Concetricity

[Richard Coody]

I know this topic has been beaten to death but i haven't seen this question answered.

I have several different types of concetricity gauges. Basicly they either index off the bullet tip like my forster case inspector or the hornady. Ot they index off the case like my 21st century. They both give very different readings off of the same loaded rounds. Seems strange to me.

Can someone explain it? Which is the more correct?

 

[CH Luke]

Concentricity. 21st Century allows you to measure off either the case or the seated bullet. Means you can check how you are processing your brass prior to checking the loaded round.

 

[T-shooter]

I know this topic has been beaten to death but i haven't seen this question answered.

I have several different types of concetricity gauges. Basicly they either index off the bullet tip like my forster case inspector or the hornady. Ot they index off the case like my 21st century. They both give very different readings off of the same loaded rounds. Seems strange to me.

Can someone explain it? Which is the more correct?

I've never found a definitive answer. I use a Sinclair runout gauge. I like to read half way from the tip to the case neck. I don't feel reading right at the neck or on the neck tells you enough. Half way out will show if the bullet is seated off center or at an angle. My best reloads have a total runout .001" or less measured this way, .0005" off the centerline. On a good day I can get about half that good and maybe 25% more at .002" or under. Rarely see over .003" unless I have a bad piece of brass. I don't feel that it's a good idea to straighten them as it loosens the neck tension. Sometimes I'll use a collet puller and pull and reseat the bullet again. Sometimes it helps, sometimes not.

 

[rap36case]

I know this topic has been beaten to death but i haven't seen this question answered.

I have several different types of concetricity gauges. Basicly they either index off the bullet tip like my forster case inspector or the hornady. Ot they index off the case like my 21st century. They both give very different readings off of the same loaded rounds. Seems strange to me.

Can someone explain it? Which is the more correct?

Without an absolute standard like gauge blocks it is hard to say which is most accurate. Concentric rounds should help predict accuracy down range. If you find the measurements cannot sort more accurate rounds from less accurate rounds, then there may be some measurement problem or the differences are negligible for this purpose. Marksmanship, dies, powder, bullet dimensions, external conditions are just a few of the sources of variation which may be more significant than concentrically loading rounds.

 

[BoydAllen]

As long as you keep in mind that they measure different but closely related things, neither is more correct. I have both. A straight (relatively) round will be straight on both, as will a crooked one. I find that the differences in readings for the same round are unimportant as long as I am comparing apples to apples, readings form the same gauge.

 

[ireload2]

Think of concentric ammo and chambers as a functional system.
What features controls the position of your ammo when the ammo is chambered and ready to fire?
That depends on how you size it compared to the chamber features.

1. If your chambered ammo is aligned by the case body then measure run out by using the case body as the reference surface. You can then look at the run out of the neck, the bullet just ahead of the neck and the bullet near the tip to insure all are on a common axis when the rifle is loaded.

2. If your chambered ammo is aligned by the case head and the neck, use those areas for the locators while you spin the case and look at the bullet.

3. You could also use the case head and the conical shoulder if your cases are controlled by the shoulder cone. You would need a conical locator that matches your chamber.

Locating on the case head diameter and the bullet tip is not how your ammo is located in the rifle. You wind up finding banana shaped ammo like that but it does not correspond to the functional locating surfaces in your rifle chamber.

If you have ran a lathe much you can think of the chamber like you are operating a lathe with a steady rest. Think of spinning a 75mm round in your lathe.

1A Chuck on the case head and use the steady rest on the case body just behind the shoulder.
If there is run out you will be able measure it at the neck and on the bullet. If the projectile is seated crooked there will probably be some run out on the bullet ahead of the case neck and even more run out at the tip of the bullet. You will be able to see the bullet tip flopping around while the case body has no run out.

2A Alternatively you chuck on the case head and put the steady rest on the NECK. Then the shoulder has some run out and the bullet still flops around if it is not straight. The neck has no run out because it is the functional alignment since it is guided by the steady rest.

