Barrel fluting post build affect accuracy?

[James Jones]

I have a couple fluted barrels that were done by the barrel maker and they shoot great , the question is have any of you guys had any seen proof of accuracy loss when having a barrel fluted after it has been shot? I've heard that some folks lost alot of accuracy , said to be from the fluting releaving strees inj the barrel and opening the bore to irregular bore size making for thight spots.
I ask because the barrel make I'm looking at doesen't offer the flute size I'd like to have

 

[Fiftydriver]

JDJones,

From my experience I would say this depends on the diameter of the barrel. If your fluting a smaller contoured barrel then I would say you will have some dimensional change in the bore. Say anything #5 or smaller.

If your fluting a heavy barrel, something with a muzzle diameter of .700 and up I have not witnessed much of a problem.

That said, I do not recommend to my customers to have a barrel fluted after it has been manufacturered. Why take the risk for minimal gains.

If done incorrectly fluting a barrel can induce severe stresses so it must be done correctly and stress relieved afterward.

My opinion is have the barrel blank fluted and if you have a factory barrel, shoot it out and then get a fluted aftermarket barrel later.

Good Shooting!!

Kirby Allen(50)

 

[ds]

Fiftydriver,

In your opinion which I value very much, will flutting wether it is done before or after laping change bore dimensions when the barrel gets warm, eg when shot.

Putting it another way flutes create thick and thin sections in the barrel. When you shoot the barrel gets hot/warm so simple physics would suggest the thin sections would expand or try to expand more than the thick sections creating an untrue and stressed bore.

Have you, or anyone noticed changes in bore dimensions on fluted barrels when warm compared to cold.

David.

 

[LB]

[ QUOTE ]


Have you, or anyone noticed changes in bore dimensions on fluted barrels when warm compared to cold.

David.

[/ QUOTE ]

If the question suggests that the bore diameter increases when "warm", irrespective of fluting, then the simple physics answer is no, unless cherry red, perhaps?

The flute function is to increase surface area to facilitate cooling, as with the fins on a radiator. It is also said to increase stiffness and reduce barrel whip. Generally speaking, the bore remains as straight as it was drilled; in cross section, although it may warp. However, the temperature at which this occurs is beyond warm. Also, most fluting that I have seen is either an odd number, or positioned so that the thin sections are not directly opposed. But, I have not seen enough examples to say that that is a true statement, anything is possible?

Actually measuring the bore when "hot" would seem to be a difficult assignment, but I could visualize the metal expanding in every dimension, length, outer diameter as well as interiorly, creating a slightly more snug bore. LB

 

[ds]

LB

Thanks for your reply,

I am unsure that fluting aids cooling anything much, it was part of the marketing hype (fire 20 shots and see how long it takes to cool back to starting temperature, with or without fluting I think it is a long wait). Also fluting will not increase stiffness but decrease it and increase twisting due to torque (that was one of the reasons interupted fluting was promoted) if a fluted and unfluted barrel are of equal diameter. However if barrels are of the same weight then fluting can increase stiffness because a larger diameter can be used for the same weight, but you still get torque.

The question I asked on heated barrel measurement was far from easy to answer, that is why I thougt Fiftydriver would know if anyone, but all help is appreciated.

Interesting thought about barrels gaining length when heated. It has started me thinking about profiles and barrel tunners again.

David.

 

[Brent]

LB,

How do you explain a press fit/interferance fit between two cylindrical parts? The part being inserted into a bore is a couple thou larger in diameter and is stuck in the freezer to cold soak, the part that it will be inserted into is warmed (not red hot or even close, just very warm) the two slip together, but when cooled must be pressed apart.

The main point here is, that the part with the bore in it is heated and its inside diameter gets larger, not smaller as it expands. I would not disagree that the part does expand and will get thicker also, BUT its circumferance expands/increases way "more" which creates the larger ID.

When freezing a shaft/race/bushing etc to be placed into a bore its circumferance "shrinks" a small amount, enough that if the interferance fit between part is small enough it will slip into the bore or be tapped in very easily... if done quickly.

Most fluting I've seen done were with even numbers of flutes so that the raised portion between flutes opposed each other, to retain the original barrel's OD stiffness but reduce weight. Stiffness would "not" be retained if a ridge was opposed by a flute and not another ridge. A barrel's stiffness can't be increased by fluting but it can be reduced in weight and retain original rigidness. Much like a steel "I" beam or "H" beam.

IMO a bore will change dimensionally when fluting because of the heat generated during the milling process. The less heat the less change... use lots of coolant and take light cuts. Now, if the barrel will warp when heating up depends on alot of things. Who knows how much stress is already in the barrel when you get it, or if it is symmetrical or not, then how much more you add to it or releive from it isn't likely to be tested by anyone to conclusion. So, cryo treat it? Probably wouldn't hurt.

We do quite a bit of fluting, 12 and 8 sided barrels but never have tested the "before and after" accuracy. I do have my concerns about it though.

 

[longtooth]

I don't recommend fluting factory barrels, if you have ever wondered why many factory barrels shoot poorly one of the many reasons is because the bore is not in the center of the barrel ,cut off an old barrel (Remington 721 barrel was the worst I have seen) about 6-8 inches from the muzzle you will find the bore is not in the center of the barrel like you would expect. This causes uneven heating of the barrel and all its corresponding problems. Barrels are also affected by the way the barrel is fluted, Horizontal milling machines do put tons of pressure pushing down on the barrel and could cause deformities, could being the important word. vertical mills put very little pressure downward, there pressure is in the cutting direction along the barrel, this reduces the luckily hood of deforming the bore.

