How many here use concentricity gages? I would like to get one but think it may be another way to drive myself crazy lusting after the perfect reloads.

I do. You should too. It Will.

Gene.............

A concentricity gauge is more important than a chronograph for accurate reloading. A chrongraph can only tell you what has already happened.

A concentricity gauge can tell you what is about to happen...

If the bullet doesn't go straight into the bore, it's going to have altered barrel time, and it's going to leave the muzzle with a "wobble" or "yaw," as it's often called.

For hunting loads on larger game at 250 yards and closer, all that really doesn't matter.

If you're interested in 500 yard shots on varmints, you need concentric loads.

I use the RCBS Casemaster, and I paid 60 dollars for mine.

Take care,

Dan

http://home.earthlink.net/~dannewberry/index.html

The more tools you have, the happier you will be. They all lead to the ultimate load. It's probably the most important tool of the bunch if your interested in extreme accuracy.

FWIW,
I agree that a concentricity guage is well worth the money. I use a Sinclair Concentricity Fixture with a Starrett .001" Dial Indicator. Works well.

Regards, Matt.

I have a NECO and find it very helpful...

Get one. I have the RCBS and love it. I can't comment on the other brands because this works just fine and never tried the others.

Hcliff

Matt brings up another problem for me. I am 30 minutes from Sinclair. That place is a expensive stop for me every time.

Go to a Benchrest tournament. Watch the shooters reload their ammo before each relay. Try to find one using a runout gauge. Nuff said?
Rusty

BR shooters seldom use scales either, but try to find chambers with more than .001" clearance at the neck. Or one who DOESN'T seat bullets either on or into the lands, which makes the load concentric. Many don't even resize brass, but none of this means that their reloads aren't or may not be concentric. With the minimal clearance they have, their loads are inherently concentric if their brass is matched to their chambers and they turn necks down the the .0000". They don't need to check. Bad example.

Hairtrigger,
I wonder if we may be using the same term in this thread, i.e. "concentricity gauge", to describe two different tools? I use a "Sinclair case neck thickness gage" to measure the uniformity (also sometimes referred to as "neck concentricity")of the unloaded case necks. On the other hand, sounds like some of the responses to your question may instead be using a gauge to measure bullet runout. Looking at the latest Midsouth catalog, it looks like the RCBS "Case Master Gauging Tool" may carry out both these functions, but I don't have one to be able to say for sure. There....have I muddied things sufficiently? Boy, am I glad Im' not as close to Sinclair as you are... I'd probably stay in trouble ALL the time if I was that close! Gary T.

Well, FWIW this BR guy advocates the use of concentricity gauges...

http://www.xtremeaccuracy.com/xtremetips.htm

From his webpage:

"A seating-depth gauge, a ball micrometer and a concentricity gauge are three of the best investments a reloader can make."

By the way, while the above quoted BR shooter seems quite knowledgeable he is also one of the most asinine individuals I've ever had any dealings with...

Maybe it wasn't necessary to say that, but... it just seemed right...

Dan

Gary,

The RCBS Casemaster does indeed have the ability to measure case neck thickness and concentricity.

However, it is tricky to set up and use for the former.

Dan

Gary T

I am refering to the tool with a dial indicator that checks for no better term the "roundness" of a reload.
If it is out of round, the way I see it it can be the brass, the way the bullet is seated or the dies used at fault.
In all reality I could care less what bench rest shooters are doing I shoot mostly varmints and paper up to 400-500 yards. I have no intrest in doing all the very time consuming things like weighing brass and sorting it. I want the best quality ammo that my progressive press can produce. No arbor press here.
My varmint rifles shoot sub moa but I feel some will shoot better, perhaps its my skills or lack of

"I am refering to the tool with a dial indicator that checks for no better term the "roundness" of a reload."

That's concentricity... You're right.

And it is certainly important, especially with thin jacketed varmint bullets hitting the lands at extreme pressures. If the bullet hits the lands off center, it will be damaged, and out of balance to some degree when it exits the bore.

Out of balance bullets, as might be expected, don't fly properly.

