Fixing chamber runout

T

thekubiaks

New member
Some of you guys may be familiar with a video put out by Grizzly's benchrest gunsmith called "Chambering a Championship Match Barrel". In this video, he goes step by step through all the steps he uses to chamber a barrel and then install it.

In this video (Chapter 23, about 54:00 minutes) into the video, the gunsmith talks about using a non floating reamer holder made by GTR.

A little further in the video after the gunsmith has completed reaming the chamber, he says "The runout is 2 to 3 thousandths, I'll have to go back and fix that.

That statement has me stumped. Once you ream the chamber, if you have runout, you cant' go back and fix that as far as I know, the chamber is already out of round. Am I missing something??

Thanks
 
There's so little taper in most chambers that if you end up with any significant runout on a completed chamber, you are probably going to be cutting the entire thing off or replacing the barrel if there is not enough length. If you catch it in the first quarter inch or so, it can probably be corrected with a boring bar.
 
thekubiaks said:
Some of you guys may be familiar with a video put out by Grizzly's benchrest gunsmith called "Chambering a Championship Match Barrel". In this video, he goes step by step through all the steps he uses to chamber a barrel and then install it.

In this video (Chapter 23, about 54:00 minutes) into the video, the gunsmith talks about using a non floating reamer holder made by GTR.

A little further in the video after the gunsmith has completed reaming the chamber, he says "The runout is 2 to 3 thousandths, I'll have to go back and fix that.

That statement has me stumped. Once you ream the chamber, if you have runout, you cant' go back and fix that as far as I know, the chamber is already out of round. Am I missing something??

Thanks
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If you establish the chamber before all other finished machine work, you can re-indicate the chamber with the indicator stylus half way in on the body of the chamber, in a sense, "splitting the difference", and cutting the run out in half in relation to all subsequent operations.

That's about the best you can do once the chamber is cut and exhibits that much run out in relation to the initial indicated points.
 
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If the video is referring to chamber runout in relation to the bore, that chamber can't be fixed. This is usually a product of bad machining practices. Cut that chamber off and start over, not doing it the same way you did that one.

When one of these videos first came out showing the operator using "range rods" the part that caught my eye was that the bullet exit point was running out an alarming amount. That was at a Shot Show in Orlando. One of this countrys very top gunsmiths called me and we had a good laugh.

.
 
JerrySharrett said:
If the video is referring to chamber runout in relation to the bore, that chamber can't be fixed. This is usually a product of bad machining practices. Cut that chamber off and start over, not doing it the same way you did that one.

When one of these videos first came out showing the operator using "range rods" the part that caught my eye was that the bullet exit point was running out an alarming amount. That was at a Shot Show in Orlando. One of this countrys very top gunsmiths called me and we had a good laugh.

.
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Jerry, there are some very successful Benchrest shooters, who do their own work, that do exactly that. ( though not with a Range Rod, they use an indicator).

They are only interested in the first inches of the barrel that interact with the case and bullet.

And yes, when they change barrels, the POI changes a great deal.

It doesn't seem to matter.
 
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A friend, who builds rifles, and shoots benchrest, has made a change in how he chambers during the last year. He drives the reamer with a center until the chamber is largely cut, and then switches to a pusher. This gives him the positive alignment and runout of the former, and the trueness to the reamers base of body dimension of the latter.

Another friend tells me that after he has taken off most of the metal for his tenon threads, that he re-indicates the barrel before taking his finish cut. He uses heavy copper wire between the chuck jaws and the barrel, and has found that the pressure of the initial cuts can move the barrel a little.

In the past I have asked several different winning gunsmiths about how they set up barrels for chambering. They were not all the same. There were smiths that work through the headstock and indicate in both ends of the barrel, and there are others, whose barrels had equal reputations, that did all of their indicating from the breach end, and let the muzzle go where it needed to to accomplish that. There was one smith that made Buckys' mounts, and had an excellent reputation, who told me that he would chamber with whatever method the customer wanted, but preferred to use a steady rest.
 
If you do the math on a barrel that is .020 out of line at the muzzle, it will be about 3.2" out of line at 100 yards. I've had barrels that will print farther off than that that were centered to .0002" or less at both ends. You can control what the ends of the barrel are doing in the lathe, but you're pretty well out of luck trying to do anything about runout in the middle of the barrel. That runout in the middle of the barrel will have an effect on where the bullet hits on the target especially when you're changing barrels.
 
thekubiaks said:
Some of you guys may be familiar with a video put out by Grizzly's benchrest gunsmith called "Chambering a Championship Match Barrel". In this video, he goes step by step through all the steps he uses to chamber a barrel and then install it.

