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Barrel fluting post build affect accuracy?

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Unread 05-01-2005, 01:00 PM
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Join Date: Mar 2005
Posts: 109
Re: Barrel fluting post build affect accuracy?

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.
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Unread 05-01-2005, 02:12 PM
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Join Date: May 2001
Location: Urich, MO
Posts: 838
Re: Barrel fluting post build affect accuracy?

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.
Chris Matthews
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Unread 05-01-2005, 03:00 PM
LB LB is offline
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Location: Upland, CA
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Re: Barrel fluting post build affect accuracy?


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
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Unread 05-01-2005, 06:35 PM
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Join Date: Jan 2005
Posts: 187
Re: Barrel fluting post build affect accuracy?

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?
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Unread 05-02-2005, 06:24 AM
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Re: Barrel fluting post build affect accuracy?

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.

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Unread 05-02-2005, 07:09 AM
ds ds is offline
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Re: Barrel fluting post build affect accuracy?


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.

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Unread 05-02-2005, 08:46 AM
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Join Date: Aug 2003
Posts: 126
Re: Barrel fluting post build affect accuracy?

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.
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