Heavier 30 caliber bullets with thicker jackets can tolerate a bit of runout with no noticeable ill effects. But if runout exceeds about .006" with the 30's, you'll see the groups open up at 100 yards. After the bullet stabilizes (longer bullets with higher BC's can "go to sleep" after the initial effects of runout) the groups will settle down. In the majority of cases where you find smaller MOA groups at 300- yards than you're seeing at 100 yards, you're dealing with the effects of runout--not twist rate/bullet weight issues.

Low BC, thin jacketed 22's don't have the luxury of "going to sleep" as range increases...

By the time your 22 centerfires exceed about .003" of runout, you will likely begin seeing larger groups, and flyers.

Keep the runout to .002" or better with the fast 22's and you'll get excellent groups--provided all else is up to snuff.

Dan

I've always heard that runout has to be less than .004" else it shows on the target. I've always asked the source of this pronouncement and never received an answer.

During the course of discussing runout with a friend that is on the current Palma Team, he shared an example; He purposely loaded ammo with runout on the bullet of 5 to 10 thousandths. He then proceeded to stack shot upon shot in the X-ring at 600 yards out of position.

I'd love if any of you could cite a source of this 4 thou runout threshhold?

He can do this at 600 yards, but his groups would likely be a tiny bit tighter with no runout in the rounds. He can probably shoot 1.5 inch groups at best, but probably more like 3 inches or so. Half MOA is quite good, of course!

But his 100 yard groups would show a much larger disparity...

At 100 yards, the bullets with high runout will not have stabilized yet. They will deviate from the intertial path imparted by the barrel for a period of time, then they'll "straighten up and fly right," on a line parallel to the line followed by the bullet with no runout (no deviation from bore line).

The above principle is sound, and proven--however, you'll find experienced shooters everywhere who don't understand what's actually happening. They refer to bullets "going to sleep" at longer ranges, but they aren't aware of the part that runout plays here. This effect is not always caused by barrel twist/bullet weight issues.
Here are a couple of links to message board exchanges where the matter is discussed in more detail:

http://216.219.200.59/cgi-bin/ubbcgi/ultimatebb.cgi?ubb=get_topic&f=13&t=000272

http://216.219.200.59/cgi-bin/ubbcgi/ultimatebb.cgi?ubb=get_topic&f=17&t=000015

Longrange shooters have known for years that they get better groups when they "index" the cartridges with higher runout so that all deviations are pointing at 12 o'clock (or "somthing" o'clock) in the chamber. This reduces the effect of runout, since those bullets deviate in the same direction from the barrel's imparted path.

Something else to consider is the obvious effect of runout on barrel throat life. The more off center the bullets strike the throat, the more excessive wear there will be to the throat. Even if it weren't for the accuracy advantages of reducing runout, barrel life considerations alone would make it worth doing.

In a nutshell, runout will affect close range groups (200 yards and under) more than it will longrange groups. It will affect longrange groups too, but the amount of MOA deviation from line of sight will decrease as the bullet stabilizes, so much so that the effects of runout on 600 yard groups would be extremely hard to measure with certainty.

Dan Newberry
green 788

Excuse the ignorance, but once you have established if or how much your bullets are deviating from perfect concentricity, how and what do you use to correct it with?

There are also a number of other ways to correct concentricity but first you have to be certain your case is concentric. You need to determine exactly where the problem lies. If the case is not the culprit it's either in the seating of the bullet or the bullet itself. Many times by indexing and pulling the bullet then rotating it 180� will resolve it. Other times replacing the bullet with another will do it. If the bullet is only very slightly off, you can "bend" it. The key is to have the case exact to begin with. If the case is off there isn't much you can do and you're only fooling yourself. Seating the bullet partially and rotating the cartridge progressively will sometimes help if you have a seater with sloppy internal dimensions.

Thank you for your reply Green 788,

quote:

---

He can do this at 600 yards, but his groups would likely be a tiny bit tighter with no runout in the rounds. He can probably shoot 1.5 inch groups at best, but probably more like 3 inches or so. Half MOA is quite good, of course!