In this video (Chapter 23, about 54:00 minutes) into the video, the gunsmith talks about using a non floating reamer holder made by GTR.

A little further in the video after the gunsmith has completed reaming the chamber, he says "The runout is 2 to 3 thousandths, I'll have to go back and fix that.

That statement has me stumped. Once you ream the chamber, if you have runout, you cant' go back and fix that as far as I know, the chamber is already out of round. Am I missing something??

Thanks
Click to expand...

At that point in the video (56:30) he hasn't chambered yet, he's just roughed the chamber with a drill and gone back to check runout before starting in with a boring bar to pre-bore before going to the reamer. And that's 2-3 ten-thousandths that he was correcting. He mentions that he has left 0.040" - 0.050" 'meat' to play with, so plenty of room for correction.

GsT
 
Mike Bryant said:
If you do the math on a barrel that is .020 out of line at the muzzle, it will be about 3.2" out of line at 100 yards. I've had barrels that will print farther off than that that were centered to .0002" or less at both ends. You can control what the ends of the barrel are doing in the lathe, but you're pretty well out of luck trying to do anything about runout in the middle of the barrel. That runout in the middle of the barrel will have an effect on where the bullet hits on the target especially when you're changing barrels.
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Well said

And I've had barrels chambered between centers that printed a lot more than 3-4" off, like over a foot!

And the runout in the (middle of) the barrel. I now clock barrels and it is my strong belief that this runout (racetrack) has a HUGE effect on "tracking" or, more specifically, how the rig recoils. It don't care whether the muzzle is "in front of the chamber" (a HUGE mis-statement) or off wandering somewheres completely out of line with the rifle, it WILL recoil like a bullwhip.

Which is good, as long as you've control of the whip.

And IMO is why "straight barrels don't shoot"
 
Boyd Allen said:
A friend, who builds rifles, and shoots benchrest, has made a change in how he chambers during the last year. He drives the reamer with a center until the chamber is largely cut, and then switches to a pusher. This gives him the positive alignment and runout of the former, and the trueness to the reamers base of body dimension of the latter.
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Not to be argumentative but I REALLY can't get with this!

IF barrel bores aren't straight, therefore IF the actual bore is running at an angle to the setup........

which, it is.....

Not only at an angle but rotating,

which it is......
 
GeneT said:
At that point in the video (56:30) he hasn't chambered yet, he's just roughed the chamber with a drill and gone back to check runout before starting in with a boring bar to pre-bore before going to the reamer. And that's 2-3 ten-thousandths that he was correcting. He mentions that he has left 0.040" - 0.050" 'meat' to play with, so plenty of room for correction.

GsT
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thank you

:)
 
Al,
If the barrel is indicated in at the breach end, that part of the barrel should be in pretty good alignment with the CL of the spindle. At that point, any offset of the tail stock center from the spindle CL will result in a chamber that is in line with the chamber end of the bore, but which may be slightly oversized at the back of the chamber. A short chamber cut this way will not be full diameter at the chamber mouth. Switching to a pusher at that point in the operation means that there is nothing pushing the back of the reamer off center and so, with the taper of the body of the case, the finished chamber will be reamer sized. This is similar to preboring on a taper before reaming with a pusher.
 
JerrySharrett said:
When one of these videos first came out showing the operator using "range rods" the part that caught my eye was that the bullet exit point was running out an alarming amount. That was at a Shot Show in Orlando. One of this countrys very top gunsmiths called me and we had a good laugh.
.
Click to expand...

If we start with the assumption that no barrel is perfectly straight, and we are cutting the chamber so that the bullet enters parallel to the bore at the start, then the muzzle should be running out when setup in the lathe, right?

When I'm on a trip and I drive into a curved tunnel in the mountains, do I drive my car into the entrance to follow the path of the tunnel? Or does it make more sense to point my car directly at the exit and attempt to drive through the wall?
 
James Lederer said:
If we start with the assumption that no barrel is perfectly straight, and we are cutting the chamber so that the bullet enters parallel to the bore at the start, then the muzzle should be running out when setup in the lathe, right?

