The Real Benefits of Barrel Fluting

12 August 2004
By Joel Avila, 1LT(P), EN, USAR

Introduction

I have been into precision / benchrest shooting for three years. I am the type of guy who believes in the saying "He alone is educated who has learned the lessons of open-mindedness." I never hesitate to ask an expert in the field about the art and science of precision shooting. If an answer is not satisfactory, I always turn to technology: The Internet. Dozens of forum sites exist for the sole purpose of educating shooters on this specific subject. From reloading to custom barrel making, the Internet is an excellent source of information.

I have learned so many things about the sport. My knowledge (albeit limited compared to a pro) developed as a result of learning from the old timers at the local shooting range, doing my own research on the Internet, and just from good old trial and error. Most information I hear or read makes good scientific sense. Unfortunately, others are more or less tribal knowledge that have been passed on from one generation to the next. These bits of knowledge are nothing more than misconceptions. In my opinion, misconceptions are like communist propaganda in a sense that the more the masses are misled, the more the people will believe what they hear is real. For instance, the topic of fluting a barrel carries certain misconceptions. Conventional wisdom states that "fluting dissipates heat quicker and it also adds stiffness."

I would like to point out that I am a Mechanical Engineer with seven years experience. I heard the "benefits" of fluting when I first got into precision shooting, and I have always wondered as to how fluting "increases stiffness." Initially, this claim did not make any sense to me because common sense dictates that reducing material on anything will also reduce its strength. One does not have to be an engineer to realize this. However, because I was new to the sport at the time I wanted to respect the knowledge of those trying to help me out. I will probably receive lots of controversy on this subject, but I am ready because science and math have my back covered.

The purpose of this article is to shed some light without confusing the reader with a bunch of boring engineering mumbo-jumbo.

Facts

These facts are supported by basic Physics 101 learned by all freshmen engineering students.

Test materials
  1. Savage 10 FP chambered in 308:
    • Outside Diameter: 0.840 inch
    • Flutes: None
    • Cross-sectional area: 0.493 square inch
    • Area Moment of Inertia: 0.024

  2. Savage 12 BVSSF chambered in 308:
    • Outside Diameter: 0.840 inch
    • Flutes: six 3/16" side, 3/16" deep, full radius
    • Cross-sectional area: 0.312 square inch
    • Area Moment of Inertia: 0.014

  3. Custom Made Rifle / Light Varmint Barrel chambered in 308:
    • Outside Diameter: 0.70 inch
    • Flutes: none
    • Cross-sectional area: 0.312
    • Area Moment of Inertia: 0.011
Results

  1. 12 BVSS Fluted (OD 0.850") versus 10 FP Plain (OD 0.850")
    • Weight: 38% less
    • Stiffness (as a function of Moment of Inertia): 43% less

    The fluted barrel is much lighter, much less rigid, but has much more surface area than a solid barrel with the same overall outside diameter.

  2. 12 BVSS Fluted (OD 0.850") versus Light Varmint (OD 0.700")
    • Weight: Same
    • Stiffness (as a function of Area of Moment Inertia): 25% more

    The fluted barrel is much more rigid, and has much more surface area than a solid barrel of the same weight.
Additional Analysis

Deflection at the Muzzle

If you are still not convinced that fluting DOES NOT increase stiffness, I offer the following theoretical explanation.

Let us take all three barrels and place them horizontally in such a way that we permanently affix one end to a fixed position and the other end will basically be free-floating. In other words, let us say that we are welding the threaded end of the barrel to the side of an M1A1 Main Battle Tank. Let us say that the weld is so strong that it is impossible to break the connection. Now, let us say that we will exert a vertical force of 500 pounds at the muzzle of the barrel. So basically, the threaded end is fixed to a tank while a 500-pound man is standing at the muzzle of the barrel. Do you want to know how much the muzzle end will deflect?

The 10FP (non-fluted) will deflect 4.4 inches. The 12BVSS (fluted) will deflect 7.5 inches and the Light Varmint will deflect 9.5 inches. Enough said.

The Ridged Roof Analogy

Others may refer to the ridged roof example. Basically, the argument is that if you take a plane sheet of metal and bend it as to make several ridges, the overall strength of the metal sheet will increase. Therefore, the argument goes on, if you close the ends of the sheet of metal forming a cylinder, the overall strength of the ridged cylinder will also be stronger than a plain sheet of metal that is also formed into a cylindrical configuration. And bingo, based on this analysis, it is assumed that fluting a barrel is similar to adding ridges to a roof.

It is true that adding ridges to a plain sheet of metal will definitely add strength. The reason being is that the Moment of Inertia of a ridged sheet has increased simply because for this type of configuration (flat and not cylindrical) the Moment of Inertia is a function of the overall width of the material. Simply put, a regular sheet of metal has, let's say, 1/8th of an inch. With the ridges added, the overall width is now at least 1 inch, depending on how the ridges are folded. The higher the overall width of the sheet, the higher the strength. So once again, adding ridges to a flat sheet of metal will strengthen its property.

