Query: Rifle bores. How true are they really? How much does it matter?

[oldred]

To me the chamber should be perfectly aligned with several inches of bore ahead of it.Bill

That's the most important thing involved, having the other end of the bore centered accomplishes nothing except having both ends of the barrel centered on the AXIS, which is nothing but an imaginary line running from the center on one end to the center on the other end. But that's what you WANT is what some will argue! But why? That axis is not representative of the bore and is just an imaginary perfectly straight line extending through the barrel with the bore wandering around it along it's path from one end to the other. I think this is the flaw to the thinking when centering both ends, confusing that AXIS line with the actual bore but it's the bore that matters! Nothing we can do is going to align a less than perfectly straight bore along it's full length but we can align the last few inches of the bore so that the cartridge is in alignment with the BORE, to heck with the AXIS because that bullet is going to follow the bore not the axis! By centering both ends of the barrel we align the cartridge/chamber with that axis and because the bore will almost certainly be wobbling by some amount at that point due to the fact it does not truly follow the axis then we end up cutting our chamber true to the AXIS but at an angle relative to the BORE. By allowing the outboard end of the barrel to locate at a point regardless of center that will have the indicator rod indicating true along it's entire length we will have centered and aligned the last few inches of the bore, if we then cut a chamber it will not only be centered but in alignment with the bore itself.

I fail to see how those who insist both ends MUST be centered can't see the problem? If we had a visible curve in the barrel it would be easy to understand, when centered on both ends the barrel would be visibly wobbling around those centered ends. With our lathe tooling square with the turning axis look what a chamber or crown would look like if cut into that wobbling barrel! It's the same thing with a normal barrel just on a much smaller scale, the error is still there but reduced to a point where we can normally get away with just ignoring it. But why do that if we can avoid that tiny error altogether and get an even more accurate cut? By allowing the outboard end to go to whatever point off center it needs to in order to get the indicator rod to run true along it's entire length then the bore is in alignment with the axis at that point on the barrel, and thus the lathe tooling also, sure the other end is off but so what? What is important is that the chamber/cartridge be pointing parallel to the bore at the point where it is located, that won't happen if both ends are simply centered so that the axis is aligned and the bore (which does not truly follow the axis) is allowed to wobble at that point which a wobbling indicator rod will surely indicate!

[oldred]

BR, the very first thing he said is exactly the point I was trying to make, the chamber should be aligned with the first few inches of bore ahead of it!

How can that possibly happen if both ends of the bore are centered on the lathe axis but the bore is meandering around that axis? You yourself clearly, and correctly, said in your first reply there is no such thing as a perfectly straight bore. Now instead of just asking me silly questions about why people don't beat a path to my door, as if I were the only one doing this the way I am talking about, why don't you EXPLAIN where I am wrong? If you can explain it then great then we would both understand why you would be right and I would then know why that wigging pin is unimportant? Can you do that?

Once more please explain why this is wrong!

We can agree a bore is not perfectly straight, right? If it's not straight then it can't be truly parallel to the axis line that extends through the bore from the center of one end to the center of the other, please explain why that would not be true? It's going to drift, as you yourself have said, all along that axis (the imaginary straight line that would be the bore IF the bore was straight). Now by simply centering both ends of the bore we are centering the bore on the axis BUT only at the EXACT point the indication is taken! As long as that indicator rod will not center along it's length then it's obviously going to be out of alignment with the axis that the ends of the bore are centered relative to, IF NOT PLEASE EXPLAIN WHY NOT? If that rod is properly fitted to the bore it is pointing along the path the bore is taking AT THAT POINT which obviously means the bore itself is pointing slightly askew of the axis line extending through the bore end centers. It may perfectly center at the point the indication is taken but if the rod will run off center when the indicator is moved outward it should be obvious it would also move off center if we could somehow indicate an equal distance along that rod inside the barrel, if one end of that straight rod will not center along it's length then PLEASE EXPLAIN how the other end extending into the barrel could be centered along it's length? If the rod is wiggling then how is it the bore is not wiggling, can you explain that? You very plainly said this about what I am suggesting, in your words,

" your method does nothing to align any more than the 2 inches or so of the indicator rod"

but that is my whole point, since that rod is a true indicator of the bore's path along the spin axis then aligning it also aligns the last few inches of the bore! If that rod is not aligned along the last two inches, and by that statement above you seem to be saying it is unimportant, then how can the length of that straight rod extending into the bore be aligned either?

