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Old March 2, 2013, 01:56 AM   #1
deepforks
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Resizing Issue - 270

so this has me scratching my head. i just bought some rcbs precision mic gauges and i'm getting readings opposite of what i should be. i checked three fired cases and all were exactly the same - one hash mark below the zero hash. ok, now that should be my baseline. i'm trying to bump the shoulder back .002". i'm using a rcbs fl sizing die. i've got the decapping pin removed. i resize all three and the mic shows them all at .002" greater - one hash mark above the zero hash?

ok, what am i missing here? how are they "growing"? this doesn't make sense to me.

image 58 is before sizing
image 61 is after sizing
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Old March 2, 2013, 03:13 AM   #2
deepforks
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well after a little research i've found some other people that have discussed this as well. seems if the die is out too far, the body of the case can be sized causing the shoulder to be pushed forward slightly. worked the die in slowly and found my .002" set back. i figured i wasnt in far enough with the die, but didn't know about the body sizing causing this. good to know. all is well.

on a side note, do most first firings cause the case to match the size the the chamber?
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Old March 2, 2013, 05:55 AM   #3
Bud Helms
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Yes. Everytime you fire it, it will expand to match your chamber, not just the first firing. An fired, unsized case will be tight on chambering and will probably not hold a bullet in the expanded mouth. The brass "flows" as a function of heat/pressure during the firing cycle to meet the chamber dimensions. Once extracted it is never an exact match due to the springiness and cooling of the brass. It is most noticeable at the shoulder, the mouth and the overall length of the case, but a caliper will show that the fired brass is different in nearly every measurement from a sized case. That is why we resize either full length, partial, or a simple shoulder bump, depending on what you're trying to achieve: accuracy: reliability or somewhere in between, and what your chamber and action of the rifle will permit.

A partially sized case will chamber, but be snug and need slightly more bolt camming pressure than a full sized case. That has accuracy advantages. A full length resized case will chamber easily, but accuracy is a dice roll. You will have to decide how much accuracy you need.

There are variations in chambers from rifle-to-rifle. Some chambers are closer to SAAMI than others. Your brass will last longer, be more reliable, and more acurate if you can determine how much resizing is required to facilitate chambering a loaded cartridge, but no more. That is another attraction of handloading, you can load for your rifle.

Plenty of caveats and assumptions here. Using a small-based die to resize for semiautos is one of them. Careful bullet selection is another. Case prep is another.

One last note: This post is full of generalities, so you will need to determine your needs and your rifle's tolerances.
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Last edited by Bud Helms; March 2, 2013 at 06:42 AM.
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Old March 2, 2013, 10:41 AM   #4
243winxb
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This works with a 270 also. #8 would apply here.

Last edited by 243winxb; March 2, 2013 at 10:47 AM.
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Old March 2, 2013, 10:51 AM   #5
RC20
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Also keep in mind that neck sizing works the neck at the same rate as FL size does.

And where does the break come? The neck.

We shoot a lot of guns so the FL sizing is by far the best but other than some effort neck sizing yields no gain other than tack driving accuracy if that is what you are after.

Have fun into some FL sizing issues and prefer to stay away from adjusting the same for neck sizing. I would rather be shooting than futzing, but thats just me.
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Old March 2, 2013, 12:04 PM   #6
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Did you screw the dies down until it sits tight on the shellholder? After that you should lower the ram and turn the die down 1/4 turn more.
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Old March 2, 2013, 04:41 PM   #7
deepforks
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pa joe, i know that is what the instructions call for, but that sets my shoulders back way too much. i pretty much did the opposite. make contact with the shell holder and back the die off. then slowly move the die in from there, checking on the mic gauge untill i found the .002" setback i wanted.

when i was asking about a first firing forming to the chamber, i read in a couple other forums where guys were saying they didn't see full case expansion until the second, and sometimes third firings. i always presumed the first shot would get you there.

i appreciate all the feedback here. i'm by no means an expert, but i love reloading and learning. i'm getting a collection of great tools and enjoy the process of getting everything so consistent and uniform, then seeing the results. it's addicting!

i've always fl sized and will be doing some research to see if neck sizing would be a route i want to explore. i've come up with some great loads for my .270 & .300 that are roughly .75". for me, thats pretty good. not saying i can't do better. getting ready to start playing with my .243 & .22-250
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Old March 2, 2013, 04:50 PM   #8
Bart B.
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Full length sizing .270 cases setting their fired case shoulder's back a couple thousandths been giving best accuracy for decades. Sierra Bullets uses Redding full bushing dies on their .270 cases testing .270 bullets for accuracy. This die doesn't have an expander ball but its bushing's a couple thousandths smaller than a loaded round's neck diameter. That's how they can shoot 1/2 MOA or better at 200 yards with their .270 bullets.

