Barrel Bore Curvature

[grit]

Because I don't see the parallel lines, I see a triangle.

It's not quite parrallel. I played with a calculator for a few minutes. An extreme example could equate to a few inches at a 1000 yards. In most cases, the curvature would be average, the timing something less than horizontal. You might see an average error of an inch or two at a 1000 yards. How would you ever know?

Don't misunderstand me, I time them. From a practical perspective, it makes little difference.

 

[J E Custom]

A little side note:

Durring World War 2, the germans developed a rifle that would shoot arround corners by bending the barrel 90o. it was accurate enough to work and was used a limited amount.

I'm sure it was rough on the bullets and the barrel, but it worked because when the bullet left
the barrel it went in the direction it was pointing.

Like Coyboy,I feel that a few thousandths would make little if any difference. but the thickness difference on a barrel would, when the barrel heated up.

I have heard that the best way to orent a barrel is with the curve up so that the weight will
tend to straighten it out. if the barrel is this weak and will not hold up its own weight I would
have to decline using it.

Straight and concentric to the bore is the only way to go. Just like the ammo we load.

J E CUSTOM

 

[Edd]

Just to make sure I understand your position.

You check a barrel to see if it passes your straightness test and if it passes you don't index it in any particular position?

 

[JRu]

An exaggerated image to hopefully clear the issue.

Any lateral offset (A) in the bore axis and LOS is introduced is compensated by erector tube angular movement to have the line of sight meet the bullet path at sight-in distance (B) , and the amount of error is relative to the sight-in distance's multiple.

So let's say at a sight-in distance of 100 yards you have compensated for an error of .1" (if the bore axis and LOS were parallel, the POI would be .1" offset to POA) . Then at 200yd you have an error of .1" in the opposite direction. At 300 the total error is .2" , etc. At 1000yd (C) the error is .9".

Shoud you zero at 150yd, the error is a multiple of 150. So for an offset X and sight-in at 150, at 300 the offset is -X, at 450 -2X, at 600 -3X etc.

This is completely analogous to when a scope is mounted offset or canted but when shot the rifle is straightened relative to the reticle. In both cases the scope adjusts to meet bullet path at sight-in distance and when looked from above the bore, scope and target form a triangle, and the amount of error can be calculated using the law of similar triangles.

 

[grit]

JRU,
Nicely illustrated. Thanks. The bowed barrel in the illustration made me chuckle.

 

[Edd]

An exaggerated image to hopefully clear the issue.

This is the image I was getting in my head when thinking about this issue. I had calculated an impact point change of approximately .15" per 100 yds per .001" of curvature. Perhaps someone would confirm those numbers.

 

[J E Custom]

Just to make sure I understand your position.

You check a barrel to see if it passes your straightness test and if it passes you don't index it in any particular position?

If this was a question for me, the answer is yes.

The reason is the bore is .0000 to .0001 end to end from the barrel maker I prefer
and the worst I have ever gotten from him on the TIR between centers on the outside
has been .0015 using a .0001 last word indicator.

I have used customer furnished barrels that were up to .005 TIR and they shot well below
1/2 MOA but I will reject any thing above that and have many times. (let's face it, you pay
good money for a custom/quality barrel and it should be strait).

Most barrel makers will gladly replace a poor barrel (But some dont want to, and I wont use
there barrels).

It is simple economics, If a smith has to spend extra time straitening and/or indexing a high
end barrel he will have to charge more for his time. Or just switch to a quality barrel maker
that stands behind his product.

Even the best barrel maker can produce a less than perfict barrel occasionally, But he should
be willing to replace it without complaining.

So I "don't'' Index barrels because I won't use barrels that may need indexing. (I don't have to)

What is the saying ? One Aww $#1T cancels all Atta boys. So to be confident of a quality firearm
It is best to reject any marginal componants.

Just the way I feel about any poor quality parts.


J E CUSTOM

 

[Edd]

What barrel maker do you use?

 

[J E Custom]

What barrel maker do you use?

I like Lilja, Kregier,hart, shillen,Brux, Border,Lawton and Pac-nor.