Quite simply if that pin has run-out on one end it absolutely MUST have an equal amount of run-out on the other end and if has run-out but is fitted to the bore PLEASE EXPLAIN how this does not reflect run-out for the bore also at that point?

Please show me where I am wrong here, CAN YOU DO THAT?

You said you were not mad but rather just frustrated because someone kept insisting their way was right but you need to look at yourself when you say that, YOU are insisting your way is right and what I am saying is wrong but I have been explaining my point and asking you to show me where I am wrong! All you can seem to do to back up your position on this is to say "Oh no, no ,no, that's all wrong because other people I know do it my way", well other folks that do what I am describing explained it to me and as I said I didn't just dream this up myself. Maybe we are talking about the same thing and it's just a misunderstanding but if you are just going to continue to take cheap shots at me and then run off without explaining your point how can we know, come on man up here and EXPLAIN TO ME WHY I AM WRONG!

It's this simple, just explain to me why or how that indicator rod centered at the bore but indicating run-out along the rest of it's length does not indicate run-out in the bore? Just explain that because that's what this is all about.

[Cap'n Morgan]

It has been a long time since I chambered a rifle, but nowadays I thread quite a few barrels for suppressors using a five-axis CNC mill. With a bore-fitting gauge pin and the machine's Renishaw probe (and a bit of math) it's fairly easy to align the bore perfectly with the Z-axis (vertical) and then mill the thread with helical interpolation. A bonus is that the barrel doesn't have to be removed from the action.

If I were to chamber a rifle (and didn't have a chamber reamer) I would use the same approach as above and helical-mill the outside of the barrel for two or three inches to make it concentric with the bore. The barrel could then be chucked in a CNC lathe on the milled area and the chamber cut with a singlepoint tool.

[oldred]

A bonus is that the barrel doesn't have to be removed from the action.

This would seem to be a (very) high tech version of what I have been trying to describe, you are aligning the actual bore at the point of machining regardless of whether or not the breech end is centered along the same axis? Or at least the breech end is aligned only as much as the bore can be straight? Correct me if I am wrong but the method you are using insures that part of the bore to be in true alignment regardless of any drift that might exist elsewhere along the bore extending to the opposite end?

That has been my point all along, the part of the bore being machined must be in alignment with the tooling regardless of where this puts the opposite end of the less than perfectly straight bore in relation to the axis line extending through both ends.

[oldred]

Oldred, I stopped replying on your posts because you cannot accept that there are other was to chamber a barrel than yours. I cannot and will not converse with someone that is that close minded. I have told you I have done what you do, I choose not to especially with today's barrels. I can chamber a barrel just as accurately as anyone, I have proven their accuracy in competition. Still, you continue I am doing it wrong. I have asked you how many barrels you have chambered that have proven their accuracy, and you ignore it.

Believe me, if you had a sure-fire way of chambering a barrel that had an accuracy advantage over the top smiths in the benchrest game, you would be a rich man.

I will not continue this.

BR once again you are doing nothing but taking cheap shots and running off! You are doing EXACTLY what you are accusing me of doing by saying i am close minded and refusing to accept more than one way of doing things!

Rather I have tried my best to explain my position and ask to be shown what's wrong with it, I will gladly accept any way of doing this if you can show me a better way than I describe, all I ask is that you EXPLAIN why it works and is better but quite apparently you just can't do that!

I have asked repeatedly if you would please explain just how that wiggling indicator rod does NOT indicate an out-of-alignment bore?

You can't can you? Does it not make you wonder why you can't explain what's wrong with that?

I have asked why you seem to think that the indicator rod not indicating center along it's entire length is unimportant, after all you clearly said that the ONLY thing that would do is align the last two inches of the rod? If that rod is an indicator of the bore's path at that point then how can aligning that rod along it's length NOT result in true alignment of the bore at that point -regardless of the other end, can you explain THAT?

Apparently you have no answers because someone has challenged something you have taken for granted without really considering all the factors and now you simply can't explain why that differing opinion is wrong! If you can then quit taking childish cheap shots at me and show me where I am wrong because if I am then, unlike you apparently, I want to know!