---

The point of that story was that perhaps you might be able to discern the difference run out cases with a bench gun, but at most shooters level with the equipment we use (even amongst the best competitive shooters) there are too many other factors that affect accuracy to a greater degree to really tell the difference. To show with statistical certainty that runout makes a difference at range would take more than a handful of groups showing the expected results.

quote:

---

The above principle is sound, and proven--however, you'll find experienced shooters everywhere who don't understand what's actually happening. They refer to bullets "going to sleep" at longer ranges, but they aren't aware of the part that runout plays here. This effect is not always caused by barrel twist/bullet weight issues.
Here are a couple of links to message board exchanges where the matter is discussed in more detail...

---

I read through the links you posted and found anecdotes of guns grouping a bit tighter at 300 yds than at 100 yds. Nothing what I was hoping for in the way of a Scientific proof source. I had the opportunity to speak to Bill Davis Jr of Tioga Engineering, a well recognized ballistic engineer and designer of the VLD bullet. I asked him about the helical path folk have been saying bullets take. He says it is there, but on a much smaller order than you'd think (a fraction of a caliber). The scientific term for that phenomenon he told me is "swerve".

quote:

---

Longrange shooters have known for years that they get better groups when they "index" the cartridges with higher runout so that all deviations are pointing at 12 o'clock (or "somthing" o'clock) in the chamber. This reduces the effect of runout, since those bullets deviate in the same direction from the barrel's imparted path.

---

I suspect you are incorrectly quoting Creighton Audette's work (and later Merrill Martin). What Audette found was that uneven case wall thickness correlated to case bases. He theorized that upon firing cases would bear unevenly on the receiver lugs causing dispersion. He measured cases to find the high side then fired then with high points supported by bolt/receiver lugs, then repeated with high points unsupported. He was able to demonstrate shifting points of impact between the groups.

quote:

---

Something else to consider is the obvious effect of runout on barrel throat life. The more off center the bullets strike the throat, the more excessive wear there will be to the throat. Even if it weren't for the accuracy advantages of reducing runout, barrel life considerations alone would make it worth doing

---

Years later Merrill Martin continuing Audette's work noted that barrels shot with indexed cases had uneven throat erosion radially that correlated with the indexed case. He theorized that the reason indexing cases worked was the "jet nozzle" effect. And this "jet nozzle" caused the uneven throat wear as well.

I would personally question the theory based on the common observation amongst Smallbore shooters that the throats on Smallbore rifles that have been shot alot will typically have throats that are washed out at the 6 o'clock position.

Again, I would for someone to steer me toward a paper or proof source with some semblance of scientific method that shows runout of 4thou or greater makes a difference on the target.

[ 01-29-2003, 21:53: Message edited by: Chris F ]

Chris F~
Very well written and accurate post. Glad you're around.

Re: "Again, I would (like) for someone to steer me toward a paper or proof source with some semblance of scientific method that shows runout of 4thou or greater makes a difference on the target." All the well tested results and documented tests I have seen or read support this conclusion. The most readily available test appears in the NRA publication of 1981, "Handloading". It also makes reference to to older publications, including Dr. F. W. Mann's "The Bullet's Flight".

The details and conclusions in the NRA publication were reached after testing 892 rounds of 30 caliber match ammunition. The conclusion is that tilts of less than .002" are good for long range accuracy. Over that tilt should be used for short ranges, under 200 yards. "These deviations become proportionately less as the tilt is reduced. Tilts over .004" do not seem to increase the dispersion of the group beyond the expected one minute". In general, the effect of tilt was concluded from target results that each .001" of tilt will increase the group spread about � minute of angle, up to a maximum of .004" as mentioned above.

Bob,
Thank you very much for that reference. I do not have that article. If you have it in electronic format and are willing to send it to me I would be very grateful. I could offer Audette's "20 shot method" in pdf format in trade if you'd like.

I do have Mann's book, and pulling it out I think I've found a chapter where he may be addressing the question. For those that are following along; I'm looking at a 1944 edition; page 249; "Causes of x-Error Located". My first pass through the chapter creates a question in my mind. Is he talking about imbalance in the bullets or runout?