But if you take a ridged sheet of metal and a plain sheet of metal and reconfigure them into a cylindrical shape of the same overall diameter, the Moment of Inertia will now be a function of the radius and not as much as the thickness. The stiffness of a ridged versus plain sheet of metal formed into a cylinder is not much significant. Moreover, fluting a barrel means removing materials; ridging a plain sheet of metal is NOT removing material but just altering the configuration. Therefore, the ridged roof analogy does not hold water because it is not an apple-to-apple comparison.

Heat Transfer

Get ready to be blown away. 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. I am sorry to say that this is absolute fallacy. 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's why:

Let us say you fire 10 rounds in 10 seconds in a hunting rifle. And at the same time your friend also fires 10 rounds in 10 seconds in a bull barreled varmint rifle. We all know that heat is generated as a result of the bullet going through the bore at a high rate of speed, causing friction and releasing energy. Now, the temperature inside both barrels should theoretically be equal, but the temperature on the surface of the hunting rifle will be a lot hotter than the temperature on the surface of the varminter. It is because the wall of the hunting rifle is a lot thinner than the wall of a varminter. The closer you are to the heat source (the bore of the rifle) the hotter it is on the surface of the barrel. Makes sense? In essence, the thickness of the barrel acts as an insulation. In short, the thinner the wall, the faster the heat reaches the surface and the faster the heat will be dissipated into and equalized within the ambient (outside) temperature. This type of heat transfer is called conduction (the other two are convection and radiation).

Here is an excellent analogy. We love to barbecue in the summer. Place an aluminum foil on the grill to cook you burgers. After you burgers are cooked, remove the aluminum foil and notice that it cools off almost immediately. Now fold the aluminum foil to make it thicker and put it back on the grill. Remove it afterwards and you will notice that it does not cool off as fast. It is the same analogy with barrels. In short, hunting rifles dissipate heat quicker than varminters do.

So how does fluting aid in heat dissipation? Basically, as materials are removed the flutes become closer to the bore. So when the bore gets hot after firing several rounds, the heat generated reaches the surface of the flutes a lot faster than a plain barrel of the same diameter.

Conclusions

In conclusion, a regular plain barrel is a lot stiffer than a fluted barrel of the same outside diameter; however, a fluted barrel is a lot stiffer than a regular barrel of the same weight. Fluting will definitely dissipate heat quickly. And it is not because the surface area is increased; it is because the heat is allowed to reach the outside temperature at a faster rate by removing materials. If your bull barrel becomes unbearably hot on the surface, it is safe to assume that the bore temperature is at a point where it can literally dissolve soft materials. This will damage your bore in the long run.

So if you wish to flute your barrel, it should be because you want to reduce the overall weight of your rifle and you want your barrel to cool at a faster rate. Fluting your barrel with the belief that it will add stiffness just doesn't make any scientific sense.



Calculations


A) Moment of Inertia of Circle:
Figure 1. Barrel Cross Sectional Detail


I = [p (r ^ 4)] / 4

Where
I = Moment of Inertia
R = Radius
p = pi (3.14)

Note: Moment of inertia of the caliber (.308 in) was subtracted from overall moment of inertia.




B) Calculate Deflection of Barrel at Muzzle
Figure 2. Free Body Diagram of Barrel


Y max = [-P(L^3)] / (3EI)

Where

Y max = maximum deflection at free end
P = load exerted at end of barrel
L = Length of barrel
E = Modulus of Elasticity (28 x 10^6 psi for steel)
I = Moment of Inertia




C) Calculate Heat Transfer from Bore to Surface of Barrel
Figure 3. Rate of Heat Transfer Diagram on Fluted Barrel


qr = [2p Lk (Ts1- Ts2)] / [ln(r2/r1)]

where

qr = rate of heat transfer
L = length of barrel
K = thermal conductivity, which is constant for steel irregardless of overall diameter
Ts1 = temperature inside bore
Ts2 = outside temperature
r2 = thickness of barrel from bore to surface
r1 = radius of caliber, in this case it's .154" (.308 / 2)

The higher the value of the rate of heat transfer (qr) the sooner the barrel's bore will cool off.



Notes
  1. AutoCad 2000 was used in determining the perimeter and cross sectional areas of the barrels.
  2. Also note that I have a 12BVSS and 10FP in my gun collection which were used in these analyses. The Light Varmint Rifle outlined, while can conceivably be developed, was used to serve as a theoretical example only. I am aware that Savage 12BVSS typically has a 26-inch barrel and 10FP has a 24-inch barrel. However, the 10FP I used in the calculations has a two-inch muzzle brake making the rifle 26 inches in length.
  3. Moreover, one may argue that the 12BVSS is stainless while the 10FP is regular steel. The two materials definitely have their own distinct metallurgical properties; however, the Modulus of Elasticity (which also determines the stiffness) between the two is very close.
  4. If you have any questions or comments please feel free to email me


Back to Articles