Are the cocentricity verification tool woth it for precision long range?
- Thread starter Evil_Dark
- Start date
My observation is that those who have the tools are almost as obsessed about getting good concentricity as they are about getting good accuracy. I'm obsessed with getting the best accuracy, but not so much about concentricity. Since I don't have the tools, I don't have a problem. Said another way my cynical view is that measuring concentricity is kind of a solution in search of a problem.
That said I put my effort into ensuring concentricity issues are minimized. To that end I use Lapua brass if it is available. I also use a bushing bump die and only partially resize the neck. The busing size is selected to minimally resize the case, and the expander ball diameter such that it just kisses the ID of the case and makes no significant expansion. This die also locates the expander ball very high in the neck so the neck is still being guided by the bushing when the bottom of the neck starts over the ball. All of this is done to minimize reworking of the brass and inducing runout. The other part of my thinking in using this approach is that when you leave the rear 1/3 of the neck unsized, it is going to fit as tight and untouched as possible in the chamber neck. This centers the bullet in the throat in my view as well as a tight neck and high concentricity.
There is also a school of thought that believes that some flexibility in the resizing die minimizes induced runout. On a fixed FL die they use an o-ring to increase flexibility to self align. In a bushing die there is clearance between the bushing and the die body allowing it to self align too.
So in short, I focus on using methods to minimize runout, but don't bother to measure it. My measure is the group size on the target.
That said I put my effort into ensuring concentricity issues are minimized. To that end I use Lapua brass if it is available. I also use a bushing bump die and only partially resize the neck. The busing size is selected to minimally resize the case, and the expander ball diameter such that it just kisses the ID of the case and makes no significant expansion. This die also locates the expander ball very high in the neck so the neck is still being guided by the bushing when the bottom of the neck starts over the ball. All of this is done to minimize reworking of the brass and inducing runout. The other part of my thinking in using this approach is that when you leave the rear 1/3 of the neck unsized, it is going to fit as tight and untouched as possible in the chamber neck. This centers the bullet in the throat in my view as well as a tight neck and high concentricity.
There is also a school of thought that believes that some flexibility in the resizing die minimizes induced runout. On a fixed FL die they use an o-ring to increase flexibility to self align. In a bushing die there is clearance between the bushing and the die body allowing it to self align too.
So in short, I focus on using methods to minimize runout, but don't bother to measure it. My measure is the group size on the target.
Think of the spaces between the chamber body and neck compared to the case and seated bullet.
The goal when hand loading is to ensure that the case holds the bullet in perfect alignment to start into the rifling perfectly straight.
If you have a full length resized case and 0.014" clearance on the diameter of the neck, then there isn't much to ensure the bullet is aligned right? Sure you can leave the bullets seated long so they jam the lands and then at least the bullet is on center even if the case isn't.
So lets walk through the fire and reload cycle for a minute...
You take a round after FL resize and shoot it in your rifle, now the case fits the chamber really nice. At least now when we resize the neck (only) the body is still a perfect match.... But what about the neck?
If the neck is cleared by 0.014" then chances are you will have runout that will have been created by the neck resizing process. In my experience, resizing a neck 0.014" will result in cases with up to 0.007" runout no matter what you do.
If you really want to control runout what you need to focus on is the neck clearance. If the neck was only clear to your loaded round by 0.004" you would automatically have runout of less than 0.002". If the neck is cleared 0.002" then you will always have less than 0.001" runout.
This gets into case selection. If you were to purchase Lapua cases which are known to be heavy, we can deduce the neck wall thickness is thicker and therefore takes up more of the gap between the neck and chamber. Try light weight Winchesters in the same rifle and the thin necks will have more clearance, which translates to more runout.
In the end, to really control concentricity you'll need a tight neck chamber and once you have that, you will not need the concentricity tool.
Checking concentricity on a slop fit rig is pretty much a waste of effort. As stated by Ron above, doing a neck only resize with a Redding bushing die and only resizing 2/3 of the neck and jam the lands is about as good as you are going to get without reducing neck clearance.
