Math, science, and barrel fluting

[MClark]

i always thought if i cant carry a rifle the weight of an M1, which GIs humped many many miles, I didnt deserve to go. these days i prob dont deserve to go.

I have a friend who is a WW-II vet. I was showing him the Garand I had bought. He picked it up and said, "It got a lot heavier in 75 years"

 

[hoz53]

This is the lightest rifle I have built. 4lb 14oz. no flutes. And a couple groups that it's own shot at 1000 yards off a bipod. I prefer to just order the contour that meets the weight requirement. An interesting side note, this one is set up with .300" firing pin fall. So much for lock time.

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what cartridge is that? those are some impressive groups.

 

[Alex Wheeler]

Its a 6mm Creedmoor. Im not too surprised that case works very well.

 

[jonbearman]

Now if they hammer forged the flutes in then maybe it would make it stiffer. Its for looks and if it shoots who cares.LOL

 

[dcali]

The trouble is that stiffness isn't by itself useful. What matters is how the barrel vibrates, it's resonant frequency (frequencies, really, but that's another topic). Frequency depends on stiffness *and* weight. This is all basic fundamental mechanical engineering and very, very well understood. It's probably important to define stiffness here as the tendency for the barrel to bend with a weigh hung off the muzzle. A stiffer barrel will bend less. It's confusing because engineers will also use the term "stiffness" when they mean resonant frequency - a higher frequency being "stiffer". It's jargon, and I won't use it here to keep things clear.

So when you cut flutes into a barrel, you're reducing the stiffness (seems pretty intuitive - less material is less stiff). You're also removing weight. Reducing stiffness *decreases* resonant frequency. Removing weight *increases* resonant frequency. So which wins? The stiffness. It turns out removing material from the outside of the barrel reduces stiffness more than it reduces weight. So the net result is that the barrel is going to slow down. Whether or not that's good is not something you can say without testing a specific rifle.

Personally, I think there are better ways to manipulate resonant frequency - simply making the barrel a little longer will slow it down quite a bit. I also think there are better ways to save weight- just turn a thinner contour.

But some people think they're cool. Can't argue with that. Aesthetics are personal.

 

[gunsandgunsmithing]

The trouble is that stiffness isn't by itself useful. What matters is how the barrel vibrates, it's resonant frequency (frequencies, really, but that's another topic). Frequency depends on stiffness *and* weight. This is all basic fundamental mechanical engineering and very, very well understood. It's probably important to define stiffness here as the tendency for the barrel to bend with a weigh hung off the muzzle. A stiffer barrel will bend less. It's confusing because engineers will also use the term "stiffness" when they mean resonant frequency - a higher frequency being "stiffer". It's jargon, and I won't use it here to keep things clear.

So when you cut flutes into a barrel, you're reducing the stiffness (seems pretty intuitive - less material is less stiff). You're also removing weight. Reducing stiffness *decreases* resonant frequency. Removing weight *increases* resonant frequency. So which wins? The stiffness. It turns out removing material from the outside of the barrel reduces stiffness more than it reduces weight. So the net result is that the barrel is going to slow down. Whether or not that's good is not something you can say without testing a specific rifle.

Personally, I think there are better ways to manipulate resonant frequency - simply making the barrel a little longer will slow it down quite a bit. I also think there are better ways to save weight- just turn a thinner contour.

But some people think they're cool. Can't argue with that. Aesthetics are personal.

Good post Damon! I agree with every word. What's confusing is that adding mass at the end lowers the frequency and yields more barrel deflection... at least in layman terms, this is synonymous with being less stiff. Hence a 1.250 straight that is 30" long is actually less stiff than say a 24" hv.
That's why I often use the Lilja calculator linked previously to show that a bigger diameter bbl doesn't necessarily equate to a stiffer barrel. Being bigger around at the muzzle obviously puts more weight at the muzzle end, creating more deflection, lower frequencies...and less stiffness. Barrels gain/lose stiffness faster by length than by diameter, over its entire length.
I hope I stated that without causing anyone confusion. Bottom line is that many people are surprised to see that their 1.250 straight can actually be less stiff than a shorter but lighter contour bbl. But yes, a bigger bbl will be stiffer per inch.