[Cap'n Morgan]

Correct me if I am wrong but the method you are using insures that part of the bore to be in true alignment regardless of any drift that might exist elsewhere along the bore extending to the opposite end?

You have it right. Some suppressors are quite long and a crooked barrel could cause a baffle strike. That said, I have never seen any modern barrels with any noticeable misalignment between the bore and the outside contour, but I once shortened an old singleshot .22 Husquarna rifle and the bore was so much off at the middle of the barrel that I had to compensate for it in the scope mount to be able to sight it in.

[oldred]

BR your repeated accusations of close mindedness (wow, talk about the pot calling the kettle black look at your own posts!), etc are just childish like your other suggestions of why don't I ask "whoever"!

I have never said what you are doing won't work I said there is a better way of doing it! You can't answer a single thing I have asked because obviously you can't. This started when I pointed out that by centering both ends of a bore/barrel a crown, chamber or other machine work will be out of square to the bore and I gave multiple explanations why this is true, and despite your insistence to the contrary (without a single explanation of how or why) it IS TRUE! You yourself stated clearly that the method I am referring to does nothing more than align the last couple of inches of the indicator rod which is total nonsense, if it's not nonsense then please explain why not! I have asked you repeatedly to explain how that wobbling indicator rod does not indicate that the bore is wobbling also but you simply can't because we both know it IS wobbling and that's just plain common sense! If that bore is not wobbling at the point the machining is being done as that wobbling rod indicates then I CHALLENGE you to explain how this can possibly be!

It should be quite apparent to anyone that machining operations done with the bore wobbling at that point is going to be a less accurate method than the same operation done with the bore running straight and true at that point regardless of the center on the other end, again how about you just drop the childish accusations and explain where I am wrong?

FACT, if that indicator rod will not indicate true along it's entire length and has increasing run-out the farther out it is measured then it IS wobbling! Being straight and snug fitted to the bore then common sense tells us that the bore absolutely MUST be wobbling also!

FACT, a chamber, etc that is then cut with that bore wobbling at the point of machining can NOT be cut truly square to that bore! However if we ignore the outboard end of the bore center and allow it to move wherever so that the indicator rod will then center true along it's length (and thus indicating the bore itself is true) the chamber, etc will then be cut true to the bore!

Centering both ends will invariably result in that indicator rod only centering true in one spot due to the less than perfectly straight bore and it will wobble along it's length which indicates the bore too MUST be wobbling, if the rod wobbles so does the bore!

By insisting that centering both ends regardless of indicator rod wobble you are insisting that machining a wobbling slightly out-of-alignment bore is the better way than ignoring the outboard center and machining a bore that is aligned and turning true at that point. If you can dispute any of that then please explain how and where I am wrong? How can you possibly argue that machining an obviously wobbling bore is better than machining that same bore running in true alignment? I have asked you repeatedly to simply show me how I am wrong and why what I am suggesting is not a more accurate method but you obviously can't do that!

Throwing out insults such as "being close mined" or cheap shots such as "ask so-n-so" or other childish things like saying "people would be beating on your door", etc is the usual tactic for a lot of people when they are confronted with a differing point of view and then find themselves totally clueless as to be able to explain and support their own position. You have done this since your second reply to me yet YOU call me close minded! I have asked you repeatedly to simply show me how I am wrong and why what I am suggesting is not a more accurate method but you obviously can't do that!

Aligning that rod and eliminating the bore wobble at the machining point IS THE MORE ACCURATE WAY, Ok now simply tell me how that's wrong or are you just going to take another cheap shot and run off again without any explanation at all?

[B R Shooter]

Hopeless

[oldred]

Hopeless

Yes you are, once again just snipe and run because you obviously haven't a clue!

You leave me no reason not to assume that you have absolutely no idea about how the axis actually relates to the bore, or do you even know the difference? Do you even know what I am talking about when I mention the axis? I seriously doubt it at this point and you make it even more apparent with each reply, I have asked you to do as I did and explain your position but all you can do is childishly throw those cheap shots and insults without even trying to explain your point and you simply are making it apparent that you do not understand the basic principles involved. You keep trying to insinuate that this is just a goofy idea that I dreamed up myself when in fact it is quite commonly done that way, as I discovered when I started asking about it when I had serious questions about the true accuracy resulting from centering both ends of that bore as I had been told at first. Listening to the description of both methods made it clear to me just why what I had already seen for myself was in fact an error after all and the way I had been told at first was a flawed way of doing this. It matters not who does it one way or the other the mechanics of what's happening are the same for everyone and no matter who insists otherwise that wobbling indicator rod does not lie, the laws of physics are what they are whether you can understand it or not!