I'll read closer when I have a bit more time and post my thoughts.

Chris~
Don't have it on computer at all. I might be able to scan it and send it. I'll try. If you post an address if I can make a legible copy, I'll be happy to send it.

As for the question of imbalance or runout, I'd think the results would be the same.

Bob,

I'm somewhat familiar with the NRA test that claims the effects of runout worsen at longer ranges, but I can't yet go along with that. If I'm wrong, though, it wouldn't be the first time!

The links I provided for Chris are, as he says, anecdotal. However, they are examples of what actually happened in these two cases, and I've seen this same effect elsewhere--only I have no additional exchanges or studies to point to at this time. Regarding those links, you'll note that runout was not even mentioned as a possible cause of the large (MOA-wise) groups at 100/small (MOA-wise) groups at 300 yards issue until I brought it up, and sure enough, when the shooters measured their runout, they found it to be excessive in both cases.

I'm still awaiting word from one of the shooters as to what reducing runout will do for his 100 yard groups, but the other shooter has already confirmed my contention that reducing the runout brought the close range groups into check.

Often, mass anecdotal evidence of a principle overwhelms scientific studies to the contrary. This happens all the time in the medical world, where the results of studies which are conducted by the best of the best end up being refuted by actuality. I can only say that at this point, the reason runout induces initial instability in the bullet seems clear to me, and my own experiences and those of my shooting acquaintances would seem to bear this out.

In a way, riflery is like "rocket science," in that the majority of what is learned is gleaned from actually firing the rockets.

My 100 yard groups with my heavy barreled .308 go from 3/8 MOA to 3/4 MOA and over with runout on the order of .005" or worse. Groups at 300 yards don't look so bad, though. Runout of .007 to .010 inches on the l-o-n-g 142 grain Swedish Mauser rounds (6.5 x 55) really shows up at 100 yards, but subsides at 300 yards. My 100 yard groups with such ammo (5 shots) are often 1.5 MOA with the Swede military surplus ammo, but 3 inches and better (5 to 7 shots) at 300 yards. The barrel twist is 1:7.8, by the way, which is more than enough to stabilize the 140's.

It is as if the bullet is destabilized on its way through the bore by the initial off center impact with the throat, but farther downrange it in fact "goes to sleep."

Velocity would be affected by runout--at least in a small way, I would think. Barrel time--critical to having the bullets exit the muzzle at the most opportune portion of the harmonic whip, would also be lengthened when the bullet hits the throat off center.

But all this much is anecdotal blather... Chris is right about that. So in the absence of any credible studies, let's approach the issue from the other end: Why would runout not affect groups? The bullet is entering the chamber off center, and is corrected with an off center slam into the throat. What would this effect be on the concentricity of the bullet itself? How would this turbulence affect a thin jacketed, light varmint bullet? How would this affect barrel time?

Thanks for your kind, thoughtful, and interesting responses, Chris. Such discussions are useful, and educating. As mentioned, you may bring a perspective to this whole issue I'd not considered...

Dan Newberry
green 788

FWIW--I'll let you shoot my ruger varmiter--box stock in .243. I'll give you 70 grain nosler bt's loaded at .007" runnout and I guarantee you that you will shoot .5" 5 shot groups with it if you are the least bit seasoned shooter. Kraky

Kraky,

That's great shooting, no doubt about it. I would imagine that if you can get the runout down, you'd have a "bug-holer" for sure!

Dan

Green788~
Re: "But all this much is anecdotal blather... " BINGO!!

In order to validly reach your conclusions, tests that are controlled need to be done in statistically meaningful numbers. You must eliminate as many outside influences as possible. Those tests published on the subject have had literally thousands of rounds fired to verify data gathered BEFORE publishing thereby reaching their conclusions.

As for, "Often, mass anecdotal evidence of a principle overwhelms scientific studies to the contrary. This happens all the time in the medical world, where the results of studies which are conducted by the best of the best end up being refuted by actuality." I'd love a couple of examples! That just is not so, PARTICULARLY in the medical field.

