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Case Cocentricity

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Unread 12-02-2008, 08:53 AM
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Re: Case Cocentricity

Originally Posted by milanuk View Post

Question for you (or anyone else using similar tooling) on the Bersin device: do you notice any issues w/ neck tension after 'correcting' a cartridge?

The reason I ask is on another forum (oriented around long range target shooting) people have theorized that in order to push the bullet over, that you must be kind of oblonging (not sure of a better term) the case neck - in short, ruining all the other work that goes into achieving consistent neck tension and release.

A couple friends of mine use the H&H tool, and swear by it - I just haven't quite made the leap yet. I haven't seen the Bersin around much, but noticed that Hornady just came out with such a device (check the video on their site for their case prep center... towards the end is a blurb for their concentricity tool) - curiously enough its marketing claims include something to the effect 'first tool of its kind to allow you to correct runout & concentricity in your ammo'. Guess they never looked very hard for the Bersin, H&H, or even some of the older concentricity gauges w/ holes in the base that you could insert and 'bend' (yikes!) the case straight.

I also use a Lee Factory Crimp on most of my loads. This seems to minimize the back and forth you can have with the Bersin. Sometimes (probably 2 or 3 out of 50) you push on one side and it creates runout on the other and the runout will go round and round the circumference and you just can't get it stabilized to center the bullet. The best you can do is keep the runout at around .002" (which is much larger on the Bersin dial where each mark is 4/10,000")

Also, if I don't have the bullet centered by the 3rd push then I will call it good and set that load aside for a sighter. When I first got the Bersin I would keep going round and round and pushing trying to get that last 2 or 3 dial mark variations out and I learned that you can loosen the bullet by repeatedly pushing from one side to the other.

There is a definite learning curve with the Bersin just like anything else but now I can usually get the amount of push right on the 1st or 2nd push.

I look forward to the 3rd or 4th case firings as those case necks have hardened enough to make them easy to stabilize perfectly and by that time at least 1/2 of the loads are perfect from the get-go.

Edit to add: Also I think the Lee Factory Crimp aids in reducing runout. When the collet close at the neck mouth to crimp, the case body is supported and I usually rotate 90* and crimp again. I don't know this for a fact and need to do some tests to see.
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Last edited by woods; 12-02-2008 at 08:56 AM.
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Unread 12-02-2008, 09:55 AM
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Join Date: Oct 2006
Posts: 624
Re: Case Cocentricity

The Lee collet will not fix the small eccentricity caused by sizing with a bushing after firing in a SAMMI chamber with .006" clearance around the neck.
Neither will the Lee Collet fit the large eccentricity caused by sizing with a sizer die with original neck that is fitted with an expander ball.

What the Lee Collet will do is keep concentric the case fired in a concentric chamber.
Likewise a honed neck sizer die [$10 mod at Forster] will keep concentric the case fired in a concentric chamber.

I broke the casting on an RCBS Rockchucker with a Lee Collet die. I sent the press back and they sent me a new one. The next year RCBS changed the casting where I broke it. I don't know how many others made the same mistake. The internal forces in an over center toggle type press are only limited by friction if a Lee Collet die is adjusted for high force right at top dead center.
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Unread 12-02-2008, 03:10 PM
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Join Date: Oct 2007
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Re: Case Cocentricity

"For a .004" displacement and a 10" rifling pitch, the tangent is 1/8 (2 pi)(.004) / 10 and the corresponding angle is 1.1 minutes."

Okay, I accept that as a math calculation and predictions against moa at the target.

What I do not get is a prediction or even a correliation pertaining to a conventional expander producing any specific tilt, incuding the .004" given as a worst case "for instance" in the article's quote.


Without a concentricity gage we are loading blind.

Using the Lee Collet Neck Die does require a bit of mechanical understanding of what's happening with it.
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Unread 12-03-2008, 12:54 AM
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Join Date: Jan 2006
Location: South Africa
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Re: Case Cocentricity

I didn't ever answer the question: Yes, in a straight chamber they should come out with virtually no runout after firing.

If you can appropriately match the bushings loaded OD and neck thickness you should be fine.

This is moving a little off your question, but I will say "REMOVING THE EXPANDER BALL WILL NOT ALWAYS UNEQUIVOCALLY GIVE YOU STRAIGHTER AMMO". You need to measure to find out and you can only measure with a runout gauge as far as I'm aware.
In my case certainly firing the thin WW brass in a SAAMI chamber is not causing eccentricity of itself - run out is less than a thou on the fired cases. It's the excessive sizing that creates the opportunity for run out as Clark points out.

But I don't size with a bushing die AND a collett die so I don't quite follow the statement: "The Lee collet will not fix the small eccentricity caused by sizing with a bushing after firing in a SAMMI chamber"; I don't doubt for a moment that this is probably correct, just that I will never know as I only want to size the case once, so I don't use the Lee Collett as a "fixer" I use it as a primary neck sizer.

The Lee Collett seems to be better in avoiding the runout created by needing to size a great deal. I sincerely believe that a regular die with an expander ball would also be better than the expanderless, bushing type in this situation - and this is my (possibly limited) experience with it.