The whole thing "in a nutshell" as the old saying goes is that either method will work because the error, fortunately, in most cases is small but why accept any error when there simply is no reason to do so? THAT'S the real question! That has been my point all along, we can do as you apparently do and perform our machining operations with that bore wobbling about the axis, as will be clearly shown by that wobbling indicator rod. Alternatively we can ignore the useless step of centering of the outboard end and instead allow it to indicate that rod along it's entire length so that we are machining a bore that is, at that point, in true alignment in addition to being centered as will be indicated by the rod running true along it's entire length.

The choice is simply do we do our machining operations with the bore centered and running in true alignment with the axis or do we perform those operations with the bore centered at both ends BUT wobbling around the center line axis?

Again either way works, at least to a point, but one way is more accurate so which is that? A bore running true and square to the machine tooling or one that is wobbling about that axis the tooling is aligned with? I think the answer is obvious and it's also become quite obvious you have simply never questioned why that indicator rod was wobbling and what it really means to true alignment. The fact you apparently have no idea about what that means to true bore alignment was made abundantly clear when you literally yelled that my method did NOTHING more than align the last couple of inches of the indicator rod, clearly (to most people anyway) it would also mean the bore too was aligned at that point instead of wobbling as it invariably will if both ends are centered. You clearly said it does nothing when it fact it does a great deal to improve the alignment and thus the accuracy of the machining operations. NO? Then once again I ask you to explain why not, can you do that?

[Willbird]

That's the most important thing involved, having the other end of the bore centered accomplishes nothing except having both ends of the barrel centered on the AXIS, which is nothing but an imaginary line running from the center on one end to the center on the other end.

The thing is as I see is, is the line drawn between the center of the bore at the muzzle, and the center of the bore at the throat very much different than a line drawn between the center of the throat, and the center of the bore 3" deeper into the bore than the throat ?? For the most part the deviation between those two lines as I illustrated with some simple math is probably within the margin of error trying to indicate in a rod shoved up into the barrel. Some use what they call "Deltronic pins" which if I understand correctly come in .0001 increments to find the one that fits just right. Others are just using whole and half minus pins, or maybe just whole thousance minus pins. Others use some kind of rod that has chambering reamer pilots mounted on it ?

If you align muzzle and breech you can directly indicate the bore surface with a .0001 indicator. So we are balancing the accuracy of the alignment method with a known inaccuracy factor (that no barrel is straight).

It would be almost impossible to do any kind of blind test of methods due to the variance in barrels one to another as far as accuracy goes. Then lots and lots of shooters probably do not shoot well enough, and learn how to track a gun on the sandbags, and use a stock that tracks well on sandbags, plus dope the ever present wind.

So if we could assign a MOA value to a long intensive practice of drilling the chamber, indicating the throat and first few inches of bore straight, taper boring the chamber to get it to position, then reaming, it it every single time would shave .1" (1/10 of an inch) from average 5 shot group size...how many shooters would pay for the chamber that is .00001" more aligned with the rear 3" of the rifle bore ??

One guy I talked to chambers his own barrels by hand, he has a friend who threads the barrel for him (Stoll Panda action maybe) and he has a hole dug under the floor of his shed, he removed a board and mounts the barrel vertical, and he reams the chamber all the way by hand with a Tap Wrench .

Bill

[oldred]

Thanks for the common sense reply directly addressing what I have been asking, I would truly like to discuss this point by point. That's what these discussion forums are supposed to be about, honest discussion leads to understanding while childish name calling and insults lead to nothing.