 

[john pruitt]

Like the man said if you do not like what other people have to say just walk on by and stand on the corner.

 

[Metal God]

I didn’t read all 5 pages but will ask , doesn’t flutes also make outside diameter closer to the bore resulting in the heat needing less distance to travel through the metal to escape. It’s not just surface area that helps reduce heating ?

I also see some barrels with very shollow flutes while others with quite deep flutes . As well as the start diameter of the barrel makes a big difference. A 1” barrel with a .224 bore can have large deep flutes which would reduce weight and increase suface area along with getting outside areas of the barrel much closer to the bore allowing for quicker heat transfer???

The reason this dead horse keeps getting beat is because there isn’t one example to work with but rather many ways the barrel could be cut which changes the results .

 

[davidjoe]

No, that's not the case. Resonant frequency doesn't work that way. Halliday & Resnick physics textbook or Feynman lectures on physics can explain it more elegantly than I. Both are definitive works on undergraduate mechanics.

These perfect resonators come to mind. I will look up that source, though. I am definitely not suggesting that conical barrels are free of vibrations, but one shape (1.25 straight) is pretty darn effective at it. A piano mutes strings by touching them with a pad, yet we free float these then chase nodes.

 

[mensajd]

These perfect resonators come to mind. I will look up that source, though. I am definitely not suggesting that conical barrels are free of vibrations. [photo deleted]

The references I cited are among the best but are calculus heavy. @damoncali already provided a concise summary above. Love to hear Toccata and Fugue in D Minor live on a good set of pipes though.

 

[46and2]

Love to hear Toccata and Fugue in D Minor live on a good set of pipes though.

You and I both!

 

[RMiller]

Ive read thru most of this thread and several like it. I do not have an answer as to whether or not a fluted barrel is more or less stiff than a non fluted barrel but I can say the weight argument doesnt hold water....ie if the barrel is fluted it becomes lighter but less stiff or it will droop less. All of that IMO isnt accurate.
If you take a sheet of printer paper and lay it flat on the table and slide it off the edge it wont be stiff enough to hold its own weight and will droop. Take that same sheet of paper and fold it in to an accordian shape or a fan we all have made one time or another and it will not only hold its on weight but a substantial amount more. Contour can increase or decrease stiffness with out changing weight to more or less. This is the idea I believe behind the "fluting increases stiffness". The shape of an obeject can change its rigidity with out a weight incress or decrease. Weight is in no way a messure of stifness or rigidity.
Just an observation.

 

[gunsandgunsmithing]

Ive read thru most of this thread and several like it. I do not have an answer as to whether or not a fluted barrel is more or less stiff than a non fluted barrel but I can say the weight argument doesnt hold water....ie if the barrel is fluted it becomes lighter but less stiff or it will droop less. All of that IMO isnt accurate.
If you take a sheet of printer paper and lay it flat on the table and slide it off the edge it wont be stiff enough to hold its own weight and will droop. Take that same sheet of paper and fold it in to an accordian shape or a fan we all have made one time or another and it will not only hold its on weight but a substantial amount more. Contour can increase or decrease stiffness with out changing weight to more or less. This is the idea I believe behind the "fluting increases stiffness". The shape of an obeject can change its rigidity with out a weight incress or decrease. Weight is in no way a messure of stifness or rigidity.
Just an observation.

Kinda like a reverse taper barrel? Picture a barrel turned around backward, with the big end at the muzzle and small end the breech. Obviously, it will "droop" more backward.

 

[gunsandgunsmithing]

Like the man said if you do not like what other people have to say just walk on by and stand on the corner.

I'm not sure who you're talking to but it sounds like good advice.

