TC barrel windage out by 20 MOA

[French Colonial]

*** Update ***

I put the barrel on a proper datum today and found that it is .020" out of straightness, in the expected direction, and biased towards the muzzle. Measuring either side was the same (opposite directions), so I think the straightness problem is real, and not just a defect in the octagon flat. I will however try all of these good suggestions before resorting to bending the barrel.

Don't be afraid of bending the barrel it works and really your straightening it.

 

[MistyMuzzy]

Don't be afraid of bending the barrel it works and really your straightening it.

Do you have any guidance on how much deflection I need to get a measurable set?

In enginnerd talk: how much deflection do I need to apply to get to plastic deformation? There are tons of variables here, but is it .010", or more like .100"? Just looking for a conservative starting point.

I guess I could model it and run FEA, but I need the hardness to find yield stress. Is HRC 25 at the low end? I assume through hardened. Maybe I'm taking this too far and just need to find a fork-ed tree..

How big is the risk of buckling the tube and wrecking the barrel relative to the amount of deflection needed to apply a set?

 

[M. De Land]

Misty Muzzy, I agree with De Land but contend where ever the bore is off on the interior the ball only goes where the bore end is pointed. Yes I blocked the ends of the barrel and used my shop press to gently bend the barrel, and not by much at that using a dial indicator to monitor the bending and check the set on the barrel. It took ok cause he reset the sights almost barrel centre and shoots a 3” group at 100 yards.
coupe

Well what happens if the bore is board on a curve or serpentine the hole in the end can be centered and still tangential to the flat orientation. In other words the bore is on an interior angle that is not linear with exterior flats hence it shoots off at the same tangent angle even know the muzzle is on center and the exterior appears straight.
I'm quite certain if one pulls the breech plug and centers the barrel in a lathe head stock then peers through the bore at low RPM that the bore will look like a jump rope and the exterior remain fairly on center. This will reveal if the barrel is bent or bored on a curve or serpentine.
Also crown filing to vector gas off bore line seldom effects good accuracy. A patched ball is more forgiving than a square bullet base but an uneven crown will usually be very hard on a patch at loading and later cause inconsistent release of the ball.

 

[Indy Durtdigger]

Here's a thread from the Gunbuilders Bench section of the forum.
Bending or straightening a barrel

 

[Bad Karma]

How the Springfield Armory does it…
https://thefiringline.com/forums/showthread.php?t=598579
And over at ALR
https://americanlongrifles.org/forum/index.php?topic=19603.0

 

[Bad Karma]

Here’s an interesting picture of a shortened barrel from the muzzle. It apparently had some runout probably not apparent until after the barrel was cut.. seems I remember being told that barrel should always be shortened from the breech end rather than the muzzle. Likely doesn’t affect accuracy at all.

 

[MDH]

I'm a mechanical engineer, so I'm familiar with machining processes and have access to granite blocks and measurement instrumentation at work. Our workplace is filled with shooters and hunters, so I get no gas for bringing in gun parts over lunch break.

If you are using standard TC sights the mounting screw holes may be out of alignment.

 

[MistyMuzzy]

Here’s an interesting picture of a shortened barrel from the muzzle. It apparently had some runout probably not apparent until after the barrel was cut.. seems I remember being told that barrel should always be shortened from the breech end rather than the muzzle. Likely doesn’t affect accuracy at all.

Yuck! I can't believe that thing left the plant like that. How far did you have to cut to see that? It must have been as ways if it was not visible at the muzzle.

 

[M. De Land]

Here’s an interesting picture of a shortened barrel from the muzzle. It apparently had some runout probably not apparent until after the barrel was cut.. seems I remember being told that barrel should always be shortened from the breech end rather than the muzzle. Likely doesn’t affect accuracy at all.

Well the trouble with shortening from the breech is all the pins or wedges, flash hole or drum no longer line up with the stock and lock plate mounts. It's always much easier to shorten from the front end and if only touching up the muzzle it usually takes less than a quarter of and inch to get into fresh muzzle profile of lands and grooves.

 

[Bad Karma]

Well the trouble with shortening from the breech is all the pins or wedges, flash hole or drum no longer line up with the stock and lock plate mounts. It's always much easier to shorten from the front end and if only touching up the muzzle it usually takes less than a quarter of and inch to get into fresh muzzle profile of lands and grooves.

Yeah, if you don’t mind having the bore off center by an eighth of an inch…

 

[Bad Karma]

Yuck! I can't believe that thing left the plant like that. How far did you have to cut to see that? It must have been as ways if it was not visible at the muzzle.

I’m sure it was cut from a much longer barrel. not my gun but it demonstrates the problem with cutting from the muzzle and also the fact that runout can occur and usually we’re never aware that it’s there.