I hope my rather crude attempt at describing the situation, the archers bow for example, makes sense of the principles involved. It is greatly exaggerated of course but in actual application when machining a barrel the same alignment error remains it is just reduced to an almost insignificant amount, key word there being almost. Yes I am aware that a range rod is not an absolutely perfect fit but properly fitted it should not be in error anywhere near as much as the run-out of a typical bore. Back when I first ran into the problem of not being able to maintain center along the length of the rod I tried different things to "fix" this since I assumed I must have been doing something wrong, after all I was doing it just as I was told. I tried turning the rod in the bore, checking the straightness on an inspection surface (using a dial test indicator on a Starrett granite surface plate) and finally even trying different rods even to grinding my own with my tool post grinder to get the fit as close as possible. Nothing changed the reading by any measurable amount so obviously I was getting a true indication of the actual bore (this has been the same with every barrel I have done) this is when I gave up and started researching the procedures. What I quickly discovered is that there are two common ways of doing this, the "center both ends" method and the "truing the rod method" with the latter just making way more sense to me for the reasons I have described in previous posts. When looking at the "center both ends" it is clearly apparent that any machining operations are going to be relative to the axis, it's just a fact of the mechanics involved since if both ends are perfectly centered in the lathe then the spin axis runs perfectly straight through from the exact center on one end to the exact center on the other end. Set up like this any tooling on the lathe and any machining operations are necessarily going to be in alignment and squared to the axis just as they would with any other rod or shaft being turned in the lathe. The error therefore lies in the fact that the bore will have some amount of error, or drift, which means it (the bore) will not be perfectly parallel to that center-line axis unless it was perfectly straight with zero drift which I think is pretty much accepted that such a bore is going to be nonexistent as some drift will be present in even the best of them. Thus by aligning with the axis we must accept any error that results by the normal bore run-out along that axis, obviously we can not accurately align that less than perfect bore along it's entire length but we can align it at the point of machining operations. By doing that we will be performing these machining operations in alignment with the bore at that point rather than the just the axis.

When set up using the center to center method the indicator/range rod is unavoidably going to have run-out due to the normal within-tolerance errors associated with the barrel making process and since it fits the bore so closely it indicates the actual true path the bore is taking at that point, if machining operations are performed with this set-up then they will be in alignment with the axis but very slightly askew of the actual bore as indicated by that rod. Fortunately for those using this method the error is small enough to not be much of a problem unless the barrel has significant drift but it is still an unnecessary error nonetheless that is easily eliminated by centering the rod along it's entire length. To me there is just no way around it, if that range rod will not center along it's entire length, as it normally will not with both ends of the bore centered, then it (and thus the bore also) will be wobbling because the bore can not be in perfect alignment with the axis and thus can not be in perfect alignment with subsequent machining operations. Now if we forget about the outboard center and allow it to go wherever so that the rod, and by extension the actual bore itself, to run true along it's entire length then that part of the bore itself will also be running true at that point and machining operations will be in fact truly square with the bore itself at that point and not just the axis.

That's my take on it and (honestly no sarcasm intended) if that's wrong and someone can explain how and why it's wrong then I would like to know. I think what I am describing is just simple machining principles and mechanics, I just don't see the logic of not believing that wobbling indicator pin when properly fitted does not indicate a wobbling bore! Although I have asked several times, and not just here but in other threads and on different forums, I have yet to hear anyone explain how it could be possible? When we consider that bore run-out/drift is normal and to be expected to some degree then run-out of that rod should also be expected as a simple representation of the normal within-tolerance bore run-out. By aligning the rod true along it's length we remove the error factor by aligning the machining operations with the last few inches of the bore, sure a chamber/cartridge may not be pointing along the the true axis and thus directly at the center of the opposite end but far more importantly it will be pointing parallel and squared to the bore at that point, after all the bullet follows the path of the bore and not the axis!

[Willbird]

I think if I were to go after aligning the 2-3 inches ahead of the throat I would find a way to directly indicate it. The guy who described his direct indication rig had found a way to make a 1:1 lever that did not have any play in it.

A deep read of the book Rifle Accuracy facts will reveal some other flaws to the whole system worthy of evaluation.

Bill

[oldred]

A tool for that might not be as complicated as it might at first seem, it only has to indicate zero and measuring the amount of run-out really wouldn't matter, we might use something like a co-axial indicator? The usual run-of-the-mill versions arn't normally dead on accurate when we are talking the kind of accuracy desired here but still there are undoubtedly versions out there that are made to the tight specs we would need.

