Bore straightness, important?

[Alex Wheeler]

I have really enjoyed working on unturned blanks lately. Mine are always turned centered to the bore on both ends after I contour them.

Just for the sake of discussion... If you hold a barrel on both ends, and both ends are concentric to the bore, the barrel is say 30" total in length, considering even a slightly curved barrel, what would be the angular misalignment of one end? Say, it has 30 thou of total curve (which I doubt) the number is incredibly small. About .06 degrees of "run out".

At 3" inside the bore that would be .003" for the example 30 thou of curve. 1 thou per inch from the starting point, if the starting point were the end of the barrel.

If your holding a barrel at both ends and both ends are concentric to the center of the bore, that bore has to be perfectly straight. It cant have a curve and also have both ends concentric. It can have a point at each end dialed in, but not concentric. I wouldnt worry about the angular misalignment, but the fact that where the throat will eventually end up will be running out if you hold the barrel between centers. How much will vary from one barrel to the next. Thats the problem, your not in control of it.

 

[urbanrifleman]

If your holding a barrel at both ends and both ends are concentric to the center of the bore, that bore has to be perfectly straight. It cant have a curve and also have both ends concentric. It can have a point at each end dialed in, but not concentric. I wouldnt worry about the angular misalignment, but the fact that where the throat will eventually end up will be running out if you hold the barrel between centers. How much will vary from one barrel to the next. Thats the problem, your not in control of it.

1 thou per inch with the example given of 30 thou of curve.

 

[Alex Wheeler]

Your assuming a consistent curve. Thats not what we are dealing with.

 

[urbanrifleman]

Your assuming a consistent curve. Thats not what we are dealing with.

Actually, now that I lay it out, the runout would be half of that... assuming any kind of arc would be a reasonable approximation.

 

[urbanrifleman]

 

[Alex Wheeler]

The amount of deviation from the hypothetical centerline is not a problem. It has no effect on accuracy. Im not sure where your going with this.

 

[butchlambert]

I've never seen a barrel bore with a flat curve. I've seen irregular spirals, but not a true spiral.
Stan, could you even manufacture a barrel with a flat curve?

 

[darkangel_r2]

The amount of deviation from the hypothetical centerline is not a problem. It has no effect on accuracy. Im not sure where your going with this.

Well...The more curvature the bore has the more effectively crooked the crown would be correct?

 

[Alex Wheeler]

Depends on how to dial in the barrel. Some methods yes, others no.

 

[darkangel_r2]

Depends on how to dial in the barrel. Some methods yes, others no.

Either throat or muzzle has to be crooked right? And if I remember correctly you prefer to have your throat true.

 

[Alex Wheeler]

I have never posted how I chamber. I have made comments but never posted how I am currently chambering. It creates too much grief talking about gunsmith processes.

 

[Stan Taylor]

I've never seen a barrel bore with a flat curve. I've seen irregular spirals, but not a true spiral.
Stan, could you even manufacture a barrel with a flat curve?

I’m going to say it like this there’s a bunch a great gunsmiths on this website that does a great job but 99% of them wouldn’t know a straight hole if they seen it I’m sorry.

 

[butchlambert]

Either throat or muzzle has to be crooked right? And if I remember correctly you prefer to have your throat true.

Sometime a "crookeder" crown shoots best. Have you chambered many barrels?

 

[urbanrifleman]

Chamber runout. Barrel curve.

These concepts as discussed on most of these forums leaves me shaking my head.

 

[Stan Taylor]

Chamber runout. Barrel curve.

These concepts as discussed on most of these forums leaves me shaking my head.

I have one question where are you find this .030 thousand curvature

 

[VA_XTC_Shooter]

I have never posted how I chamber. I have made comments but never posted how I am currently chambering. It creates too much grief talking about gunsmith processes.

Why would you not talk gunsmithing on the gunsmithing forum? I thought that's what this place is for. That's why I am here to learn and discuss gunsmithing techniques.

 

[RGRobinett]

I've never seen a barrel bore with a flat curve. I've seen irregular spirals, but not a true spiral.
Stan, could you even manufacture a barrel with a flat curve?

Butch, I have such a barrel, but, it's because the UPS people "made it that way" . . . I keep thinking I'll chamber it and just see what happens (how it 'shoots') . . . but that elliptical span keeps telling me not to waste my time! Advise to barrel purchasers: NEVER accept a barrel in a damaged tube/triangular/square container! My customer, who ignored that advice, sent that barrel back, and I replaced it. I gave up on shipping barrels as a favor, or, for any other reason. RG

 

[darkangel_r2]

Sometime a "crookeder" crown shoots best. Have you chambered many barrels?

Sometimes is a strange word. Sometimes people win the lottery. All else being equal I would bet the farm on it not being a beneficial condition. The more crooked the bore at the muzzle the earlier one side of the bearing surface of the bullet would lose contact with the crown letting gas preferentially escape on that side. That is not a good thing. I am unsure of the magnitude of this effect. But I haven't found anyone else discussing it so its hard to put a number to it. It is minimized with a straighter bore.

Is there a way to accurately determine bore direction relative to a square cut muzzle face? And I mean accurately. Both angles theta and phi known with x-y plane as square cut flat muzzle face and origin as center of bore at muzzle.



If there is you could use a cutting tool where the cutting head spins instead of spinning the barrel in a lathe to make a crown. If that's too difficult to do accurately a flat cut perpendicular to the bore axis at muzzle might be easier. Then after the flat cut it should be possible to modify the crown in other ways as you can center coning cuts off the flat face centered at the bore. That way both throat and muzzle could be closer to square to bore at their respective ends. Index your max angles to 6 and your ready to go.

I have not chambered any barrel, so feel free to tell me if what I am suggesting is incorrect or mechanically impossible.

After reading Tony Boyers book and how he describes the different grades of barrels from hummers to junk I've been thinking about what mechanical explanation there could be for those grades as he describes their properties. I think a big part of the effect he describes is achieved by reducing epicyclic bullet motion at the muzzle keeping the meplat truer to axis of rotation.

Mass asymmetry and tricyclic wobble motion assessment using automated launch video analysis

This paper describes an approach to identify epicyclic and tricyclic motion during projectile flight caused by mass asymmetries in spin-stabilized pro…

www.sciencedirect.com

I think square cut even lands so the bullet has even engagement and good alignment at the start is one part (helps make bullet exit timing and bullet alignment consistent). I think having a crown that is close to true to bore is another part (imparts less pitch and yaw at muzzle exit). And I think a harmonic profile that heavily favors a single plane is another. There's many many more (general internal dimensional accuracy, internal surface finish, ect..).

 

[urbanrifleman]

I have one question where are you find this .030 thousand curvature

It was just an assumed "bend".

Dont get me wrong, I have no way to measure whatever that would be.

People indicate in one end of the barrel then go to the other end and talk about the curve indicated on the other end.

I don't. But people do.... and they make a big deal about it.

 

[Dusty Stevens]

It was just an assumed "bend".

Dont get me wrong, I have no way to measure whatever that would be.

People indicate in one end of the barrel then go to the other end and talk about the curve indicated on the other end.

I don't. But people do.... and they make a big deal about it.

Hes asking if you are talking about if you had the barrel between centers and measured it in the middle or if it was indicated into the throat and you see it at the muzzle bore. Hes just asking so he can better explain what is happening