Spiral fluting

[Cosmoline]

Can anyone tell me what advantage there would be in spiral fluting on an AR barrel, or any other barrel for that matter? I would also like to find some data on the subject but cant seem to find any. I thought I heard Costa talking about it awhile back in either an article or video but cant remember where. If I recall correctly, the talk was that they have found the rigity of a barrel is more important than previously thought and that the more rigid the barrel is, the better the accuracy under high heat conditions. I'm wondering if it indeed is advantageous or just a passing fad. I see the popularity of spiral fluting on the bolt of a bolt action rifle and am unsure of that theory either. Thanks guys.

[InF1uxx]

As far as i am aware the spiral fluting makes the barrel lighter due to the removed material and also reduces heat due to the increased surface area, increasing accuracy, this can actually increase the heat of the rifle due to removed material though. Unless you are running under sustained automatic fire i don't think the heat of the barrel will matter all that much and you will most likely worry more about recoil and muzzle rise than the opening of the group. It has its purpose but for the average consumer i wouldn't suggest it other than for looks, I would stick with a thicker barrel profile.

[markm]

Fluting on a Bolt gun's bolt does about nothing. Even the homies on accurateshooter admit as much. It's essentially a way of showing off the fact that you're not shooting a factory action. Fluting on a barrel is about as useless. The heat myth was busted somewhere. The rigid claim too.

The only gain is you shave weight and make your gun look gay as hell...

[Cosmoline]

Fluting in general has been bounced back and forth as to it's benefits, if any. I am specifically questioning the spiral. I cant seem to find any real data that can lead me to believe there are any physical advantages.

I have to agree with MarK on the looks. It reminds me of the old rod-iron decorating twist.

[Stickman]

It reduces barrel weight, and looks different than what everyone else has.

[FastIndy]

This is coming from an engineering standpoint, so you can make your own decisions on the relative worth of these points, but here's the breakdown. This is assuming identical materials for everything.

First, the fluting seen on alot of the hard duty bolts are for debris and/or ice clearance. As the bolt is rotated it gives a free channel for the debris to hopefully fall into instead of jamming up between two low-clearance parts. The deeper and more extreme the fluting, the more you can count on the fact that it is a look-at-me item.

Fluting on barrels is a little different. Its main function is to decrease weight, so you really need to compare it to turning a barrel down to a smaller profile. First, let's consider heat factors.

Heat:
Fluting DOES increase the surface area of the barrel. Just remember that parts retain heat alot like batteries retain electricity. For an event that's nearly instantaneous like firing a round, the barrel will increase in temperature by a fixed amount. This difference in temperature that the barrel goes through(deltaT) for each shot is determined by the mass (interchangeable with volume in our comparison) of the barrel. For a material that conducts heat quickly, the relationship between mass and this deltaT is very linear. Double the mass and you can expect to see half the temperature increase. As the barrel increases in temperature the deltaT for each shot will become smaller. Cooling is much more time dependent since the difference between the temperature of the air and the temperature of the barrel is much smaller than the barrel and the combustion gases. So while the mass of the barrel will determine how many rounds you can fire in a string without a material failure, the cooling rate (influenced by surface area) will determine the sustainability of fire, or the time between shots. If the barrel material used was very resistant to heat, at some point the thermal radiation/conduction to the air would match the heat input by combustion byproducts, and you could maintain a high rate of fire indefintely. The problem is that the cooling rate is very low, so for most materials used for barrels you can count on a failure long before then.

Structure:
A fluted barrel will have increased stiffness over a nonfluted barrel of the same weight. In other words, if you want your barrel to be lighter and you have enough cross sectional thickness to not worry about overpressure from the bore blowing your barrel up, you should have it fluted instead of turned. Accuracy for us is all about consistency. What this means is minimizing variations and the magnitude of vibration present in the firearm when the bullet has yet to leave the barrel. If the loads we shot were 100% consistent and the structure of the firearm in the same position every time, then it wouldn't matter if the barrel had the rigidity of a wet noodle and flopped around 6" at the end on every shot. But in the real world it comes down to needing to minimize deflection of the barrel. This usually means pushing vibrations farther up the frequency spectrum in order to bring down total deflection. So it is safe to say that the barrel's degree of resistance to bending will have an effect on accuracy. So if you're trying to design something to resist bending, think of it as a beam. Vibrations will distort the barrel in much the same way as if you used it as a ladder rung, bending it in the middle. So in this case the barrel's stiffness is proportional to its moment of area (and the material of course). Moment of area is a geometric property that (in our case) is most important when applied to the cross-section of the barrel, and it is critical to beam bending.
http://en.wikipedia.org/wiki/Second_moment_of_area

Optimization:
So, we want low weight (or maybe not after reading the first part of this post) and high stiffness. Consider the cross-section of a barrel, looking right down the bore. The weight (or weight per length) is controlled only by the area of the cross section. Moment of area is also controlled by area (maybe the name gives it away) but more importantly by the location of the material in the cross section. A portion of the cross section that's farther away from the axis that it's bending around will contribute more to the stiffness of the structure. It's like turning a 2x4 on edge to better resist bending. It still has the same weight and cross-section, but by lining up the direction of bending with the axis of the 2x4 that has the highest moment of area, it is much stronger. What you may start to realize is that if you're only after increasing stiffness in one direction, you can simply make something thinner and thinner while maintaining the same cross-sectional area. The rub appears in the fact that a barrel needs to be stiff in every direction. This, however, is even still possible. By increasing the diameter and decreasing the wall thickness of a tube, the same cross-sectional area is maintaned while moment of area increases and is the same in every direction. So the compromise comes from the fact that we still have to launch a projectile with this thing, and that means that we have a fixed inner diameter. The strength of fluting comes from the fact that the material of a rib is still significantly farther away from the axis of bending than if it were a plain tube, and so is able to carry a larger part of the load. A larger outer diameter with a constant cross-sectional area will constrain the ribs to be thinner and thinner, but moment of area, and therefore stiffness, will still increase. Coincidentally, this also maximizes surface area.

[Cosmoline]

Very interesting. Thank you for posting that Indy.

[TehLlama]

Fluting on or around contact/friction points can increase tolerances - stuff like dust custs are exactly that. Fluting on the exposed face of a bolt doesn't accomplish much functionally, but can look cool.

If you're constrained by a fixed weight for the rifle and need to maximize accuracy, there is an argument for it. If you're constrained by cost, you'll still be ahead with a boring cylindrical tube in a smaller diameter but of higher quality than taking a larger one and removing material.

[markm]

We went into Sportsmans Warehouse Saturday. They had a LWRCi turd rifle with Spiral fluting. I always get a chuckle when the imbecile behind the counter tells you that an LWRCi gun is a good AR!