Neck Run out

S

Snipe

New member
Good Morning,

I have started to turn my necks with 21st century lathe. I am using new Peterson 6.5 cr srp brass. After turning the brass I have .014 all the way around which is perfect, but I have .004 run out on the neck using 21sr century run out gauge. Why? Is this because i have not fired them yet? Any ideas?

Snipe
 
I ran them thru a standard redding die, trimmed, debur, turned then thru a redding s bushing die and im still getting run out. I confused the only thing i can think of is they need to be fired then resized and checked again. now when i use my Hornady run out gauge it measures .001 or so but that gauges uses the head of the case not the body.
 
That's possible with new cases. Full length resizing them should straighten them up some amount.

I think people making runout gauges for cartridges headspacing on their shoulder are ignorant of the fact that the case body between the shoulder and pressure ring next to the extractor groove doesn't touch the chamber wall when fired. The case shoulder is centered in the chamber shoulder. Any out of round the case body near the shoulder has doesn't matter. The case pressure ring is usually pressed against the chamber wall by the extractor claw. Therefore, all rounds are a tiny bit crooked in the chamber before firing. And very repeatable from shot to shot.
 
Yes i had .002 run out on the brass before i turned them. im fire forming them today and rechecking. Its the body forsure i rechecked a finished round on my Hornady gauge which uses the head to hold the round and it measures .001
 
Bart B. said:
That's possible with new cases. Full length resizing them should straighten them up some amount.

I think people making runout gauges for cartridges headspacing on their shoulder are ignorant of the fact that the case body between the shoulder and pressure ring next to the extractor groove doesn't touch the chamber wall when fired. The case shoulder is centered in the chamber shoulder. Any out of round the case body near the shoulder has doesn't matter. The case pressure ring is usually pressed against the chamber wall by the extractor claw. Therefore, all rounds are a tiny bit crooked in the chamber before firing. And very repeatable from shot to shot.
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Extractor claw ? You don't even know what firearm he is shooting them in . Also , not all rifles have an extractor claw that contacts the brass while in battery . That myth needs to be gone.

Also, a properly fire formed case sure as heck can be centered in a chamber with a slight crush fit . Neck sizing dies are a wonderful thing .
 
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another problem that sounds like a snipe hunt :D jking of course

the way the Sinclair operates is the neck slides onto a mandrel which is inserted into the cutter holder. The case needs to be FL sized to the correct ID and the necks trimmed before turning, Videos are available on Youtube and the Sinclair website
 
Proper full length resizing of bottleneck cases has proved best for accuracy since the 1960's. And been my observations. Best accuracy is the size of the largest groups fired as they happen when all the variables add up. Smallest groups typically happen when the variables cancel each other out.

New cases usually produce better accuracy than neck only resizing.

All with the bolt closing freely without any binding or crush fit. Such cases are centered in the chamber shoulder when the firing pin impact drives them there then fires the primer.
 
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For neck turning to be done right the distance from the shoulder to the case mouth has to be consistent so full length sizing and a trim is necessary before turning. Otherwise doughnuts will occur due to either not cutting into the shoulder slightly or neck separation because of cutting too far into the shoulder.

Getting it right is not really hard, you just have to be patient and sneak up on the cut when doing the case stop adjustment. Once you have the stop set it is mind numbingly easy

OP's issue is really confusing, the way these things operate the case would have to mean the neck was off kilter before he ever turned it since the cutter cannot cut a angle in relation to case neck due to how they operate

http://bulletin.accurateshooter.com/tag/neck-turner/

this shows a K&M but they all work the same. The neck slides over the mandrel and the cutter takes .005 to .001 off until the case mouth hits the stop. Take your time adjusting the stop and if the neck ID is sized correctly and the cases are trimmed correctly it is pretty much a idiot proof operation
 
Reloading dies work off the outside surface of the brass. The outside surface geometry establishes one axis.

Neck turning is based on the mandrel fit to the inside diameter of the case neck. That diameter ,or cylinder,has its own axis. That axis may or may not be coaxial to the outside of the case,including the shoulder taper Bart refers to.

If the brass,as drawn,has non-uniform neck wall thickness,the case OD,which is what the die and the chamber reference,will not be co-axial to the neck ID,which is what the neck turn mandrel references.And the bullet.

So long as the neck thickness is not uniform,a resizing operation will only deflect the expander spindle . The OD and ID will not be coaxial.

Turning the neck will make the OD of the neck coaxial to the ID of the neck.

The OD of the neck is no longer coaxial to the outside geometry of the case.

If the brass is sized again after neck turning,this might be corrected.

But if you resize virgin brass,neck turn, then charge and seat bullets, the neck and bullet may present off center to the bore as the bolt is closed into battery.(Assuming there WAS unequal neck thickness,corrected by turning)

I can't say for sure how that will go as pressure builds,the bullet enters the bore,and the case fireforms.I assume the fireform will correct the problem.But the bullet may be offcenter as it enters he throat.
Without a resizing operation after neck turn,the first firing of neck turned ammo might see some variability.

I don't neck turn. I'm just thinking it over.
 
Neck turning is based on the mandrel fit to the inside diameter of the case neck. That diameter ,or cylinder,has its own axis. That axis may or may not be coaxial to the outside of the case,including the shoulder taper Bart refers to.
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which is why you need to use a sizing mandrel before neck turning. That pushes any imperfections to the outside of the neck and ensures the inside of the neck is perpendicular to the case head. The case rides on the mandrel and all is right with the world

1. FL size the case so the ID is .002 or .003 smaller than the mandrel
2. Trim cases so necks all the same length
3. use the sizing mandrel to bring the ID to the diameter of the neck turning mandrel or .0005 smaller
4. lube the turning mandrel with Imperial or Kiwi mink oil
5. adjust stop so cutter just barely cuts into shoulder as shown in the link
6. turn cases taking off .0005 to .001 each pass until you are satisfied

it is about as simple of a operation as it gets if you follow the directions included with the tool. No need to overcomplicate. If I could figure it out, anyone should be able to. I ain't no rocket surgeon


http://www.xxicsi.com/arbors-and-mandrels.html
 
After uniforming neck thickness, I've expanded them then full length sized the cases so the necks are centered on the case shoulder. Verified by the case mouth is centered in the case gauge neck when case shoulder is centered in the gauge shoulder.
 
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