I have had a few issues with all but the Liljas. (My personal favorite because of consistency
barrel to barrel). On several occasion's,Lilja has called and told me that I would have to wait
another week or two because they did not like the barrel they made for me.(They would not
tell me why just that it was not within Specifications or appearance standards. I like the fact
that They do the rejection instead of me.(This helps the schedule and tells me that they are
looking at every barrel that comes out of their shop.

With the others they send there barrels to me and after I check them I have to send them some
of them back if they are over my acceptance level. They may be rejecting some of them but they
dont call to explain why my barrel will be a delivered little later.

The ones I listed have always been happy to exchange the barrels for me. and normally send
me a great barrel in its place. I am sure there are other barrel makers that will do the same
but I have ether not used them, or had problems with their quality or service and wont use them.

Barrels are the key to accurate rifles and no matter how good the smith is he can't make a rifle
shoot great if the barrel is not true, strait and of the best quality. The barrel will always limit the
rifles ability.

J E CUSTOM

 

[375 CHEYTAC]

I agree with what J.E Custom said through out these posts.
I dont index a barrel but for those of you that do, you should be cracking down on quality control. Do you measure the minutes and seconds when you instal these bent barrels or just get the curvature close and there fore no rifles shoot straight.

 

[orwapitihunter]

375CT,
I find your post condescending. You make it sound like those of us that set up our rifles to account for bore curvature, are using substandard blanks. The barrels that I fit and chamber aren't "bent". Do you measure the curvature of the bores in the barrels that you fit/chamber? Putting a barrel between centers and measuring the run out on the exterior of the blank only tells you how true the external dimensions are in relation to those two points of the bore. And I know that some will argue that using a center to locate true center of the bore is not precise enough to put you in the sub .0005" range. I have never bothered to check the validity of their statements for myself but if a smith was so inclined it would be easy to check the repeatability. If you can't break it down and set it back up and get the same measurements, then that range of measurements will show you the accuracy of your set up. But I'm getting off track.
What percentage of the barrels that come in your door do you return to the manufacturer? I use a lot of the same barrel makers that were mentioned, Lilja, Hart, Brux, Broughton, Shilen, Benchmark, PacNor, and MRC. My return rate is less than 5%. Sometimes I have the customer supply the blank, so I don't get much input as to the manufacturer of the tube. I have also picked up bargain barrels (MRC) that showed lots of bore curvature, washed out lands at the breech and oversized dimensions at the breech, (hard to use a center to locate center). I used those barrels on a couple of my personal builds, and they both shot in the .2's and .3's.
My personal feeling is that the slight bore curvature found in barrels has little to no effect on accuracy. But if it might show up as a right/left component at long range in the bullet's flight, I want to eliminate it. And if I pick up a couple of MOA in scope travel that is a win also.
I also feel a lot of curvature that results in serious wall thickness differences can cause uneven heating and a shift in POI. One more time, how are you checking bore straightness?
With all that said and the steam level back to normal, I'm off to indicate another barrel, cut the chamber as straight and centered with the bore as possible, and indicate the muzzle so that the curvature of the bore is timed so that any true of theoretical problems are nullified.

 

[Edd]

How much would it cost to have a gunsmith check a new barrel blank? I would like to have the high end of the muzzle marked and would like to know how much the bore is curved.

 

[orwapitihunter]

Edd,
I won't comment on how much another smith might charge you for that service, but if you approached me with that request, this is what I would tell you.
It doesn't work like that.
#1. Until you cut the blank to your finish length you can't determine where the bore is pointed.
#2. I have no way of measuring the actual amount of curvature. Think of it like this, how can I reach 10", 15" or even farther into a hole the size of our common bores, and make a measurement?
A person could slide precisely ground undersized pins through the bore and determine some level of straightness, but what that really tells you is questionable.

We are all very fortunate that there are so many great barrel manufacturers. The closer those barrels get to having a perfectly straight bore the less all this matters.
With that said it still amazes me to see some of the dog legs that come on factory rifles and shoot more than adequate for general hunting purposes. I have shortened numerous factory barrels for their owners, and cut up numerous factory take offs for thread protectors and such. It's crazy, but not uncommon to find a bore .030" to .050" of an inch from the center of the barrel.

 

[Edd]

What would it cost for you to mark the direction the barrel is pointed at it's current length?

If you have no way to tell how much it is curved, what do you use as your basis to reject the barrel and return it?