Measuring concentricity

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dantiff2

Guest
I placed this thread over on the 6mm site too but thought someone here would chim in......


where do you all put the point of the indicator to measure run out?? I place on the middle of the neck and got next to nothing, 1/2 a thou .0005 but when I move to around the ogive of the bullet I get a .003 to .0035 TIR...

But i'm taking that as the necks are straight. I know the lenght will exaggerate but when someone says they have 1 thou runout is that at the neck?

thanks

Dan
 
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This is a good question. Is there a "standard" for how far out the bullet to place the indicator?? the closer you get to the brass, the better it looks.
 
Think of the case as having one axis in it's body, one axis in the neck, and the bullet as having another. If the bullet axis lies exactly on the case axis then you will detect no run out no matter where you put the indicator tip.

Now shift or bend the neck or bullet slightly so that the three axises don't line up or lie on one another. This means the bullet axis may be anywhere in space relative to the case axis. If you have this condition then you can never get consistency in your readings off the bullet. To make sense of this condition you have to take readings off the neck to determine if this is where the run out is coming from.

If you happen to have the neck axis coinciding with the case axis then the bullet axis will intersect the case axis at some point and you can get consistent readings. At the intersection you will get zero run out. (neglecting ovalty) If you move a small distance from this then you will get a higher reading. If you go to double the distance then you will get twice the reading. If the bullet point is concentric then it makes no difference where along the bullet you place the tip.
 
HUH?

My reloads are rarely without some run out. I just read statements that say I have so much [say .003] run out on my loaded ammo.

I wonder where on the bullet they position the tip of the indicator to come up with figure. Obviously, the further away from the neck you get the more it exaggerates this run out- just like using a very long indicator stylus minimizes the readings.

I try to go one bullet diameter out from the end of the neck. That keeps it simple for me and it is a repeatable system that lets me check my technique.
 
Ogive

I measure on the ogive. Although I've never measured on the case neck I would expect there would be variance, unless you turned the case.
 
FWIW, it is probably best to measure runout on the case neck after the bullet is seated AND measure on the bullet ogive where the ogive contacts the forcing cone (leade).

The measurement at the case neck will tell you if your sizing process is correct. The measurement at the bullet contact point will tell you if your seating equipment is doing its job.

How much does runout effect accuracy? IMO, very little, IF, you are shooting turned neck brass that does not have over 0.003" total clearance and if your bullet is seated into a freebore/leade that is concentric with the barrel bore.
 
I asked RCBS where they recommend measuring the runout, and they said the best place to measure it is just in front of the case mouth. I was talking with someone else, and discussing runout, and we decided that to a certain extent, the further you measure from the case mouth, the more likely you are to be measuring the concentricity of the bullet makers manufacturing ability, rather than how straight you are seating the bullet into the case.

I have been switching seating dies and measuring runout. My average runout of the sized case is .0005. In 308 caliber, with a Redding competition die, my runout at .0015 and less was between 91 and 92%. With a Hornady die, my runout at .0015 was 22-23%.

Although it isn't an apples to apples comparison, an RCBS gold competition seating die in 300 Win mag gave me 48% loads with .002 runout and less.

Although all this should have been done with just one caliber, I didn't have access to RCBS 3078 dies. But a 70% difference in the 308 dies sure surprised me.
 
noone said:
I asked RCBS where they recommend measuring the runout, and they said the best place to measure it is just in front of the case mouth. I was talking with someone else, and discussing runout, and we decided that to a certain extent, the further you measure from the case mouth, the more likely you are to be measuring the concentricity of the bullet makers manufacturing ability, rather than how straight you are seating the bullet into the case.
Click to expand...

I think just the opposite is true- too close to the case neck and you are checking the quality of the bullet- closer to the ogive you are checking the the degree to which the case neck is out of the centerline of the case body- albeit somewhat exaggerated. It is sometimes beneficial to exaggerate things in order to understand them. It it were perfectly straight, no amount of exaggeration would result in run out.

If I were a die manufacturer, I would also want the finished result created by my product to be measured in a manner that would shed the most favorable light on it!
 
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Has anyone tried to measure the runout on a bullet? I was thinking the same about the bullets adding to the runout. It makes sense that measuring on the neck reflects the sizing die and the bullet reflects the seating die but I also agree that 1 thou on the neck is problably 3 thou (.003) on the ogive... What gives?