The goal when hand loading is to ensure that the case holds the bullet in perfect alignment to start into the rifling perfectly straight.
If you have a full length resized case and 0.014" clearance on the diameter of the neck, then there isn't much to ensure the bullet is aligned right? Sure you can leave the bullets seated long so they jam the lands and then at least the bullet is on center even if the case isn't.
So lets walk through the fire and reload cycle for a minute...
You take a round after FL resize and shoot it in your rifle, now the case fits the chamber really nice. At least now when we resize the neck (only) the body is still a perfect match.... But what about the neck?
If the neck is cleared by 0.014" then chances are you will have runout that will have been created by the neck resizing process. In my experience, resizing a neck 0.014" will result in cases with up to 0.007" runout no matter what you do.
If you really want to control runout what you need to focus on is the neck clearance. If the neck was only clear to your loaded round by 0.004" you would automatically have runout of less than 0.002". If the neck is cleared 0.002" then you will always have less than 0.001" runout.
This gets into case selection. If you were to purchase Lapua cases which are known to be heavy, we can deduce the neck wall thickness is thicker and therefore takes up more of the gap between the neck and chamber. Try light weight Winchesters in the same rifle and the thin necks will have more clearance, which translates to more runout.
In the end, to really control concentricity you'll need a tight neck chamber and once you have that, you will not need the concentricity tool.
Checking concentricity on a slop fit rig is pretty much a waste of effort. As stated by Ron above, doing a neck only resize with a Redding bushing die and only resizing 2/3 of the neck and jam the lands is about as good as you are going to get without reducing neck clearance.
Last edited:
Ok thanks for your input.
I've noticed that one of the good upgrades that had made a nice difference is the neck sizing. It helped a lot to have a more consistency in the velocity, the groupings were a lot better. So I don't think that the cocentricity will improve as much... I'm better purchase a bushing die instead!
Regards,
Dark
I've noticed that one of the good upgrades that had made a nice difference is the neck sizing. It helped a lot to have a more consistency in the velocity, the groupings were a lot better. So I don't think that the cocentricity will improve as much... I'm better purchase a bushing die instead!
Regards,
Dark
- Location
- Somewhere on the Hudson Bay Coast
Accuracy is all about consistency, anything that can be measured can be controlled, and the tighter you can keep the tolerances, the tighter your groups will be; but its seldom just one thing. Whether your powder scale has an accuracy of +/- .1 grain, or +/- .05 grains in itself doesn't matter unless you have taken steps to uniform the powder capacity of your cartridge cases, uniformed the bullet weight and meplat, uniformed the bullet pull weight, uniformed the case neck run out, uniformed the primer pocket depth and shape and the flash hole size, and uniformed the primer seating depth and seating pressure. Those are a lot of steps to go through if your intention is to shoot for score on a 300 yard target with an AR-15. On the other hand, if you don't shoot more than a few groups a day through a bolt gun, perhaps you can justify it.
I agree. If you start chasing concentricity, you will then want to buy new cases, and then a neck turner, and when you have done all that, you will want a new barrel with a tight neck. Much less costly to buy a bushing die. I have a Forster Bushing Bump Neck sizing die. I believe I gave you the link above. It has two significant advantages over the Redding. First it bumps the shoulder as well as sizes the neck. It is adjustable so you can bump the shoulder 0.001" to help ensure your cases fit the gun, and at the same time only resize 2/3 of the neck to keep the case concentric in the neck. The second advantage is that although the Bushing Bump die does not come with an expander ball, you can add it. There are various oversizes, but I found the standard one works best. My view is that the last contact with the case should be the ID of the neck with it just bumping up the ID of the neck 0.001" at the most. You control how much sizing the ball has to do with the selection of your bushing. Forster locate the ball high in case, so the combination of minimal resizing and having the ball high creates minimal runout. No, it will not correct cases that have uneven neck thickness, but since you put the expander ball through last, the ID is good and round. I use powdered graphite to lubricate the ID of the neck when sizing, and leave it in there of course. I thing the whole combination gives a very uniform bullet grip force. I also jam my bullets to ensure they stay concentric on loading. Not all loads are suitable for that though.