Where I'm headed is that everthing I'd seen indicated that expander balls were from the devil, so I bought the option that worked without it when I had the money and the chance. It was the wrong choice FOR MY CIRCUMSTANCES (the thin brass and large chamber). What I'm saying is not all that you read or see in opinion is a universal fact, as it always "depends" on things like the combination of chamber size and cases I've used as an example.

I made an expensive error and would never have known were it not for the run out gauge (and I'm embarrassed to admit that I shot the ammo like that for a year and results were pretty good!). I agree, without it you are "loading blind".

Also had I borrowed dies and loaded up some ammo and shot some and measured I'd have known how much I was sizing and that the bushing die was the wrong choice and would have ordered a different set of dies with an expander from Redding in the first place and saved money and frustration.

So I believe at the least you need a vernier (cheapish should do) and a micrometer (to measure sized and loaded round OD) and preferably a run out gauge (you can borrow these last two from time to time if need be) and a bullet comparator for measuring AOL. You can get by without a ball micrometer if you don't sort cases. Then measure and decide from there what is best for your purposes.

Next time around I'd go with a Lee Collett and Redding Body Die. Seater? Preferably something like a Forster or Redding that's an in-line type set-up.

Kirby posted some great info in a recent thread on run-out or cocentricity. Do a search and see if you you can find it. He also outlines how to set up regular dies for the least run out. Others also made good comments in the thread (sorry guys I don't recall your names). It's a really great read.

Read the tips in the Redding tech section too, they are good.

Good luck with it!

Last edited by LRHWAL; 12-03-2008 at 01:16 AM.
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Unread 12-03-2008, 01:48 AM
JRu JRu is offline
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Re: Case Cocentricity

I keep a local copy on my computer so I'll just post it right here. It's excellent information and deserves to surface in search results as often as possible.

Originally Posted by Kirby Allen
the expander stem creates about 85% of all Run out problems in ammo using conventional dies.

Here are a few tips to try using conventional dies that will help with your run out issues.

First do as Uncle B stated and try sizing some cases in the FL die with the expander stem removed, it will surpise you!! Generally run outs drop to 0.001" or less instantly. Only problem is these dies reduce the neck diameter to much and require the case necks to be expanded back to the proper dimension.

First thing is to get the case deprimed without sizing the case. You can do this by dropping the expander/depriming stem as low as possible to deprime the case before it contact the die body, or use a smaller diameter expander stem. Whatever you use, we need the cases deprimed before starting this process.

Now, with the cases deprimed, and the expander stem installed back in the FL die, adjust the FL die down to size the proper amount on your case, generally I like 2/3 to 3/4 the neck sized when partial full length sizing.

First thing to do now is to determine how the FL die body is effecting neck run outs. Try sizing some cases. With the die lock ring loose, run a case up to the point that you want it sized to. Lock the die body lock ring down and then withdraw the sized case. This should hold the die in proper alignment.

Size another case and then measure the neck run out on this case. If it is 0.001" or less, your ready for step 2. If it is not, untighten the die lock ring and repeat the test but with the lock ring loose instead of tightened. SOme die threads are not perfectly true, as are some press threads off a bit. If they are and you tighten the die lock ring down you will pick up run out.

Run a properly lubed case up into the FL die until the ram is at the top of its travel.

Then raise(unscrew) the expander stem up until it stops against the inside of the case mouth that is in the die. Then turn the expander back down 1/2 rotation just to give it some breathing room.

DO NOT tighten the lock ring on the expander stem!!! Very important, let her float!! THis will allow it to self align with the case as it is pulled over the expander ball.

The benefit here is similiar to a sliding sleeve inline sizing die. The Inside of the case mouth engages the expander ball before the case is released by the die body in the neck area. Basically, the die body is true in most cases, since the case held in position by the die body, the expander stem is required to center itself to the position of the held case mouth. Leaving the stem floating allows it to do this. And generally the finished expanded case neck will have a dramatically lowered neck run out.

Keys are:

-Deprime cases without sizing the case
-Adjust the die body so it is producing good low runouts
-Leave that expander stem floating so it can self align

YOu will see a dramatic decrease in your neck run outs using this system with conventional dies.

The next issue is the conventional seating die. We need neck run outs as true as possible and the case mouths perfectly square and evenly chamfered or bullet runout will increase during the seating process.

Using conventional seaters, you can expect to see the neck run out value at least doubled in the bullet run out. Point being you need your neck run outs as low as possible for any hope of getting sub 0.003" bullet run outs.

There is no real good way improve the conventional seating die. Basically, get that brass as square and even in the case mouth as possible, get the neck run outs as low as possible and you get what you get.

To greatly improve the bullet run out, you really need an inline seating die. Forunately, Forster makes a great inline seater that is very reasonably priced. I think they run about $30 for the more popular chamberings.

These will really help the run out of finished ammo.

When using an inline seater you will generally see the same bullet run out as the neck run outs you started with, or in some cases slightly better.

There are more expensive inline seaters like the Redding Comp dies but they all do the same thing, hold the bullet and case in the same axial alignment while the bullet is seated. This is key to low bullet run outs!!

Try that set up with your sizing die and I assure you your neck run outs will drop significantly on average.

You've started down that slippery slope of run out!!! Kind of like getting a chronograph. All was well before you knew the true!!!

Kirby Allen(50)
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