fluting process.

Is this nit-picking? Absolutely
Is it measurable in controlled conditions? Absolutely

With the extra cost, lack of real world applicability to the heat flow increase and the weight reduction being minimal I'd just as soon pack a slightly heavier rifle or chop the barrel 2" shorter than pay for the fluting.

None of my comments are meant to offend anyone, they're rooted from doing some calculations, dealing with the barrels first hand, and seeing the various configurations out there in hunting and range conditions.

My bottom line on fluting:

Do it if you like it, don't do it because you think it's going to improve the performance of the rifle. Save the money for fluting and spend it on components for more trigger time.

 

[J-Ham]

Re: Rifle barrel Fluting?

bohem, that post kicked ass! Very well written. I think I've just double the amount of knowledge I've learned this year. Thank you.

 

[bohem]

Re: Rifle barrel Fluting?

Thank you, sorry it was so long, I know that loses people sometimes, myself included.

 

[TrooperBrian]

Re: Rifle barrel Fluting?

I found the answer to this through a long course or research on "barrel breakin". And like barrel break in, barrel fluting is largely a trend.

Barrel break in only came around when certain higher up manufacturers started shipping these "break in procedures" with a barrel. It caused people to clean their barrels so aggressively, they would be forced to buy more barrels sooner.

And like fluting, one person did it, and then everyone did it, not because it actually did anything, but because the disease made everyone THINK they did something.

Shoot it, clean it when you're done.

I challenge anyone who thinks fluting is worthwhile to give evidence of it, everything so far has been inconclusive at best.

Save your money.

 

[VKC]

Re: Rifle barrel Fluting?

Thanks bohem for great explanation, I also learned a lot.

At least for me I don't think the cost of fluting a barrel after factory production is worth it for me. Agree if you really want, just got the barrel shortened and that will do the trick for cheaper with better balance.

As far as barrel vibration harmonics, I believe that fluting does affect this. Even, odd, spiral, straight, etc...Would have to effect it is different ways...but not sure if it effects accuracy adversely if done correctly.

Any thoughts on atypical fluting like the Knight's Armament PDW barrel with divots???

 

[bohem]

Re: Rifle barrel Fluting?

vkc- I don't have any experience with them, I can't comment on it, sorry. They do look pretty cool though.

 

[Trevor]

Re: Rifle barrel Fluting?

Barrel fluting does it have benefits, I believe so. Either with the ability to cool the barrel at a higher rate or to improve rigidity.

** When I say that it will improve rigidity I mean it in the context that a light varmint barrel would not be as stiff as a heavy varmint barrel that has been fluted so the fluting would equal the diameter of a light varmint barrel.

Here is an article that is an interesting read


supported evidence
A. Weight is directly proportional to cross-sectional area. For instance, the weight of the barrel is equal to the cross-sectional area multiplied by the length of the barrel, and finally multiplied by the material density of steel.
B. Stiffness (rigidity in engineering lingo) is directly proportional to Moment of Inertia.
C. Surface area is directly proportional to the outside perimeter. For instance, the surface area of the barrel is equal to the perimeter multiplied by the length.
Heat Transfer
There is another misconception about fluting in relation to cooling the barrel.
Some people believe that fluted barrel cool off faster than regular barrel because the surface area of a fluted barrel is greater than a plain one. Fluted barrels indeed cool off faster than a plain barrel of the same diameter, and not because of surface area, it is because of other factors.

here is the entire article
http://www.snipercountry.com/articles/realbenefitsbarrelfluting.asp

 

[bohem]

Re: Rifle barrel Fluting?

Trevor, that's an interesting article although I do feel it necessary to point out a couple of points on keeping mass in the barrel.

Some considerations:

1) In a string of fire is it reasonable to expect that the rifle barrel will cool off from an airstream? Few would be able to say yes except under very specific considerations.

Instead of removing material from teh barrel that can conduct heat away from the bore quickly, let's leave it in there adn allow the thermal mass of the barrel to absorb this heat quickly as opposed to conduct it a short distance to the root of the flute where it now has to bridge the air gap to cool further.

Correct, if you take a light barrel and a heavy barrel and heat soak them to the same temperature the light barrel cools off faster. Let's think about it in reverse, put both barrels in the freezer and heat them up. How long is it before the barrel warms to room temp?

The author of that article is correct in stating that less mass of a given material will cool faster, however, the way in which it cools is suspect. Large agressive flutes to remove a large portion of the mass in the barrel will cut the time to cool, however it will also greatly increase the rate at which the barrel heats. Additionally, the heat flow from the bore to the outermost surface of the barrel is going to be almost the same as the previous, full cross section. Now you have non-uniformity across the barrel.

2) The stiffness difference in a fluted vs. a non fluted barrel is easy to show based upon cross sectional moment of inertia. However, let's talk about specific stiffness. IE, stiffness to weight ratio. The stiffness of a pinned/free column is dependent upon the length^3. If you take a 24" barrel and cut it back to 20" you have (5/6)^3 change in stiffness, specifically, 1/.58 = 173% increase for a straight taper barrel. A contoured barrel gets even higher improvement because the lightest region of cross section are being cut off.

I'd like to see a safely fluted barrel of a reasonble contour that comes up with a 173% specific stiffness increase.

The real benefits of barrel fluting is written well, however, from my engineering expertise I would rate it as "close, but not quite close enough". It's possible that I'm wrong and I would love to see that happen, I'll flute the barrels of my 17lb long barreled 7mm and have something reasonable to carry around the woods as well as shoot competitions with it.