3A The last example is chucking on the case head and running the bullet in a female cone center in the tail stock with no steady rest. The area between the case head and the bullet tip flops around if the ammo is not straight. But you do not get a sense of how much the bullet tip is off center when the round is loaded in the chamber. This is how some concentricity checkers are designed but they are not functional gages since they DO NOT function as the chamber and ammo function.

This stuff is covered by ASME Y14.5M Geometric Dimensioning and Tolerancing which is an American standard document on the subject. This is the document where datums come from.
The theory behind design of tooling is that it should be functional and use the same locating features as the functional item being gaged if possible.

For the curious ASME Y14.5M is a handbook type document.
http://www.machinedesign.com/datasheet/new-asme-y145-2009-standards-gdt-pdf-download

 

[Richard Coody]

Think of concentric ammo and chambers as a functional system.
What features controls the position of your ammo when the ammo is chambered and ready to fire?
That depends on how you size it compared to the chamber features.

1. If your chambered ammo is aligned by the case body then measure run out by using the case body as the reference surface. You can then look at the run out of the neck, the bullet just ahead of the neck and the bullet near the tip to insure all are on a common axis when the rifle is loaded.

2. If your chambered ammo is aligned by the case head and the neck, use those areas for the locators while you spin the case and look at the bullet.

3. You could also use the case head and the conical shoulder if your cases are controlled by the shoulder cone. You would need a conical locator that matches your chamber.

Locating on the case head diameter and the bullet tip is not how your ammo is located in the rifle. You wind up finding banana shaped ammo like that but it does not correspond to the functional locating surfaces in your rifle chamber.

If you have ran a lathe much you can think of the chamber like you are operating a lathe with a steady rest. Think of spinning a 75mm round in your lathe.

1A Chuck on the case head and use the steady rest on the case body just behind the shoulder.
If there is run out you will be able measure it at the neck and on the bullet. If the projectile is seated crooked there will probably be some run out on the bullet ahead of the case neck and even more run out at the tip of the bullet. You will be able to see the bullet tip flopping around while the case body has no run out.

2A Alternatively you chuck on the case head and put the steady rest on the NECK. Then the shoulder has some run out and the bullet still flops around if it is not straight. The neck has no run out because it is the functional alignment since it is guided by the steady rest.

3A The last example is chucking on the case head and running the bullet in a female cone center in the tail stock with no steady rest. The area between the case head and the bullet tip flops around if the ammo is not straight. But you do not get a sense of how much the bullet tip is off center when the round is loaded in the chamber. This is how some concentricity checkers are designed but they are not functional gages since they DO NOT function as the chamber and ammo function.

This stuff is covered by ASME Y14.5M Geometric Dimensioning and Tolerancing which is an American standard document on the subject. This is the document where datums come from.
The theory behind design of tooling is that it should be functional and use the same locating features as the functional item being gaged if possible.

For the curious ASME Y14.5M is a handbook type document.
http://www.machinedesign.com/datasheet/new-asme-y145-2009-standards-gdt-pdf-download

So using the 21st century gauge indexing off the case body gives the best results

Not any that index off the case base and the bullet tip?

 

[ireload2]

Yes I agree with your statement. In order to give you be best results the sized case should be supported in your chamber the same way.
I don't like the bullet tip to support a loaded round being checked since the bullet tip is hanging out in the air when it is chambered. The bullet tip support makes you think the case is crooked when it is the bullet that is seated crooked.

Believe it or not if you train your eye a little you can roll your loaded rifle ammo across a smooth flat surface like a marble counter top or a granite surface plate and see run out down to about .003 at the bullet tip. The longer the bullet the easier it is to see the run out.

So using the 21st century gauge indexing off the case body gives the best results

Not any that index off the case base and the bullet tip?

 

[Dave Berg]

I've always taken the position that the concentricity I'm most interested in is the point where the bullet meets the rifling. Understand that this is benchrest prepped brass and the bullets are seated slightly into the lands.