When I'm on a trip and I drive into a curved tunnel in the mountains, do I drive my car into the entrance to follow the path of the tunnel? Or does it make more sense to point my car directly at the exit and attempt to drive through the wall?
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James, I like your sense of humor! :D It's not often that I just burst out into laughter but this did it.

Thanks for the good laugh.

Gene Beggs
 
James Lederer said:
If we start with the assumption that no barrel is perfectly straight, and we are cutting the chamber so that the bullet enters parallel to the bore at the start, then the muzzle should be running out when setup in the lathe, right?

When I'm on a trip and I drive into a curved tunnel in the mountains, do I drive my car into the entrance to follow the path of the tunnel? Or does it make more sense to point my car directly at the exit and attempt to drive through the wall?
Click to expand...

Are you making the assumption that the bullet will straighten the crooked bore by your car in the tunnel analogy? The bullet will take the exit path of the last 3/4" or so of where that part of the bore is pointed. If that last fraction of bore is not pointed in the direction of the recoiling rifle, then what?




.
 
Boyd Allen said:
Al,
If the barrel is indicated in at the breach end, that part of the barrel should be in pretty good alignment with the CL of the spindle. At that point, any offset of the tail stock center from the spindle CL will result in a chamber that is in line with the chamber end of the bore, but which may be slightly oversized at the back of the chamber. A short chamber cut this way will not be full diameter at the chamber mouth. Switching to a pusher at that point in the operation means that there is nothing pushing the back of the reamer off center and so, with the taper of the body of the case, the finished chamber will be reamer sized. This is similar to preboring on a taper before reaming with a pusher.
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Boyd, please refine "indicated in at the breach end"........ are you speaking of using Gordy Gritters' method but doing the "pre-bore" using a roughing reamer instead of drilling/boring?
 
JerrySharrett said:
Are you making the assumption that the bullet will straighten the crooked bore by your car in the tunnel analogy? The bullet will take the exit path of the last 3/4" or so of where that part of the bore is pointed. If that last fraction of bore is not pointed in the direction of the recoiling rifle, then what?




.
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hmmm, this is a new one on me. I've never heard of anyone setting the last 3/4" of the muzzle to run "parallel with the centerline of the recoil"

I don't consider myself to be a machinist but I'm REALLY good at setting up and solving problems and I can't, in short order, even come up with a starting point from which to build a system for accomplishing this........

And even if I COULD find a static parallel......my muzzles bend up and down off centerline in the order of up to 1/4" gross movement, flex like Calfee's antennae and rotate like Elvis' pelvis....

just cain't get wit' it...
 
There is more than one way to do what Gordy's method accomplishes, but yes, that. You could use a roughing reamer, but my friend does a variety of chambers rather than a lot of one, so he has done the method that I described with a finish reamer.
 
Boyd Allen said:
There is more than one way to do what Gordy's method accomplishes, but yes, that. You could use a roughing reamer, but my friend does a variety of chambers rather than a lot of one, so he has done the method that I described with a finish reamer.
Click to expand...


OK, that's probably going to work some of the time but.....in practice it's common for the breach end of the barrel to be wobbling by a couple thou once the ogive section's dialed in and the lapping bell is normally noticeably eccentric. My guess is that all lappers favor one side. In other words the reamer will start out cutting an eccentric wallowed hole.......ellipsed or "egged" in the common parlance.

I guess one could cut off the lapping bell as the center-to-center guys do and that would help...but I most often end up parting this section off AFTER having established a centered hole. This solves the old conundrum of HOW and WHEN to get rid of the egged out portion, and makes it so you don't have to throw it away, you can use it for something nice...


I'd be interested to learn of other ways to find a parallel section of bore 3"-5" from the end.
 
Al,
Why would anyone use the parts of the barrel that are belled from lapping to indicate from? Before (and even after) the Grizzly (or Gordy) rod came along, some used Interapid type indicators with very long contact points, or range rods of various types. I have even read of the use of precision ground pins. After chambering, the barrel is reversed in the headstock and dialed in the same way before cutting the crown, so that it is square to that end of the bore.

Here is a good thread on dialing in a barrel. Gordy Gritters wrote some good information about his method in post #19.
http://forum.accurateshooter.com/threads/indicating-in-barrel.3746886/
 
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