With your enthusiasm you would do well to read a few good books on this subject. Older issues of Precision Shooting cover a great deal of what you are interested in but some of the publications mentioned earlier are even better and more detailed.

The firearms you list upon which your conclusions have been reached sound like ample factory chambers. First place for introducing error and lack of consistency from the gitgo. Scopes, range conditions, ammo, etc., all enter into the picture. As much as possible the shooter should be taken out of the equation. I've not read the recent links provided as I found the originals on this subject faulted and meaningless. See the first line of the post.

To ignore the tests of the published ballisticians, and dismiss the data in the tests published by the NRA publication quoted earlier, and fail to agree only because you are only "somewhat" familiar, doesn't lend much credence to your conclusions. "Damn the facts, I know better?"

Re: "Why would runout not affect groups?" It does. Whether it's significant to the shooter is only based on each shooter's criteria.

Re: "The bullet is entering the chamber off center, and is corrected with an off center slam into the throat. What would this effect be on the concentricity of the bullet itself?" It might have great effect depending on the deformation. Who says the bullet tilt is corrected? There is no way a grossly deformed projectile will fly true if that be the case. Sorta like throwing a rock. However, a slight tilt would probably be negligible, after all the entire ogive is slamming into the rifling. If the bullets are seated on or into the lands, there is no "slamming" at all as they are already there at ignition.

Re: "How would this turbulence affect a thin jacketed, light varmint bullet?" At any significant velocity bullets have been known to fly apart and never reach the target. What causes this is pure conjecture. Usually either a defective bullet or a very high twist rate in the barrel. Deforming or obturation could also be a factor

Re: "How would this affect barrel time?" Probably no more significantly than the accumulated fouling in the barrel would. Obviously there would be some effect, but then so would the fourth, fifth or later shots in a string. Fouling varies from shot to shot.

When the effects of tilt, as tested in controlled environments has been determined to be � minute of angle for every .001" of runout. For .001" that's a quarter inch at 100 yards. Without good test conditions and benchrest equipment I doubt many shooters have the ability to see and know the difference and determine the cause of any variance that small.

As for stabilization of the bullet down range, it's pretty well accepted that most bullets stabilize earlier than 100 yards. A few, depending on many factors, including velocity and rifling twist, stabilize a bit past the 100 yard mark. Those few that do can produce accuracy at longer ranges beyond their MOA accuracy before stabilization. However, they don't magically return to the point of aim after deviating significantly from their earlier trajectory. Beyond 300 yards too many outside factors influence the trajectory and without statistically meaningful numbers, what happens in a few random shots has little or no meaning.

If all the findings over time by those ballisticians before us are accepted, the tilt of the bullet can affect groups as much as 1 MOA. This makes little difference in hunting to 100-200 yards. Longer ranges definitely demand more precision than that.

Bottom line, if someone is interested in only good accuracy for hunting or plinking, much of this discussion is meaningless. However, if extreme accuracy is the goal, this is just one of the factors that must be considered.

Bob,

I'm not sure if I made myself clear above, but I agree with you that runout effects accuracy. The bullet deformation you mention is plausible--I totally concur.
However, in my own experience (and that of acquaintances, only two of which I referenced) I believe the effect is dimished over longer ranges. The evidence shows this. Take some old Lake City match stuff with runout on the order of .005" and group it at 100 yards, then group it at 300 yards. Even if the barrel twist is optimal, the MOA will drop at 300. (I think we actually agree on this, but you may differ in that you wouldn't attribute this effect to the runout).

As for the medical profession analogy... Do you know what Viagra was originally designed to do? According to the scientists that made the pill? And I'm sure you're familiar with Thalidomide and it's effects... But scientists said it was great for morning sickness in pregnant women. Low fat diets are the way to get to great health, but not according to some medical scientists...

Bob, you and I are about 80% in agreement on most issues. If we disagree on one or two items, that's really not so bad...

At least not to me, anyway...