I use a case with a bullet seated to the depth of my match loads that has been chambered. The engagement marks are usually pretty fine so I use a Magic Marker on the bullet and use that to set up my Sinclair runout gauge.

I sort the loaded rounds based on runout as everything else I can measure has been done (powder to 0.02 gn, case volume sorted, bearing surface sorted, shoulders bumped .001", etc.). I use the least concentric as fouling shots and early sighters and then the good stuff for final sighters and score. My "worst" are normally less than 0.002" TIR. With the best the needle never moves when the cartridge is rotated.

I do it because it makes me feel better with one more variable minimized. However, I'm pretty much convinced that it doesn't matter much if at all. More than once I've picked up the wrong ammo box when leaving home and found that what I brought was the "bad" and "mediocre" stuff but I didn't notice a bit of difference on target. I still do it anyway but mostly because it's the way I've always done it....

 

[chkunz]

Dave, it seems like each generation has to learn this for themselves.

 

[Richard Coody]

Dave, it seems like each generation has to learn this for themselves.

I say that quite often

I am 62 and still learning. Just starting to get back into benchrest shooting after 30+ years devoted to work. I am enjoying the adventure.

 

[R.Morehouse]

T.Boyers book has about 3/4 of a page dedicated to the subject that keeps it pretty simple. I personally am in the lands for my LV 6ppc and soft seat for the 17lb. 6.5-06imp. Things seem to be going pretty strait....Or at least as strait as the wind seems to be blowing it...

 

[dkhunt14]

I've always taken the position that the concentricity I'm most interested in is the point where the bullet meets the rifling. Understand that this is benchrest prepped brass and the bullets are seated slightly into the lands.

I use a case with a bullet seated to the depth of my match loads that has been chambered. The engagement marks are usually pretty fine so I use a Magic Marker on the bullet and use that to set up my Sinclair runout gauge.

I sort the loaded rounds based on runout as everything else I can measure has been done (powder to 0.02 gn, case volume sorted, bearing surface sorted, shoulders bumped .001", etc.). I use the least concentric as fouling shots and early sighters and then the good stuff for final sighters and score. My "worst" are normally less than 0.002" TIR. With the best the needle never moves when the cartridge is rotated.

I do it because it makes me feel better with one more variable minimized. However, I'm pretty much convinced that it doesn't matter much if at all. More than once I've picked up the wrong ammo box when leaving home and found that what I brought was the "bad" and "mediocre" stuff but I didn't notice a bit of difference on target. I still do it anyway but mostly because it's the way I've always done it....

I also believe when the bullet is engaged in the rifling it tends to straighten it. Matt

 

[dkhunt14]

Yes I agree with your statement. In order to give you be best results the sized case should be supported in your chamber the same way.
I don't like the bullet tip to support a loaded round being checked since the bullet tip is hanging out in the air when it is chambered. The bullet tip support makes you think the case is crooked when it is the bullet that is seated crooked.

I have seen a lot of great 10 shot groups at 1000 with new cases that were small in the chamber. If you are in the lands it aligns things. I tend to size my competition stuff harder then most. I also know a bunch of other BR guys that do the same. It is old time beliefs that a case has to fit the chamber tight to shoot good. JIM HALL, who worked for Sierra said a case should fit a chamber like a rat turd in a Violin case. MATT

 

[ireload2]

You have fallen into the "relative" thought pit. A rat turd fit in your world is a very close fit in the normal non OCD world.

Do you ever shoot anything besides a benchrest rifle? If you work with near perfect systems you might be able to claim almost anything works. The near perfection of everything allows you to shoot with something a little sloppy and get away with it. So try your experience out on a large sample of rifles, both military and commercial hunting rifles and let us know how your sloppy ammo theory works. Perhaps you can show me a #4 MK 1 Lee Enfield that shoots in the zeros because the loaded Pakistani rounds are so much smaller than the chamber. I bet Jim Hall never won a bench rest match shooting an arsenal built #4 Mk1 Lee Enfield nor have you.