[Willbird]

Well he and I talked on the phone, and he described what he had made. It was a 1:1 lever as I recall, that would reach maybe 3" up into the bore. He controlled one axis of motion with the pivot, then it ran alongside something like a dowel pin to control any side play. It worked well for him to actually measure the grooves themselves, and find a mistake in a cut rifled barrel. Two grooves were 0.0000 to 0.0000 and the other two were 0.0000 to 0.0003 or some such...the mfg replaced the barrel. The guy who shooting a tricked out 30-30 rifle in some kind of BR for score match....he took it to one of the BR for group ranges/matches and did better then they thought he would...but not in the hummer type groups.

With a 1:1 lever that was pretty rigid (carbide would be ideal...) one can just use a .0001 plunger type indicator to do the rest of the job. I have a far bigger lever system my dad build to allow indicating a shotgun barrel straight, the lever went down in the bore, and you used a .001 indicator at the muzzle, it allowed you to indicate two sections of bore at the same time.

The long indicator tips reduce the sensitivity of the test indicator so they are not doing what we need to do really. Double the tip length and you halve the sensitivity of the indicator.

[oldred]

I think I can visualize that setup and it sounds as if would indeed work quite well.

[Willbird]

I think I can visualize that setup and it sounds as if would indeed work quite well.

Chances are that some mechanism already exists that is at least a start in the right direction .

Hmm wild eyed pondering whether a rigid non contact airgauge setup would work, make a carbide mandrel say 4" long that was .210 for most of it's length with the end being spherical and say .215" dia...then make an air hole in one side, and balance a steel ball on the air flow, then rotate the barrel.

The carbide dies one past employer made to cold head the hex on spark plug bodies they used an air gauge to measure the final size across the flats. Could use the other methods first, then use the air gauge as a final check...the closer to running true everything is the closer to .218 (for a .224 centerfire barrel) the ball on the end can be and not touch anything.

[bob208]

the bores are as accrete as the man machining them and the method he uses. when I worked with hoyt. we would drill the blank. then hold it between centers to get the outside running straight and true to the inside. then ream and rifle them. then there is a trick to milling the flats.

in a book about harry pope. a shooter took one of wis barrels and shot it for group . then trund the barrel 90 deg. and did it again until he made it all the way around. it shot in the same group every time.

[country gent]

Look at the id conversion for the old starret dial imdicators, These were the dial with a mounted stem and plunger out the back. The Id conversion clamped on the 3/8 stem around the plunger and one point rode the plunger the other the part.

[Willbird]

the bores are as accrete as the man machining them and the method he uses. when I worked with hoyt. we would drill the blank. then hold it between centers to get the outside running straight and true to the inside. then ream and rifle them. then there is a trick to milling the flats.

in a book about harry pope. a shooter took one of wis barrels and shot it for group . then trund the barrel 90 deg. and did it again until he made it all the way around. it shot in the same group every time.

Well if you think about the geometry involved there . Aligning muzzle to breech, then creating the breech mounting point using that line from muzzle to breech, the only error will be the curve in the final few inches of barrel. If the barrel was aligned to the rearmost 4" of bore, and that same mounting point created, the muzzle end of the bore would run out more in relation to the mounting point.

Bill

[oldred]

Well if you think about the geometry involved there . Aligning muzzle to breech, then creating the breech mounting point using that line from muzzle to breech, the only error will be the curve in the final few inches of barrel. If the barrel was aligned to the rearmost 4" of bore, and that same mounting point created, the muzzle end of the bore would run out more in relation to the mounting point.Bill

This has been my point all along and this fixation with aligning the muzzle and breech ends ignores the fact the bore does not perfectly follow that axis line! Sure by centering both ends and then turning the outside of the barrel will give us a straight barrel with the bore being centered on each end but where is that bore going to be located in relation to the outside of the barrel at, for instance, the middle of the barrel? By centering both ends we get the bore dead-centered with the axis on the ENDS but it is still going to be out of alignment by varying degrees along it length! Again by simply centering the ends we get the ENDS of the bore dead centered at the point of machining but we are ignoring that all important ANGLE at that point, how much angle is determined by the amount of error or drift that is inherent to all bores. Center both ends as closely as possible but then indicate a couple of inches back inside the bore and see what we get, that wobbling indicator/range rod is proof the bore also is wobbling at that point and when both ends of a barrel are centered that rod will wobble.

By aligning the last few inches of the bore we have that part running in true alignment regardless of where the other end may be but having the part being machined in alignment with the tooling is what is important and that can't possibly happen if the last couple inches of the bore is wobbling even if it does indicate center exactly at the end.