Have a good one (and tell Hotcore I said "hey"!)

Dan

Green 788~
Hotcore? Huh? What's that about?

As far as I know I've never had ANY "discussion" with you such as to bring any disagreement. Furthermore, I seldom, if ever, post opinions or conclusions that aren't clearly labeled as such. It's almost always facts, accepted and published conclusions, or long experience. To say you agree with me 80% of the time is very telling.
I generally disagree with your conclusions 100% of the time, if that tells you anything.

By the way, I really like the basic unit from Sinclair where you use the dial indicator. I got the indicator off of ebay. I had the NECO unit. OK but pricey. I think there's one for sale over on varminthunters.com classified...

Roger

"I generally disagree with your conclusions 100% of the time, if that tells you anything." Bob338

I thought we agreed that runout negatively affected accuracy. Guess I was wrong!

And that's what I meant by the reference to Hotcore!

Dan

Dan 788 and Bob338,
Thank you for your kind words of welcome.

I've searched for a credible answer to the question of whether runout matters for a while now with little success. I've come to several conclusions;
1) Most folk are happiest trying to keep their runout under .004" whether they have seen it on target or not. To them it makes sense and makes them feel better. More power to them.

2) If I want the answer at a level that I'd like, I'll have to invest in a rail gun and a 600 yd temp controlled shooting tunnel.

Green 788, case observations are not valueless in drawing a conclusion and I apologize if I gave that impression. I will say that anecdotal observations are difficult to draw conclusions from because of their uncontrolled nature. There are too many confounding variables to control for. A well designed testing protocol that eliminates as many variables as possible and isolates the variable you'd like to study (in our case runout's effect on accuracy) is best and what I was asking for references to. Well beyond the desires and capabilities of "hobbiests" like ourselves, but my hope was that some an article based on arsenal data as Bob338 seems to.

Aggregating data or "mass anecdotal evidence" can help increase your body of data, but are prone to the same as above. And Meta-Analyses are not viewed as favorably as well controlled trials. I am not aware of any cases where anecdotal reports proved well designed Controlled Trials wrong, but am open to the possibility and would love to hear of true examples.

Your medical examples of Viagra and Thalidomide are not failings of Scientific Method but rather validation that Scientific Method works. Thalidomide was inadequately tested for side effect (and most notably teratogenic effect). A US FDA Pharmacologist stalled the approval of Thalidomide in the US due to this inadequate safety data. The rest of the world was not so lucky and the relaxed standards of the rest of the world continue. (The US had a bout of this during the Clinton/Kessler years).

Viagra was found in trials to be not as effective as Pfizer had hoped in treating angina...we all know what it was found to be effective for. It is a great example of a serentipitous discovery. But wait, it only works in approx 60% of patients. Many men got excited by the glowing anecdotal reports coming in only to be disappointed when they found they were in the unfortunate 40%. Data from controlled trials would have tempered their enthusiasm.

When there are dissenting views in Medicine, Randomized Controlled Trials are classified as the highest level of evidence (Level 1A). Anecdotes are as I recall Level 3 or 4.

On another subject, has anyone realized that in a non-bench rest gun, you have several factors that might negate the pains you take to minimize runout.

1) In "plunger" type ejector bolts (Rem 700, Sako, AR15, Post64 Win70, but cartridge itself is "cocked" to the side of the chamber by the spring loaded ejector. That's gotta induce more tip of the bullet relative to the centerline of the bore than 4 thou.

2) Same situation in bolts without plunger except the "tip" is created by gravity.

Non benchrest chambers have slop to allow easy function. This "slop" allows the above situation.

Yes, jamming a bullet into the lands might counter-act this effect, but are we able to do this for every load and do we know for sure?

So I'll pose another question; How many of you that are experiencing better accuracy with 4 thou or less runout are shooting with the bullet jammed into the lands, or shooting fireformed and neck sized cases? How many of you are not?

My apologies for another very long post.

Tom Jones,
How much would shipping for one of those "Holes" run to Hawaii?