So you size your ammo harder? I have no idea what that means? Does it mean you push the shoulder back .020? I bet it doesn't does it? Does it mean that you squash your brass down .015 smaller than your chamber in diameter? I bet it doesn't.
Or maybe you let your shoulder clearance float around any old value from round to round, and maybe you let your bullets run out .040 at the tip? Did you ever consider that the cone of the chamber shoulder keeps your round centered in the chamber? And did you ever consider that that centering is considered a close fit for purposes of controlling the location of the loaded round before it is fired? After all you control your shoulder bump to what? About .001? Yet this is considered sloppy?

It is not old time belief - it is current practice that the ammo has to fit well. That is fit better than the current SAAMI engineering standards. Otherwise we could all shoot factory rifles with factory ammo. What do you think you are doing when you control the diameter of the chamber and the case head, the diameter of the neck, the size of the throat and the free bore, the fit of the bullet. Then on top of that you manage the shoulder clearance to the nearest .001. You are using a barrel blank that cost $400 to $600 that is a perfect fit for your bullets. Why not just use some old tube from one of Midway's low buck sources?
No I don't really think you have a clue as to what sloppy ammo really is. In your world sloppy might be .002 to .004 clearance - on a diameter. If you think it is more then you should be able to get bench rest accuracy out of any old .30-30 with factory ammo.

I have seen a lot of great 10 shot groups at 1000 with new cases that were small in the chamber. If you are in the lands it aligns things. I tend to size my competition stuff harder then most. I also know a bunch of other BR guys that do the same. It is old time beliefs that a case has to fit the chamber tight to shoot good. JIM HALL, who worked for Sierra said a case should fit a chamber like a rat turd in a Violin case. MATT

 

[mike a]

This may sound dumb but, with regard to brass fitting the chamber on not. Wouldn't a spring loaded plunger type ejector push the round to the side? I'm FL resizing.001 bump and mag length.

 

[mikecr]

I have several different types of concetricity gauges. Basicly they either index off the bullet tip like my forster case inspector or the hornady. Ot they index off the case like my 21st century. They both give very different readings off of the same loaded rounds. Seems strange to me.

The Forster/Hornady type(I call em neck benders), are completely different than the 21st Century/Sinclair gauges.
By pinning the bullet tip in their measure the neck benders provide very small portions of reality.

While marketed as concentricity gauges, the Sinclair type is really a total indicated runout(TIR) gauge. This type tells you when your loaded ammo is finally straight, and your ammo isn't until measured so with this type.
Straight is the best you can do. Straight is both low in runout and concentric (which are separate different attributes).

I feel that straight ammo produces least chambered tensions, and that it comes into play more & more as chamber clearances are reduced.
In other words, runout won't matter in a sloppy chamber fit, so where you produce bananas for ammo, a sloppy chamber would probably provide relief for this condition. Both the banana and the clearances are produced by heavy sizing, and most often you see them together. Folks who believe you need big clearances, will size more, and then they actually do need bigger clearances(perpetuating)..

As far as bullet alignment with ITL relationships, binding bullets to force such alignment represents a chambered tension, and could as well be counterproductive.

 

[dkhunt14]

You have fallen into the "relative" thought pit. A rat turd fit in your world is a very close fit in the normal non OCD world.

Do you ever shoot anything besides a benchrest rifle? If you work with near perfect systems you might be able to claim almost anything works. The near perfection of everything allows you to shoot with something a little sloppy and get away with it. So try your experience out on a large sample of rifles, both military and commercial hunting rifles and let us know how your sloppy ammo theory works. Perhaps you can show me a #4 MK 1 Lee Enfield that shoots in the zeros because the loaded Pakistani rounds are so much smaller than the chamber. I bet Jim Hall never won a bench rest match shooting an arsenal built #4 Mk1 Lee Enfield nor have you.