 

[M. De Land]

Here’s an interesting picture of a shortened barrel from the muzzle. It apparently had some runout probably not apparent until after the barrel was cut.. seems I remember being told that barrel should always be shortened from the breech end rather than the muzzle. Likely doesn’t affect accuracy at all.

All that looks needed here is a new crown cut. Looks like the crown is off center not the bore.

 

[IanH]

In enginnerd talk: how much deflection do I need to apply to get to plastic deformation? There are tons of variables here, but is it .010", or more like .100"? Just looking for a conservative starting point.

Don't go enginerd on it. That's coming from an ME, for what it's worth.

First, unbreech it and read the light rings to discover exactly where and how much the bore is curved.

Ignore the outside of the barrel, it is meaningless because all gun drills wander a bit and any barrel which has not been straightened the old fashioned way relative to the bore will have deviations, some of them egregious.

To straighten the bore, you will need a sturdy bar or workbench, some wooden blocks, a big C-clamp or ball joint press, and a 1" dial indicator and stand. Place your blocks equidistant from the apex of the bend and your clamp ON the apex of the bend. If the bend doesn't coincide with a flat, use angle blocks or shims or notch the blocks appropriately to rotate the barrel as required to attack the bend exactly on its plane. Depending on barrel material, length of curved bore, and wall thickness it might take 1/8" of deflection to reach the elastic limit or it might take 3". Likely much less than 3" but I had a 20-gauge barrel with a hook just past center and again near the end in a slightly different direction and the middle one required an unbelievable amount of movement to reach the plastic point. Bend on it and release, measure the result. If nothing, repeat in, say, 50 thousandths increments until it does. It doesn't take long. When you begin to see some permanent results via your indicator not returning quite to zero, take the barrel loose and read the light rings again. Repeat until the bore is straight.

Clean up the crown and go shooting. If it's still misbehaving, repeat the bending until it isn't. There's no way you'll collapse that barrel unless you block it on the ends and drive over it with a truck. A thin shotgun barrel is a different story, especially if you don't use shaped blocks with a lot of surface area to distribute the stress.

 

[MistyMuzzy]

Don't go enginerd on it. That's coming from an ME, for what it's worth.

....

Yes! Thanks for the starting point. What you outlined was pretty much my plan, except I was going to use an press instead of a c-clamp. I'm not going to de-breech the gun until I have to, because that fit is perfect right now. I'm going to start by taking the .020" bow out that I can measure externally and see how it shoots. I'll update when I get to it in the next few days.

 

[M. De Land]

*** Update ***

I put the barrel on a proper datum today and found that it is .020" out of straightness, in the expected direction, and biased towards the muzzle. Measuring either side was the same (opposite directions), so I think the straightness problem is real, and not just a defect in the octagon flat. I will however try all of these good suggestions before resorting to bending the barrel.

Your barrel probably is made of 12L14 which should bend easier than 4140-50 or 1134-7 barrel certified steel.
The whack over a hard wood block will get most of the bend out and sometimes is all that is needed because it seems to de-stress the original bend but I usually have to do the final adjustment with my V blocks and a heavy c clamp taking 1/16 inch advancements until it corrects.
I've also been told but have not tried that when a barrel is under tension in V blocks to correct a bend that a dead blow with a lead hammer will also relieve stress. I don't think they are striking the barrel itself but rather the mount or table the V blocks are on. Apparently it's the shock wave doing the work.
If the bore is bored crooked than orientation to vertical is the best fix in my experience because bending to correct a crooked bore stresses the steel which may or may not remain stable with heat changes .

 

[Bill M.]

If I were going to try to "straighten" a muzzle loader barrel I would rig it so I could apply incremental pressure and with a dial indicator to tell me when I passed the elastic limit and got to the permanent set range. And...I probably would not try to "straighten" a barrel that shot well otherwise. I would work on the sights and go forth.

 

[Bad Karma]

If I were going to try to "straighten" a muzzle loader barrel I would rig it so I could apply incremental pressure and with a dial indicator to tell me when I passed the elastic limit and got to the permanent set range. And...I probably would not try to "straighten" a barrel that shot well otherwise. I would work on the sights and go forth.

One of my Green Mountain barrels is somewhat bent in windage. I centered the foresight and adjust windage via the Williams peep rear. Still bothers me slightly when I glance at the off center windage indications… I just try not to look.

 

[mjthornhill]

Here’s an interesting picture of a shortened barrel from the muzzle. It apparently had some runout probably not apparent until after the barrel was cut.. seems I remember being told that barrel should always be shortened from the breech end rather than the muzzle. Likely doesn’t affect accuracy at all.

Looks like the bore itself is not that far off. The chamfer at the end is what is off center. If you have calipers or a micrometer try measuring down inside the barrel past the chamfer. The difference from side to side in where that chamfer starts may be a lot of the problem.

 

[mjthornhill]

All that looks needed here is a new crown cut. Looks like the crown is off center not the bore.

Amen!

 

[gustaf]

How they do it at the factory.