Thanks for staying with me here Chris. (Hotcore is the court jester, but he's harmless enough, and during the times his wife unlatches the cellar door and lets him roam freely about, you'll see entertaining--and sometimes accidentally informative--ramblings from brother Hotcore )~!

Regarding the amount of error in factory rifle chambers, etc.

What would you think of the notion that the custom built tolerances of the BR type actions actually tolerate runout better than the factory "sloppier" chambers?

Wouldn't you say that the built in error factor in a factory rifle would benefit from low runout even more than the BR rifle? True, the crookedness of this part or that part might act to negate a particular amount of runout in some cases (depending on the indexing), but odds are--I think you'll agree--that the overwelming majority of the time the "stars won't be aligned," so then...

I would say the the factory barrels would benefit from lack of runout more than the BR rifles would. In a BR rifle, .004" of runout is equivalent to .004" of runout. In a "sloppy" factory chamber, with a .003" error in the chamber cut, depending on the position of the cartridge when it enters the chamber, we could be looking at a total of .007" of runout. Am I making sense? (I don't always make sense, just ask Hotcore!)

I agree with you that controlled studies are beneficial, but my main problem with them is that they often don't address all the variables in the real world, which is where the "mass anecdotal evidence" has value. If the study on runout were done with a super-tight rail gun (in order to get the perceived level of accuracy needed to come to a viable conclusion) that rail gun by virtue of its tight dimensions might totally overcome the effects of the runout. The conclusion would be that runout doesn't matter. However, with a factory rifle, the accumulation of error could be what causes the runout to have a greater effect.

One other thing... We would also need to define runout. See my other post "Types of bullet runout" for what I mean by that...

This is fun. I'm enjoying it. Let's keep talking!

Dan

Chris~
Re your questioning whether runout matters, there is no doubt in my experience that it does. The real question is "How much does it affect a shot?" I'll accept the findings of those more learned than I. While I don't have a rail gun, and my age and eyes no longer enable me to shoot with the precision I could some years back, I did a bit experimenting recently on the effects of runout. It bore out information learned earlier in casual experimenting on its effect.

I work with a custom gunsmith friend in shooting and testing guns. He had a client send in a 1000 yard gun chambered for 7.82 Warbird. The customer claimed it wouldn't shoot. He turned it over to me last August and I just returned it to him with my findings. It WILL shoot. In the course of the shooting it was imperative to eliminate as many outside factors as possible. The gun had a tight necked chamber but was throated too short for optimal use of the 240g Sierra MatchKing which the owner was shooting. In spite of the problems with the chamber I went forward.

The first thing I discovered was the very low quality of the brass available, and its high cost. About $2 per case. After all the prep of brass, sorting it and turning necks to a .015" wall thickness to yield .002" clearance, I was able to get about 9 cases, out of twenty, that were 'near' perfect. The others had some inherent problems with the necks or case volume. On those pretty good cases I was able to seat bullets with zero runout. On the other cases the necks themselves caused runout of between .001" and .004". The bullets had all been sorted and culled. The cases were marked appropriately and tracked through the shooting. When they were checked on the concentricity gauge, the high spot of the tilt was marked on the cases and tracked throughout the shooting. I fired approximately 200 rounds through the rifle annealing those case about every 4 reloads. All loads were chronographed on an Oehler 35P. When those eccentric cases were used, the high spot was always inserted at 12 o'clock. The near perfect cases were put in randomly. The bolt had no ejector.

In every case of using the eccentric brass there was some vertical stringing. Not so with the 'good' cases with zero runout, which printed without any directional bias. At one point in the shooting, mostly at 300 yards, I chambered the cases with runout at the 3 o'clock position and sure enough, the evident stringing was horizontal. Wind was mostly 0-2mph from my back.
Groups for the cases with zero runout were mostly under 1.5" inches at 300 yards, the others with the 'straightened' runout exceeded that with some groups running near 3"+. (This was after load development.) The evident stringing was always in the orientation of the high spot.

This effect was studied in the article mentioned earlier. If you want copies of it, I have scanned it and can send it if you will let me have a valid E-mail address.