 

[longtooth]

I was in the proses of posting and missed two good posts from ds and Brent Moffet. ds is correct in saying that the barrel is no stiffer then before just lighter, you gave me a idea about torque and I will see if I can design a experiment to test that. Mr Moffit is correct on the cryo treating it will relive stress from machining, My barrels are stress relived before rifling and after as well as after machining and chambering is done. I know that seems excessive to some but I have the facilities at my disposal and the cost is free.

 

[chris matthews]

I'll throw in my two cents worth as I have done a considerable number of barrels before and after....

Yes, fluting does alter bore dimensions. The bore opens up a bit in diameter in the fluted section. You can feel this with a tight patch or lead lap after fluting.

Here is when a couple of barrel makers do their fluting:

Rock: After drilling, before reaming and rifling.

Broughton: After buttoning, but then it is stressed relieved (by heat, not cryo) and then lapped.

Schneider: about the same as Broughton.

All of them recommend re-lapping the bore if you flute after build up.

I have fluted several barrels after rifling-but before chambering and they took longer to break in and settle in before they started shooting well.

There is a bunch of smoke and mirrors BS out there about fluting- what it does do and what it doesn't do- you make your own decision there, but if you want to flute a barrel that is already chambered, have it lapped at least to help maintain accuracy.

 

[LB]

BM,

I am aware of the technque in fitting pistons, but, you know, this interference fit involves IDs and ODs measured in inches, not usually small decimal fractions, so I believe the actual "gain" in bore diameter (if there is one) is so negligible as to be insignificant, regardless of fluting....assuming we are talking about a "warm" barrel? Besides, the process usually involves heating one part and cooling the other part.

One definition od warm might be: able to grasp the barrel without pain. And, I admit, my knowledge of auto mechanics is not any greater than my knowledge of the physical characteristics of rifled barrels, at various temperatures.

Fluting, whether even or odd, I defer. Fluting adding stiffness during recoil, I also defer to our experts. Advantages of fluting, versus disadvantages, I'll sit that one out, as well.

As to actually heating a barrel enough to slightly expand the OD, I believe it may increase the length, as well. Should we have a metallurgist lurking about, perhaps we can learn if the various alloys expand in known, and predictable dimensions?

For instance, I heard, once that the SR71 gains length, in the air at speed, as the skin heats up, and when it lands, and cools off; all the fuel tanks leak badly. Okay, poor example, perhaps?

Does the bore increase in size, unevenly (WHEN WARM) due to fluting, that is the question.

Not only am I of the mind that it doesn't increase enough to actually measure, (or worry about) but I am also skeptical of the suggestion that what expansion "might" exist, is UNeven expansion, because of the flutes. And, which might turn the "warm" barrel into one with a polygonal (and rifled) bore; for lack of a more accurate term.

If the barrel were made of bread dough, and baked in an oven, would the bore size expand, as would the outer diameter? Fortunately, barrels are not made this way, but the bore would be smaller, would it not? It may not have the slightest application, in this discussion, but it does illustrate the thought behind my ignorant statement.

Good hunting. LB

 

[keithcandler]

It's a bad deal, JD to flute after the bbl has been finished. Bob Hart told me that in the situation that you are talking about that a bbl that had a tight lap would change to a sloppy fit lap.

Also, there are different processes to flute a bbl.
Someone that does not know what they are doing can ruin a bbl introducing stresses into the bbl that cause the bbl to warp. It is best to flute a bbl before it has been lapped, and even then smaller dia bbls can be changed.

Do you feel lucky?

 

[Chawlston]

When considering bore expansion and constriction, how much does one expect the bore to constrict with fouling from numberous shots. I guess this would explain why some shoot better clean and some shoot better less clean.

Chawlston

 

[ds]

LB,

Below are figures taken with a digital meter from what I understand you attach a probe or something to a digital meter and it reads out temperature, useful for putting in a barrel so it seems like a fair working definition of warm. This was info from a thread on snipers hide. I did not copy the whole thread.

" starting barrel temp 87
after 1 shot 91.1 cool of(approx.) 1.2 minute 90.1
1 shot 93.2 cool of(approx.) 1.2 minute 92.3
1 shot 95.1 cool of(approx.) 1.2 minute 94
1 shot 97.9 cool of(approx.) 1.2 minute 96(?)
1 shot 99.9 cool of(approx.) 1.2 minute 97.7
1 shot 101 cool of(approx.) 1.2 minute 99
4 shots maybe 15 sec apart up to deg 112
20 minutes cool down to 91, ambient temp was 83
9 shots approx 10-15 sec apart up to 126.7
30 minutes cool down to 94.1 ambient temp at 81.6
10 shots approx 10-15 sec apart up to 122
10 minutes cool down to 115
20 shots up to 152 deg 5 minutes cool down to 130 ambient temp at 82
10 minutes more cool down to 122
10 more minutes cool down to 108 ambient temp at 83.6
16 rds up to 147.7"


I don`t really follow the bread in oven theory, ovens heat from the outside in, barrels heat from the inside out.

David.

 

[Richard338]

A steel object that expands due to heating will expand in ALL its dimensions by a constant factor.
That means that the length will be multiplied by (say) 1.001, so will the barrel thickness, and the bore.
The bore enlarges, not constricts. I once worked this out for someone who thought their tight necked chamber was constricting onto their brass.
In fact the bore enlarges (but only very slightly at realistic temperatures).
Brass has a larger coefficient of expansion and will expand slightly more at the same temp. So with really high temps if you let your cartridge soak up a lot of heat, it could be slightly tighter fit.
Realistically this should never pose a problem. The situation is similar but slightly more complicated with differential heating/cooling.

Bottom line, very little difference in bore at realistic temps. What difference there is is an enlargement.