looking thru the bore after its dialed in for chambering

K

kman

Guest
my friends got his barrel dialed in thru the head stock with 4 jaw chuck and cat head on the other end after dailing in he looks thru the barrell and notices the hole kinda wonders off center line in the middle but looks ok on either end,his question does this effect accuracy,my friend john says he has noticed this on production barrels he has cut off and set back on his varmit rig but hasnt had much experience with custom barrels before,what do u guys think
 
Some are straighter than others, I wouldn't worry too much about it. I would probably index the high spot (muzzle end) at 12'oclock when threading.

I had a 6.5 Bartlein barrel that looked like a friggin' jump rope that I darn near sent back (worst one I have seen), but sure am glad I didn't. That barrel turned out to be a true Hummer. YMMV
 
Kman, EVERY barrel will show the exact same thing, some to a greater degree than others.

This is a product of the deep hole drilling proccess. The Gun Drill will tend to wonder off in different directions at different points in the blank, if you look close, you can often see spots that tend to run out in one direction, other spots just the opposite.

That is why many of us use the method of chambering that employs finding two spots that we want to run dead true, then single point boring the third spot true with these.

It is a surprise when many chuck up a barrel and look through it as it is turning. Makes you wonder how they shoot as well as they do.........jackie
 
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eddief said:
Some are straighter than others, I wouldn't worry too much about it. I would probably index the high spot (muzzle end) at 12'oclock when threading.

I had a 6.5 Bartlein barrel that looked like a friggin' jump rope that I darn near sent back (worst one I have seen), but sure am glad I didn't. That barrel turned out to be a true Hummer. YMMV
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straight barrels don't shoot
 
Al, I have a 30BR 1-18 Krieger, probably the straightest barrel I have ever chambered, that would argue that point.......jackie
 
When you wonder about the process of pushing that poor little gun drill through a barrel blank, especially with 'sub' calibres, that they are as straight as they are!

I wonder what the barrel makers reject rate is like, just for the gun drilling process?????
 
Darren White said:
When you wonder about the process of pushing that poor little gun drill through a barrel blank, especially with 'sub' calibres, that they are as straight as they are!

I wonder what the barrel makers reject rate is like, just for the gun drilling process?????
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That's why we have different bore sizes. Odds are pretty good that one drilling will work okay. The barrel maker might even get a couple of chances to redrill a bad barrel that's already rifled!
 
Welcome to the current crop of barrel steel. It may get worse before it gets better.

Recently I chambered a 1980's McMillan blank and it was very straight. This week I was at Terry Leonards shop and he was chambering a several year old Hart blank and that bore is straight.

This crooked bore thing we are seeing now may be a combination of barrel steel quality and manufacturing process.

The worst shooting barrel I have chambered, period, has a bad kink about 3" ahead of the chamber.
 
Darren White said:
I wonder what the barrel makers reject rate is like, just for the gun drilling process?????
Click to expand...
Zero...

AeroE said:
That's why we have different bore sizes. Odds are pretty good that one drilling will work okay. The barrel maker might even get a couple of chances to redrill a bad barrel that's already rifled!
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Gun drilling down an existing hole? Ya think? :confused:

Re-Ream? perhaps, but I'd even be surprised if they did that.
 
The problem...

or at least ONE of them, in drilling a straight hole is that the barrel blank material (usually un-contoured bar stock) is not really straight to begin with, and/or not perfectly symmetrical. Then, when you drill the blank (perhaps 30" or more in length) at 3000RPM or more, its unbalanced and unsupported middle tends to swing out from the center of rotation (this can be offset to an extent by supporting the bar in the center, or several places along its length, though most barrel makers do not, apparently, do this). The gun drill is designed and shaped to center itself quite accurately WITH THE CENTER OF ROTATION of the bar - which for reasons just explained, can be well off the center of mass. The net result is a hole which is not straight to some extent.
Modern barrel making practice frowns on straightening the barrel during manufacture, as this can introduce undesireable stresses leading to a finished barrel which changes shape when heated by firing, while a slightly crooked bore may have no observable effect on accuracy.
In practice, a carefully made barrel, even though not truly straight, can and will shoot very well indeed, if otherwise properly fitted.
It is easy to ream-up an existing bore, (the gun drill is not used in existing holes) but the reamer will follow the original bore, so will have no effect on the straightness or otherwise of the enlarged bore.
If you buy barrels from reputable makers, you can expect them to perform well, and to be backed-up by the makers.
mhb - Mike
 