Until that happens, I am going to save my money on fluting and buy more barrel blanks and components to shoot them out with.

 

[sigma2chi]

Re: Rifle barrel Fluting?

As far as fluting being good or bad, in a "Do" or "Don't Do" it scenario...I'de say a barrel with cut rifling should/could be fluted without affecting accuracy. A button pulled barrel should never be fluted because of the of the inconsistencies (did I spell that right?) in pressure relief left in the barrel from the button pulling process.

just my 2 cents

 

[VKC]

Re: Rifle barrel Fluting?

Thinking off how fast a barrel cools off is one way of looking at it. But another way to think about this, is how fast a barrel heats up. The more mass a barrel has the slower it heats up. So if you take a barrel and then flute it, it has less mass. In another words, the non-fluted barrel actually heats up slower then the fluted barrel. Or a fluted barrel heats up faster than a non-fluted barrel.

Now if you take a fluted barrel of equal mass to a non-fluted barrel, that is a different story.

The next factor to consider is that a fluted barrel has uneven heat dissipation compared to a non-fluted barrel. The question is whether or not this effects or alters accuracy with uneven heating and cooling of a fluted barrel. Maybe Bohem can answer this from engineering perspective.

 

[bohem]

Re: Rifle barrel Fluting?

I think you hit it pretty well, I was making a point about keeping the barrel closer to ambient longer, which the extra thermal mass of a non-fluted barrel will do.

If the flutes have a proper symmetry there should be little difference, however, as I mentioned in the VTR triangular barrel thread, you should not flute a barrel in such a manner that it allows for a "soft axis" that the rifle can bend/vibrate about.

 

[sotexhill]

Re: Rifle barrel Fluting?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: bohem</div><div class="ubbcode-body"><snip> In a windy environment...the shape of the barrels flutes (specifically the round cross section and sharp edge on the flutes) turns the flow into turbulent. The heat flow in turbulent boundary layers is much less than a laminar boundary layer. We're talking 25% rates.
<snip> </div></div>

I am surprised that the fluting tripping the boundary layer flow results in reduced heat flow from the barrel to the air. With automobile radiators, laminar flow of the coolant and air through and around the tubes is deliberately avoided so that cooler air is put against the outer surface of the tubes and hotter coolant is put against the inner surface of the tubes. The increased temperature differential between the air and the coolant increases the heat flow from the coolant to the air. I would expect that breaking the laminar flow around a rifle barrel would also improve cooling. But no engineering background or test data to back up my intuition/speculation.

 

[karagias]

Re: Rifle barrel Fluting?

Here is an interesting thought regarding a "soft-axis" and vibration and how flutes can play a role.

So long as the rifle’s center of mass does not coincide with the bore axis along which the recoil force will act, the rifle will undergo translation and rotation about its center of mass as the bullet and gas accelerate down the barrel. The angular acceleration imparted by the recoil causes the barrel to vibrate. Designing the rifle so that its center of mass coincides with the bore axis, thereby eliminating the angular acceleration responsible for much of the barrel vibration, is usually impractical, especially because the mass of the rifle changes as ammunition is consumed. We must therefore find some way to better deal with the barrel vibration.

It is conceivable that by fluting a barrel we can constrain the muzzle to vibrate at low frequency on a plane upon which lie both a “soft-axis” and the rifle’s center of mass. A low frequency is important as it offers the best chance for us to consistently pass the bullet through the muzzle while the muzzle is either at the top or the bottom of the wave characterizing its vibration.

People often assume that stiff barrels will result in better accuracy. That may be true if the barrel is stiff enough to adequately control the muzzle position during firing. But a stiff barrel will likely vibrate at higher frequency than a compliant one making it more difficult to consistently time the exit of the bullet with a unique position of the muzzle as it vibrates. When you work up a load by varying powder charge, you are effectively changing this timing. The Browning Boss does the same thing but in a different way.

Refer to Chapter 4 of Harold Vaughn’s Rifle Accuracy Facts for more on barrel vibration. No discussion of rifle accuracy can be complete without reference to Vaughn’s book.

I think a strong case can be made for improving the accuracy of a rifle by intelligently fluting the barrel to “soften it up.” As for the heating and cooling effects of fluting, I will leave that up to Bohem. He’s is undoubtedly our resident expert on heat mind mass transfer and I would like to thank him for the engineering rigor exhibited within his posts. Nice job Bohem.

 

[pjparker]

Re: Rifle barrel Fluting?

New to this forum, but will toss in my two cents worth....I shoot both midrange and longrange F Class and a 20 shot match with two sighters can heat up a barrel quickly, since I tend to shoot fast to minimize the number of condition changes to have to dope. I'm shooting an RPA Quadlock with a Krieger #17 heavy varmit contour barrel with Krieger's 10 groove straight fluting.
Barrel's chambered for .260 rem. Other than a one pound reduction in weight, I have not seen any other advantage to having ordered the barrel with flutes. It might cool a little quicker once it's put in a verticle rack and convection can draw cooler air along the length of it, but that's subjective. The barrel is still blistering hot when firing is complete so I,m of the opinion that if you want a cooler running barrel, look elsewhere.

Paul Parker

 

[karagias]

Re: Rifle barrel Fluting?

I too shoot high-power with a Jim Cloward built 308 match rifle, RPA Quadlock + 34" Krieger barrel #14 Heavy Palma contour, no flutes. Barrel is warm to the touch after a 22 round match, perfect for warming hands on a cold day.