After a new bottleneck case is fired, its body expands to the chamber shape then shrinks back a couple thousandths in diameters as well as head-to-shoulder length; its neck expands almost to the chamber neck then shrinks back a tiny bit larger than before. It'll easily go back in the chamber if fired from a bolt action rifle. If that were not reality, then neck-only sizing would never have been so popular.

Any case sizing die/process that ends up making the case even the tiniest bit hard to chamber (i.e., partial full length sizing that moves the shoulder forward a bit) hurts accuracy. This is why benchresters that neck only sized their cases had to full length size them after 4 or 5 firings so the bolt would close freely. Most benchresters have moved over to full length sizing every time now as the accuracy is better overall. Sierra Bullets' first ballistic tech proved this back in the 1950's trying every fired case sizing tool and technique known to man.

Folks believing that neck sizing fired bottleneck cases allows them to fit the chamber more perfectly centered need to realize that all rimless bottleneck cases, including new and full length sized ones, center up front at their shoulder when fired. It doesn't matter how much smaller in diameter the case body is relative to the chamber as long as there's clearance enough for those out-of-round cases to fit in out of round chambers without binding. There's no such thing as perfectly round cases and chambers. The firing pin drives the case forward before it detonates the primer perfectly centering the case shoulder in the chamber shoulder. After the case shoulder's hard into and centered in the chamber shoulder, the primer fires. If that were not true, then brand spankin' new unfired cases would never shoot 1/4 MOA at 100 yards or 1/2 - 2/3 MOA at 600 yards; they do that quite nicely.

I think you're on the right track with that RCBS die and setting the case shoulder back a couple thousandths.
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Last edited by Bart B.; March 3, 2013 at 10:56 PM.
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Old March 4, 2013, 05:20 AM   #9
Bud Helms
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Thanks for the post, Bart.

You pointed out a couple of things I had forgotten and a couple I had not heard before.
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Old March 5, 2013, 12:57 PM   #10
243winxb
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deepforks, the Redding Competition Shell holders may be of some use to you. http://forum.accurateshooter.com/ind...pic=3789175.15Competition shell holders will square the die in the press when the shell holder makes contact with the die. Tighten lock ring after contact. The amount of shoulder set back will be more exact. From link, Redding email
Quote:
Thank you for using Redding Reloading Equipment. If your Full Length Sizing Die is adjusted to just make contact with the shell holder, and you run a case up into the die. You will see a gap between the bottom of the die and the top of the shellholder. What happens is when the case pushes up on inside of the die and it removes the play between the threads of the die and the threads of the press. There is also some flex in the press itself, even Redding Ultramag Press will flex. Each piece of brass will have a different amount of resistance to resizing. So this gap will vary, which gives you a variation in shoulder setback.

Redding's Competition Shellholders allow you to adjust your die for a cam-over (screw the die down another 1/8 to 1/4 of a turn) and eliminate this gap. You then adjust the shoulder set back by using the Competition Shellholders. You start with the +.010 and work back toward the +.002 shellholder until you achieve the shoulder set back you want or your bolt handle will close freely. You may still see some variation due to difference in spring back of each piece of brass.

I hope this helps.

Regards
Chris Fox
Customer Service

Redding Reloading Equipment
I find FL sizing to be more accurate than neck sizing using standard dies. The Redding Type-S FL bushing dies are even better.
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Old March 6, 2013, 10:17 AM   #11
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The Lee Collet neck sizing die is something to consider. If you are reloading for one rifle, then it is not clear to me why you need to bother with full-length resizing. Just something to consider. I reload 270W for one rifle and I only neck-size.
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Old March 6, 2013, 11:07 AM   #12
Bart B.
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totaldla, the reason folks getting best accuracy these days use full length sizing dies is threefold.....

First, case necks are better centered on case shoulder. Rimless bottleneck cases center in the chamber shoulder when fired. That better aligns the bullet with the bore.

Second, reducing a fired case's body diameter and setting its shoulder back .001" ensures there will be not binding of the bolt when it closes. Neck only sized cases soon start interfereing with the chamber and that offsets them in the chamber when fired causing abnormal barrel whipping and wiggling compared to when there's some clearance around them.