mhb said:
or at least ONE of them, in drilling a straight hole is that the barrel blank material (usually un-contoured bar stock) is not really straight to begin with, and/or not perfectly symmetrical. Then, when you drill the blank (perhaps 30" or more in length) at 3000RPM or more, its unbalanced and unsupported middle tends to swing out from the center of rotation (this can be offset to an extent by supporting the bar in the center, or several places along its length, though most barrel makers do not, apparently, do this). The gun drill is designed and shaped to center itself quite accurately WITH THE CENTER OF ROTATION of the bar - which for reasons just explained, can be well off the center of mass. The net result is a hole which is not straight to some extent.
Modern barrel making practice frowns on straightening the barrel during manufacture, as this can introduce undesireable stresses leading to a finished barrel which changes shape when heated by firing, while a slightly crooked bore may have no observable effect on accuracy.
In practice, a carefully made barrel, even though not truly straight, can and will shoot very well indeed, if otherwise properly fitted.
It is easy to ream-up an existing bore, (the gun drill is not used in existing holes) but the reamer will follow the original bore, so will have no effect on the straightness or otherwise of the enlarged bore.
If you buy barrels from reputable makers, you can expect them to perform well, and to be backed-up by the makers.
mhb - Mike
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Oh Yeahhh??? And whadda' YOU know about drilling holes Mike??

:) :)

al
 
Shoot!

Ah'm glad yuh smiled when yuh said that, pardner.
mhb - Mike
'Not Quite the Last Hole Bender'
 
A fellow who had a long history doing contract machining for the aircraft industry once told me of a time when the company that he worked for was having trouble meeting a straightness spec. for IDs of some long hydraulic cylinders, so he took a job with the competition. (Yes, that would be industrial espionage.) What he found was that they were counter rotating the gun drill and the work piece. It has been some time since that discussion took place, but at the time, he told me that two barrel makers were drilling that way, Krieger, and Douglas. Anybody else know anything about this?
 
Not all...

The older-generation deep-hole drilling machines still in use by barrel makers, as opposed to more modern or altered machines, rotate only the barrel blank. At least we (those of us who still use antediluvian machinery) have progressed beyond hand-forging the barrel blank around a mandrel...
mhb - Mike
 
Mike,
My guess is you've at least been following the discussions about barrels with big curves in em? There's all sorts of folks who seem to think the bullet needs a straight start and that putting the chamber in at an angle to the od seems to be their cure. etc etc etc. Whatever. For the typical amount people say they are off in the first 2 inches, I'd have to think if it was caused in the drilling process that the center of the blank must have been eccentric by anywhere from .030 to .050 or more. There's been some talk of some serious 'bent barrels'. So, my question to you is, don't you think some of these figures are exaggerated? I should think that if the barrel blank in a gun drill is eccentric by the amounts needed to cause the errors these people say they see, it would shake the machine out of its concrete base. I personally have cut barrels into 2" sections to see how much the hole wanders, and I've not seen one wander .002. This on barrels I tapered from straight blanks in a 15" lathe (two operations).

I don't know first hand, the order of operations bbl makers use, and for all I know they may vary. I could see where a bent piece of stock would cause some problems (throw weight off center making relaxed part bent). But, for them to be off the amounts people claim, let's just say I'm skeptical.

What's the ole saying? "I'm from Missouri..."
 