RangerPark
CGN frequent flyer
- Location
- Near Edmonton, AB
I also believe in checking runout. That being said I also believe annealing case necks does more to achieve consistent neck tension and accuracy than anything else. There's only so much you can do to control total runout. A good reloading process will make most rounds under .001" TIR but not all of them. If I'm shooting for groups I'll sort my rounds by TIR, over and under .002". That being said I've shot equally good groups with .005" TIR rounds than .0005" ones.
I use a concentricity gauge so I can narrow down where the issues, if any, are coming from. Is my sizing die causing it, the seater, the press itself, the rifle's chamber, etc, etc? Once I've determined where the runout is coming from, I can then make a decision whether I want to or if it's worth doing something about it. I like having that information and is why I recommend the tool.
In most cases run-out can be caused by the expander in the sizer dies. Eliminating the expander in the process really helps. Also, a word of caution about tight necked chambers. I have found that they cause more issues than they give any benefit to. That idea came from the benchrest crowd who only use a few cases in the rifle over the course of the barrels life. If you are shooting a match or hunting rifle, I would strongly suggest staying away from them. They can be dangerous.
If you don't use an expander in the sizer die, then you have to use a bushing die or honed out neck sizer die that sizes down the neck much less. Perhaps the real cause of runout is not the exapnder ball, but really just the over sizing down, and then back up again. The expander ball gets blamed, but the damage has already been done before the expander ball gets into the act.
Why are tight neck chambers dangerous? Please elaborate.
Know your chamber neck diameter and turn your necks accordingly. Pretty simple. I'll prepare 100's of cases for a season. Because my barrels always have the same neck dimension, I could prepare 10 years worth of cases if I wanted to. In a typical match I'll shoot no more than 200 rounds using a batch of 20 cases. I usually retire those cases after 2 matches. So that's what, 60 cases if I retire a barrel at 1200 rounds? Ok I think I see where you might be coming from.
I suppose if someone was a scatter brain you could jam an unturned neck into your tight neck chamber and be in for a nasty surprise. Is that your point?
Last edited:
Tight neck chambers are not unsafe if you know what you're doing.
The biggest problem is what they call the dreaded donut which is a reduction in the inside diameter of the neck at the neck to shoulder junction. It is caused by the forward flow of brass under pressure when the round is fired. Eventually after enough firings the brass will flow enough to form this donut.
This can create a problem only if the bullet is seated into the donut. If you have the barrel throated so the bullet bearing surface is forward of the neck to should junction about 0.04" then the donut will never have an effect.
Plus as a sanity check you can always use a bushing from your neck sizing die as a go gage on loaded rounds. If you have a .337 neck and the .336 bushing slides freely over the neck of the loaded round, you are good to go.
If you have a tight neck and feel resistance closing the bolt on a live round, then pull it out and inspect. Do not fire the round. You have a QC problem.
The biggest problem is what they call the dreaded donut which is a reduction in the inside diameter of the neck at the neck to shoulder junction. It is caused by the forward flow of brass under pressure when the round is fired. Eventually after enough firings the brass will flow enough to form this donut.
This can create a problem only if the bullet is seated into the donut. If you have the barrel throated so the bullet bearing surface is forward of the neck to should junction about 0.04" then the donut will never have an effect.
Plus as a sanity check you can always use a bushing from your neck sizing die as a go gage on loaded rounds. If you have a .337 neck and the .336 bushing slides freely over the neck of the loaded round, you are good to go.
If you have a tight neck and feel resistance closing the bolt on a live round, then pull it out and inspect. Do not fire the round. You have a QC problem.