Here is a guy that tells you what the effects of runout are at 600 yards

http://yarchive.net/gun/ammo/bullet_runout.html
 
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Concentricity

vs accuracy.
Not sure I can clarify or confuse this. But, in another life, I balanced large steam turbine rotors and generator fields, that weighed tons. One of the primary considerations contributing to vibration in 3600 rpm (or any rpm, for that matter) is the difference in center of mass, versus the center of rotation. The greater the offset, the greater the vibration.
My thoughts lead me to wonder about this difference in projectiles that are rotating much faster than 3600 rpm. Any difference in cm vs cr in bullets flying downrange, certainly must translate into a difference in POI versus POA. Problems with consistent accuracy just could be due to immeasurable differences in distribution of mass, rather than TIR measured at various points on the loaded round.
Someone with a solid engineering background just might address this, and tell us if perhaps moment weighing bullets is practical, usefull, or a search for a solution to a nonexistent problem.

Getting my flame retardent suit ready.

Dennis
 
degmon said:
vs accuracy.
Not sure I can clarify or confuse this. But, in another life, I balanced large steam turbine rotors and generator fields, that weighed tons. One of the primary considerations contributing to vibration in 3600 rpm (or any rpm, for that matter) is the difference in center of mass, versus the center of rotation. The greater the offset, the greater the vibration.
My thoughts lead me to wonder about this difference in projectiles that are rotating much faster than 3600 rpm. Any difference in cm vs cr in bullets flying downrange, certainly must translate into a difference in POI versus POA. Problems with consistent accuracy just could be due to immeasurable differences in distribution of mass, rather than TIR measured at various points on the loaded round.
Someone with a solid engineering background just might address this, and tell us if perhaps moment weighing bullets is practical, usefull, or a search for a solution to a nonexistent problem.

Getting my flame retardent suit ready.

Dennis
Click to expand...

Dennis,

There's been a lot of work done in this area. You're right but not the first to have considered this ;)

You'd probably enjoy the book "Rifle Accuracy Facts" By Harold Vaughn. >> http://www.amazon.com/Rifle-Accurac...=sr_1_1?ie=UTF8&s=books&qid=1232689252&sr=1-1 << Harold builds both static and dynamic testers in the book. Much work has been done, much of it is ref'd in the book, but for pure reading enjoyment and solid information it's hard to beat Vaughn. "The Bullet's Flight" by Franklin Mann is more comprehensive (and shows that this is not new research) but a slog to read. >> http://www.amazon.com/Bullets-Fligh...r_1_11?ie=UTF8&s=books&qid=1232689108&sr=1-11 <<

I recommend RAF to any aspiring shooter.

al
 
Vern Juenke produces a machine that measures the rotational imbalance of bullets. It is used by bullet manufacturers to check their production methods and product.
.
 
Rotational effects

Al, recalled the Vaughn writing after I posted, but decided to let it go. Never meant to imply that my thoughts were original.
Started reading it again last night, for the umpteenth time. Sometime ago, I went after the Mann stuff, but it was laborious.....kinda like reading James Joyce.
Just got to love trying to get so many contentric circles to align.

Winchester, is there a source to read about the Juenke work?


Dennis
 
I measure the case after sizing. My preference is to measure the seated bullet as close to the bullet tip as possible. If you get a zero or a very low -1 to -4,000th's inconcentricity at the bullet tip, then that's going to be one premium hand-load!
(as you know, readings on the Sinclair gage is actually half of what is indicated. ie; 4,000th's is actually 2,000th's)

I have each slot in my plastic box labeled "zero" to -10 with each cartridge placed in its corresponding spot.

I have found that cartridges with -1 to -5 inconcentricity reading on the gage provide me with 1/4" MOA or better. Cartridges with -9 or worse are used for fouling the bore or sighing-in.
 
Now just a minute VaniB... Readings on a Sinclair runout gage are half value? Not sure i'm with you on this one..?

Dan
 
dantiff2 said:
Now just a minute VaniB... Readings on a Sinclair runout gage are half value? Not sure i'm with you on this one..?

Dan
Click to expand...



Ok....on the Sinclair concentricity gage, the case and bullet is NOT spinning on it's axis. It's spinning on its side. Therefore, one complete 360 degree revolution of the case on side is actually registering twice the actually deformity.
 
Since someone mentioned the Sinclair gauge, I'll throw this one in for you guys:

Where do you feel is the 'right' place to support the cartridge when check for concentricity?

The Sinclair supports the case at just in front of the case head, and just below the case shoulder.

The destructions that come with the NECO gauge show that as one alternative, but recommend putting one of the vee-block knife-edges just in *front* of the case mouth, and the indicator tip at approx. the bullet ogive.

Interestingly, the two readings don't always agree...
 
Yeah I was pondering the same thing.. Would you get a reading similar to that of a sinclair gauge if you used v-blocks or even a gauge like Hollands.
 
Several years ago Skip Otto and I were on the 200 yd sight in range at the Whittington Center and engaged in this same discussion. He went into his vehicle and pulled out his Neco Concentricity gage and showed me how he used two gem indicators on it to measure. He put one tip on the neck and the other near the ogive of the bullet. This way made it easier to determine where any runout might be.
 
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