 

[chkunz]

Ive read thru most of this thread and several like it. I do not have an answer as to whether or not a fluted barrel is more or less stiff than a non fluted barrel but I can say the weight argument doesnt hold water....ie if the barrel is fluted it becomes lighter but less stiff or it will droop less. All of that IMO isnt accurate.
If you take a sheet of printer paper and lay it flat on the table and slide it off the edge it wont be stiff enough to hold its own weight and will droop. Take that same sheet of paper and fold it in to an accordian shape or a fan we all have made one time or another and it will not only hold its on weight but a substantial amount more. Contour can increase or decrease stiffness with out changing weight to more or less. This is the idea I believe behind the "fluting increases stiffness". The shape of an obeject can change its rigidity with out a weight incress or decrease. Weight is in no way a messure of stifness or rigidity.
Just an observation.

If you study an engineering textbook on strength of materials and look at the properties of beams you will find an engineering term "moment of inertia" and the beam with a higher moment of inertia will deflect less under load. The further an element of mass is away from the center of mass the higher the moment of inertia, think of how an I beam is shaped: that shape is to maximize the moment of inertia.

As the beams get thinner and thinner the equations for beam stiffness not longer works, they are no longer considered beams as your sheet of paper example illustrates. The equations for the properties of beams apply to rifle barrels and the way thin sheets behave does not apply to rifle barrels.

 

[RMiller]

If you study an engineering textbook on strength of materials and look at the properties of beams you will find an engineering term "moment of inertia" and the beam with a higher moment of inertia will deflect less under load. The further an element of mass is away from the center of mass the higher the moment of inertia, think of how an I beam is shaped: that shape is to maximize the moment of inertia.

As the beams get thinner and thinner the equations for beam stiffness not longer works, they are no longer considered beams as your sheet of paper example illustrates. The equations for the properties of beams apply to rifle barrels and the way thin sheets behave does not apply to rifle barrels.

I in no way said that a beam that gets thinner wont be less ridged. What I said was weight alone is not a messure. Angle iron is far more ridged than flat stock. Even lighter weight angle can be stiffer than heavier flat stock.
The shape of an object is far more important than weight alone. Students built bridges from cardboard. Not because of the weight of the cardboard but because of the way it was folded. Thats my only point. To say that a barrel that is fluted has the be less ridged because it weighs less is wrong IMO. I could be wrong.
To be honest I only mentioned any of it because I always believed thats where the concept of it being stiffer came from. The shape of it. Again I could be wrong.

 

[chkunz]

I in no way said that a beam that gets thinner wont be less ridged. What I said was weight alone is not a messure. Angle iron is far more ridged than flat stock. Even lighter weight angle can be stiffer than heavier flat stock.
The shape of an object is far more important than weight alone. Students built bridges from cardboard. Not because of the weight of the cardboard but because of the way it was folded. Thats my only point. To say that a barrel that is fluted has the be less ridged because it weighs less is wrong IMO. I could be wrong.
To be honest I only mentioned any of it because I always believed thats where the concept of it being stiffer came from. The shape of it. Again I could be wrong.

I agree that stiffness has all to do with shape and that is what the moment of inertia term in the engineering text book equations for beam properties is telling us. Early on someone made the point that for barrels of the same weight the fluted barrel is stiffer and that concept seems to be easily overlooked by some in this type of discussion.

 

[hpshooter]

Would modify that to the following:

of the same weight and length the fluted barrel is stiffer and that concept seems to be easily overlooked by some in this type of discussion.

 

[Dusty Stevens]

I can solve this debate- if you think it looks cool and are not shooting short range br then get it. No need in worrying about stiffer- thats not always better you know. Matter of fact the most responsive tuner equipped barrels are way less stiff than youd think, like a diving board, so dont concentrate on stiffness. A barrel is going to shoot if its going to shoot no matter if its a 1.450 railgun barrel or a 26”#1 mountain rifle taper.

 

[Intheshop]

Are I beams tested in a single plane of deflection? If so,is that a reasonable comparison to a barrel that vibrates(bends) in a more circular fashion?

I say no,but could be wrong.