Third, while the size of the smallest groups shot are the same as what neck only sized cases produce once in a (great) while, the largest ones are smaller. All the fired groups range from 1 to 4 units of measure with full length sizing instead of 1 to 7 units using neck only sizing. This what the benchresters learned a few years ago. Other rifle competition disciplines have known that for decades.

Call Sierra Bullets and ask them what dies they use to reload cases used to shoot their best bullets into 1/4 MOA in their 200 yard test range.

Of course, any die type and use techniqes may well end up makeing good ammo for someone if they don't use the best ones correctly.
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Old March 6, 2013, 12:05 PM   #13
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totalda,

The reason for the sizing is to allow self-centering. It's the same reason the Redding Competition Bullet Seating Die has a floating bullet seating stem. The reason is to overcome imperfections, as Bart suggested.

The NECO and RCBS Case Master gages have extended anvils that let you measure case wall runout back inside the case. When you get to the thicker case body wall brass near the head, I find it typically has about twice the runout the thinner brass near the neck does. When case brass expands, it inflates more on the thin side than on the thick side, pushing the whole case off center in the chamber and tilting the neck in the chamber. If you haven't narrowed the case body during sizing, as when neck sizing only, you are inserting the case in the chamber with that tilt error already in place.

Because cases are thinnest at the front, they expand there first, so the bullet is released and underway before the worst of the unevenness back at the thicker part of the wall starts to expand and further tilt the case neck off alignment with the bore axis. So when you start with a narrowed case from full length resizing, you get about half the tilt misalignment that firing the unsized body starts you with.

Some chambers are more immune to bullet tilt than others, but there is also an asymmetry of bolt thrust that results, and Harold Vaughn showed that can introduce off-axis recoil moments that introduce muzzle deflection that opens groups up.

Like you, I like the Lee Collet Die for necks, and consider it a better mouse trap than other neck dies. This video shows why. But in light of the wall thickness issues and unless I know I have a dead perfect chamber and perfectly square bolt face, I follow the Collet Die with the application of a Redding body die to narrow the case and set the shoulder where I want it.

One thing you can do to mitigate the error magnitude, if you like to neck size only, is to use one of those gages I mentioned to identify the thin side of each of your cases. You then file a small notch in the case rim at the thin or thick point (your choice; just be consistent about which it is). Then when you shoot, always place the cartridge in the chamber with that notch at the same point on the clock. This will cut the error introduced on the target in half as compared to randomly orienting the cases in the chamber.


Deepforks,

When folks talk about setting a shoulder back one or two thousandths, that's starting at the size the case returned to elastically in the chamber. The problem is that how much that size differs from the size of your chamber differs with the actual firing pressure and with how much the steel in the gun stretched. When you load so hot you get sticky bolt lift, you have actually stretched the steel so much from bolt face to shoulder that it has, being more elastic than brass, allowed the brass elastic limit to be exceeded past the chamber's no-pressure length and then the steel snaps back over top of the brass to slightly size the shoulder back. If you fire a very mild load, by contrast, it can come out of the chamber already a couple thousandths shorter than the chamber.

When you resize, the case body is narrowed, which pushes the shoulder forward. You then set the case shoulder back with the die shoulder. How much is best to set the shoulder back should not actually be a fixed value, but one that depends on how much elastic rebound your fired cases actually had. If you have a case coming out of the chamber a couple thousandths shorter than the chamber, you may well get best precision on target just by setting the shoulder back, after narrowing, to the length it emerged from the chamber with initially.

How to tell? Well, I've fiddled with this on and off. One experiment was with averaging brass growth as load workup increased pressure, looking for convergence on a chamber length value. But while that would be ideal because you are doing a load workup anyway, it takes some math and curve fitting to work out and it varies with the brass lot and requires a fair amount of averaging to get past shot-to-shot variation to establish.

By far the easier thing to do is to fire a few rounds with your established accuracy load you intend to use. Measure the shoulder data to head lengths of several of the cases and use Mr. Guffey's trick of employing the cases themselves as gages. Strip your bolt and use shim stock between the case head and the bolt face until you find the shim thickness that causes the case to just kiss the end of the chamber as the bolt completes closing. Cases are not hard like a steel headspace gage, but with a light touch and no ejector pressure on the case, you can figure the chamber length out pretty well, then subtract a couple of thousandths from that to determine how much shoulder setback you really need.
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