Barrel fluting post build affect accuracy?

[ds]

One error and one omission in my last post.

Error, the sensor looks to be on the ouside of the barrel not inside.

Omission, calibre I think is .308 Win (AR 10 rifle).

David.

 

[LB]

[ QUOTE ]
20 shots up to 152 deg 5 minutes cool down to 130 ambient temp at 82


[/ QUOTE ]

What would be interesting is data on accuracy, for the second half, but 152º is beyond warm, since you cannot grasp the barrel at that temperature without burning your hand. And, that is with a cupcake chambering. Anyone that continues to shoot a hot barrel will eventually be rewarded with an erroded throat and degraded accuracy. I always put my hand on my barrel to judge whether or not I can continue. I hope it is a common practice, especially on warm days? In any case, stainless barrels are the only kind I buy.

What I see from the previous post is that the change in barrel ID is negligible, if at all, and the ID increases, if it does. It does not address fluting a previously manufactured barrel, or the question of uneven expansion in the area of the flutes. Since it does not, I will continue to assume that it is (at best) negligible, and probably not a valid concern.

Good hunting. LB

 

[ds]

This I guess is going beyond my means, but an idea would be to have three digi meters with three sensors, one probe type to monitor internal barrel temperature and two external one on the ribs of the flutes and one in the flutes, and monitor temperatures over time over number of rounds fired.

It would show if the fluted areas got hotter than the non fluted areas and if the fluted areas cooled down quicker. Interperating the results is where I am out of my depth.

David.

 

[longtooth]

The linear expansion of the barrel would be negligible for most purposes.The Thermal Coefficient of Expansion for 416R works out to about 0.00002 per degree starting at 68F/20C (thats the std. reference temp for ISO and ANSI) so a 20 degree increase(88F) would cause the length and dia of the barrel to grow by 0.0004. As to stiffness Varmint AL has a study on that, www.varmintal.com/aflut.htm that looks interesting.

 

[Brent]

So, from 68* up to 118*, a 50* increase = .001 larger bore diameter?

 

[Brent]

DS,

Infra red laser thermometer, they're pretty cheap these days and you can check the temp at any location, even up in the throat if you're a good aim.

 

[Richard338]

The thermal expansion coefficient for steel in the temp range of 20C-100C is from (1x10^-5)/C to (2x10^-5)/C. Longtooths post says 2x10^-5/C for 416 steel.
This is a fractional change. So if you went from 20-100C the fractional change is 0.0016.
This means that a 30" barrel lengthens by 0.048 inches. If the barrel is 1" thick, it will become 0.0016 inches thicker.
A .2640 bore will open up by 0.0004 inches (less than 1 thou).

 

[Brent]

Never have slugged a fouled bore to measure the change but, I do know that the buildup will increase pressure substantially.

I've measured a drop in pressure of what I want to say was 5-8 Kpsi depending on the bullet fired, this after cleaning a Lilja bore that was fouled with nearly 100 rounds.

At the time I was wondering if the cold temp was influencing the bore diameter, shrinking it and causing what were somewhat hot loads to become way too hot.

Roughening of the bore over the last couple hundred rounds obviously increased the pressure of what used to be quite medium loads. I also measured a pretty substantial loss of efficiency, at the same pressure (reducing my loads to do so) lost anywhere from 50-150 fps on all my loads since break in was complete. Probably around 300 rounds through the bore at that time.

 

[koginam]

What would happen if you had the barrel stock, drill, reamer, and button at say 150 degrees and they were still at spec. and you built the barrel, I would think the barrel would shrink at 80 degrees and not open up until the barrel reached over 150 degrees. maybe this is a new manufacturing process, those of you who build barrels should try it and cut me in if it works.

 

[ds]

Brent,

Thanks for the idea with the tehermometer. I will try to find one.

Richard 338 / Longtooth,

I am sorry I am really thick, but can you help with this,
from what you said a .264 bore will expand in diameter .0004" if heated froom 20c-100c, if the barrel diameter is 1" thick (hope I hvae understood right).

What will the bore expansion be if the barrel is 1/2" thick? any different, if so if you think about the two differences in diameters as one fluted barrel would expansion differnces create stress in the barrel.

Thanks for bearing with me,

David.

 

[Richard338]

DS,
To work out the expansion for any size is easy.
Take .00002 multiply by the temp change (say 80C). That gives 0.0016 as the fractional expansion.
All you do now is multiply the size of the object by 0.0016, and thats how much it expands.
The nice thing is that it is unitless. If your object is 1" it expands by 0.0016 inches.
If your object is one meter, it expands by 0.0016 meters. So a 1/2 inch barrel expands by 0.0008 inches (1/2 x 0.0016)
The bore is the same in your case, so it expands the same (.2640 x 0.0016)=0.0004

 

[ds]

Richard 338,

Thank you very much for the info. I am trying to learn.

David.

 

[Popapi]

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.

Well I have a Lilja Hand-lapped button rifled barrel(270WSM) that was fluted by a smith AFTER it was finished and it shoot BUGHOLES, now I called Lilja and asked what their process is as per fluting and they say they flute AFTER the barrel is finished when I say finished meaning fluting is the LAST thing they do after it has been rifled, contoured, heat treated, lapped etc.