Chamber runout - excessive accuracy?

S

skipkh

New member
I've often wondered - so now I'll ask your thoughts...

I set up and machine meticulously enough so that my chamber has no more than 2 tenths runout when done. I am told Clay Spencer guarantees 1/2 a tenth. I'm wondering, though, does it make any real difference? I used to measure every powder charge on an Itin precision lab scale - guaranteed accurate to .05 grains. On the suggestion of a board memeber, I did a test where I simply threw the charges using a Harrell's Culver (using a very consistent throw technique) and my groups didn't change. So, in the end, the accuracy afforded by the scale didn't make any practical difference. I think the same may be true in the chamber. 50 millionths? The case walls of the brass aren't true to that level! I haven't tried this, but I suspect a chamber cut to .00005" runout and one cut to .0005 will not show any difference downrange.

Thoughts?
 
Well, I suppose my first though better be that I'm not a gunsmith. I have done some of my own work, but these days, Joel Pendergraft does the driving & I do the sitting . . .

That said:

At some level, you have to have a standard. How the rifle prints isn't quite good enough, there are too many variables. Where that standard should be is up in the air. I have seen in print (not on target paper) where someone did a barrel with .0005 runout, shot it, wasn't satisfied, redid the chamber to .0002 shot it, and was happy. Well, sample size = 1, and "happy" was the critera. Not what we need.

.0002 has become a standard. Whether or not it is needed, no one knows for sure. It would take a sample size of about 70 to get good Standard Deviation numbers. That's 70 with .0002, another 70 with .0003, etc.

Finally, runout measured how? You've got two points to use with your set up. Which two did you pick? Would others have made a difference?

In the last analysis, it doesn't take that long to indicate in to .0002.

FWIW
 
I agree it doesn't take THAT long to indicate to 2 tenths, and I'll always do a job as accurately as possible - this is not a field for cutting corners. That being said, it really would take a large sample, as you noted, to be statistically significant.

When I set up for chambering I use a precision piloted range-rod and I care primarily about the runout in the neck/lede region. I try to strike a balance where I can get the least average runout in the base and the lead, but when I have to compromise, I favor being true in lede.

At the end of the day though, I'd love to have enough barrels to actually test this out. I'm a betting man, and I bet 2 tenths is overkill :)
 
As for dialing in on the neck -- well, we use one of the gretan rods (& dual spiders through the headstock). After indicating the barrel in that way though, we then use a long-stylus interapid dial indicator at (about) the middle of the neck and (about) the bullet ogive, and those become the two final points. I wouldn't trust this without first indicating in the whole barrel, just as I wouldn't trust only a range rod. If things don't seem to be working out between the two measurements with a barrel, better figure out why. Sometimes it is just a crooked bore, sometime you can pick a better spot for the chamber.
 
Is what you are measuring really what your indicators say?

Most folks can read indicators. But do they know what the runout is in the bearings of their lathe and do they keep the bearings loaded during the entire chambering process.

Do you run your lathe for 15 minutes in each direction prior to indicating your barrels? Are your bearings warmed up?

Are the jaws in the chuck cleaned and lubricated so they hold consistent preasure from all 4 or 6 sides? I have seen worn out jaws loose their tension during the machining process.

Are you using a soft metal bushing between the lathe jaw and the barrel? is it getting compressed and damaged during the turning process?

The same applies for the spider end of the spindle. Are the set screws holding constant?

If you start out with undetectable runout you will end up with undetectable runout. An interesting test is to dial it in on one machine. Machine it and then dial it in on another machine and check it for runout. You will be surprised what you find.

Just some things to think about.

Nat Lambeth
 
im confused

just for curiositys sake, is the use of two tenths a misnomer?

in my train of thought two tenths is 2/10 or .2 and .0002 would be two ten thousands or 2/10,000.:eek:

guess it must be smith lingo two tenths may be slang for "ten" thousanths?;)

one thing for sure i pay the smith and he runs the machine. yepperee;)
Fred
 
Fireball Fred said:
just for curiositys sake, is the use of two tenths a misnomer?

in my train of thought two tenths is 2/10 or .2 and .0002 would be two ten thousands or 2/10,000.:eek:

guess it must be smith lingo two tenths may be slang for "ten" thousanths?;)

one thing for sure i pay the smith and he runs the machine. yepperee;)
Fred
Click to expand...


yuppereeeeee, "two tenths" is .0002
 
"Chamber runout - excessive accuracy?"

I have never had an issue with "excessive accuracy" in any of my guns.
Ted
 
If you are within .0002 you are good. If the thickness of a hair is .003, how much is .0002? Your finger temp can change dimensions .0002. 1/2 of a tenth is a joke as it would take a controlled environment and very expensive measuring equip. to verify it. You would gain a hell of a lot more by going out and practicing.
Butch
 
Not to pick on the man - I've seen Mr Spencer's work first hand and it's top shelf all the way! But the statement is still there on his web page in black and white....

"We guarantee the runout of our chambers to be less than 50 millionths of an inch."

I won't dare challange his honesty or integrity, but I think someone sold him a .001" indicator with a .0001" face on it :)
 
it would take a controlled environment and very expensive measuring equip. to verify it. You would gain a hell of a lot more by going out and practicing.
Click to expand...

Couldn't have said it better myself...........;)
 
He doesn't mean me...

skipkh said:
Not to pick on the man - I've seen Mr Spencer's work first hand and it's top shelf all the way! But the statement is still there on his web page in black and white....

"We guarantee the runout of our chambers to be less than 50 millionths of an inch."

I won't dare challange his honesty or integrity, but I think someone sold him a .001" indicator with a .0001" face on it :)
Click to expand...

Really!
 
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Butch Lambert said:
If you are within .0002 you are good. If the thickness of a hair is .003, how much is .0002? Your finger temp can change dimensions .0002. 1/2 of a tenth is a joke as it would take a controlled environment and very expensive measuring equip. to verify it. You would gain a hell of a lot more by going out and practicing.
Butch
Click to expand...

I use a 7X14 lathe with a 6 jaw. the lathe itself has runout of about 0.0005 and i routinely set up with a 4 jaw to around 0.002. yeah, no zero missing there. my rifles shoot to around 1/2 - 5/8 at 100 yards -- and i am not an experienced rifle shooter by any means.

these are hunting rifles, not BR. while i have not done 70 rifles, the ones i have done have been consistently good shooters in that range, for a wide variety of calibers.

i only mention this to give you an idea of what you get at this end of the spectrum.
 
Honest..

Answers, that's what we need.

AMMASHOOTA
"I use a 7X14 lathe with a 6 jaw. the lathe itself has runout of about 0.0005 and i routinely set up with a 4 jaw to around 0.002. yeah, no zero missing there. my rifles shoot to around 1/2 - 5/8 at 100 yards -- and i am not an experienced rifle shooter by any means."
 
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