So you size your ammo harder? I have no idea what that means? Does it mean you push the shoulder back .020? I bet it doesn't does it? Does it mean that you squash your brass down .015 smaller than your chamber in diameter? I bet it doesn't.
Or maybe you let your shoulder clearance float around any old value from round to round, and maybe you let your bullets run out .040 at the tip? Did you ever consider that the cone of the chamber shoulder keeps your round centered in the chamber? And did you ever consider that that centering is considered a close fit for purposes of controlling the location of the loaded round before it is fired? After all you control your shoulder bump to what? About .001? Yet this is considered sloppy?

It is not old time belief - it is current practice that the ammo has to fit well. That is fit better than the current SAAMI engineering standards. Otherwise we could all shoot factory rifles with factory ammo. What do you think you are doing when you control the diameter of the chamber and the case head, the diameter of the neck, the size of the throat and the free bore, the fit of the bullet. Then on top of that you manage the shoulder clearance to the nearest .001. You are using a barrel blank that cost $400 to $600 that is a perfect fit for your bullets. Why not just use some old tube from one of Midway's low buck sources?
No I don't really think you have a clue as to what sloppy ammo really is. In your world sloppy might be .002 to .004 clearance - on a diameter. If you think it is more then you should be able to get bench rest accuracy out of any old .30-30 with factory ammo.

Yes I size my ammo smaller them most. My buddy made me a big diameter die to do it. I can size all the way to the ejector groove. It is way more then the .001 that most size the the shoulder. Yes I bump the shoulder more then .005. Do I recommend this no. Because you can get case head separation if not careful. Why would I want to shoot an old rifle like them. I have no interest, except in the utmost accuracy. After all this site is called ACCURATE SHOOTER, not military rifles.

You don't need your fitted cases to shoot small groups and I can prove that. Jim Hall worked with Remington so I would imagine he shot his share of factory rifles and factory ammo. I have seen many factory rifles with factory ammo shoot small groups. I have also seen many guys fireform a 308 Baer which is taking a Weatherby radius and blowing it out to 35 degrees and taking taper out also. Some of these groups are incredible at long distance, like 10 shots in 3 to 5 inches. Just because you can't think it can be done doesn't mean it doesn't happen on a regular basis. Some of the F-Class guys even Fireform in matches. You don't need tight cases to shoot small. I have heard many other guys say it and do it also. Matt

 

[LHSmith]

Another example of a loose fitting case producing accurate results- the 6 PPC when initial FF using a 6 mm bullet, granted the necks are turned to allow 1 to 2 thou radial clearance to the chamber neck, but the .220 Russian case taper leaves a lot of room in the chamber. Loaded long with a match bullet to push the case head against the bolt face, it is not uncommon for these initial FF loads to agg in the 2's.

 

[LHSmith]

Do you ever shoot anything besides a benchrest rifle? If you work with near perfect systems you might be able to claim almost anything works. The near perfection of everything allows you to shoot with something a little sloppy and get away with it.

Nothing could be further from the truth. A true Benchrest rifle in the hands of an experienced competitor who has spent time developing loads that particular rifle system likes (aka TUNING) will readily show subtle discrepancies (your so called slop) in the reloading process by its results on the target. This happens only because in a tuned Benchrest rifle a shooter can gain trust in what the rifle is telling him > feedback....there is no second guessing of where shots will go BEFORE the trigger is even pulled. A 0.005" difference in seat depth is immediately apparent on target....as is a .001" change in neck tension, a 0.4 grain powder differential (short range BR), a 0.0003 difference in neck thicknesses, need I go on.
Also, if the coned breach is the end all answer to case alignment in the chamber why do most SR BR competition shooters net better accuracy with turned necks? The answer is it's there for positive feeding....not to control concentricity.
The only way a tuned BR rifle can be considered "forgiving" is if one has that rare exceptional barrel. Other than that, one had better make sure his loads and loading technique are consistent to the "nth degree".