4Mesh:

I don't know that anyone has measured and recorded the actual runouts in barrels they have observed to be 'crooked' or 'bent' as received from the maker. I certainly haven't. Neither do I know exactly how much displacement occurs due to dynamic imbalance of the blank in drilling it - It can't be very great, or, as you have observed, the vibration would shake the shop. When I say the bar stock is not straight, I mean that it is as-received from the steel supplier, in 12 to 20 foot lengths, which are 'straightened' (by the steel maker) in the manufacturing process, but are not, and cannot be, truly straight when cut into proper lengths for barrel making. It can be considered straight enough, from a practical standpoint. In drilling the typical 30" blank I mentioned earlier, the runout observed at the exit end after drilling is normally less than .010" TIR. And, in the shop where I've worked, the barrel is contoured to near-finished exterior dimension BEFORE reaming and rifling, centered on the drilled bore at both ends. The final exterior diameter and contour requires removal of only about .025", by grinding under coolant flood, and a minor turning operation to establish the contour of the transition (if any) from the breech cylinder to the straight-taper run to the muzzle.
Other makers do, of course, follow differing procedures.
From my own observations, I doubt that any barrel with runout or curve in the bore of more than a very few thousandths would ever get out of the shop of any of the reputable makers - certainly I've never seen a modern custom-made barrel that exhibited anything like a runout at breech or muzzle approaching .010", much less the amounts you postulate. I have to believe that experienced barrel makers examine their barrels at each step in manufacture: if the drilled bore in the blank is very much out of true, it will be obvious on inspection; even more so after reaming. In making cut-rifled barrels, I'd be afraid to try to rifle a blank with a badly curved bore, since I think it likely that the rifling head (about 6" long, no smaller than .002" under the bore diameter) might literally stick in the bore.
On the other hand, I have actually encountered both military and commercial barrels that, when shortened at the muzzle end, resulted in the bore being visibly eccentric at the new muzzle, and some of them in the neighborhood of .025" or a bit more - enough to be obvious to even the casual observer. How that happened, I don't know, but it isn't rare in older barrels.
Perhaps some other forum participant has more specific information...
mhb - Mike
 
Mike

Mike, I have actually measured the amount of runnout of different spots in a barrel. Follow me here, this is rather convoluted.

It stared when I got a new blank ready to chamber. Looking through it, I could see some shadows that I did not like, so I put the blank in the lathe, chamber end in a steady rest, and chucked the muzzle in a three jaw buck chuck. I looked through the bore as it turned, and it was really bad, at least two spots looked like those clowns in a circus riding the bicycles with the wheels running out.

I was sort of mad, thought about boxing it up and sending it back, but instead, I thought I would see just how bad it was.

First, I put the blank between centers, and the turned profile ran within .002 any where on it. So I took and scribed a straight line down the blank, and then cut it up in four inch pieces. (each piece represents about $40).

Now, I could put each one of these pieces between centers and see how much the OD ran out with the ID.

One particularilly bad spot, about 7 inches up from the chamber end, now had that spot run out .011 to .012 inch. That shows that the Gun Drill took some sort of weird turn at that spot. Other pieces would run out as much as .003 to .004, some darn near dead true.

I marked the runnout spots in their relation to the OD, laid the pieces on a table with that scribed line all back in line, and hardly any of the spots showed the same runnout in relation to that line. In fact, on other bad spot, about 10 inches up from the muzzle end, was about 90 degrees to that really bad spot up by the chamber.

So yes, I destroyed a new blank, (probably a hummer),:D just to satisfy my curiosity. I sent all of the pieces back to the manufacturer with an explanation as to what I had done, just for their own knowledge. They sent me a new blank. And Thanked me.

This is another reason I think the idea of barrel indexing is a farce. Barrels are not "bent", as in a banna style bend. They have various spots in the ID that for what ever reason the Gun drill decides, tend to wander in different directions.

Since I know what darned near .012 looks like, I can now look through a bore and pretty well get a good idea to my own satisfaction as to just how "straight" the thing really is. I have a pretty good "machinist" eye for this sort of thing........jackie
 
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