Please post your thoughts, opinion, beliefs, observations on how a rimless bottleneck round positions itself in the chamber with the bolt closed then what happens when the firing pin strikes it. For each part of the round (head, primer, body, shoulder, neck and bullet), state what differences there are in what happens between a neck only and full length sized case.
Neck Only vs. Full Length Sized Cases: Differences When Chambered then Fired?
- Thread starter Bart B.
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Bart,
I'm not sure what you're looking for in terms of info.
I see it this way:
You full length size when needed. For example when brass won't chamber unless you FL size.
You neck size only when brass will chamber, but neck is too big to hold a bullet. Once fired in a specific rifle I will neck size if it is adequate.
I full length resize when making ammo to fit any rifle (SAAMI spec). AR15 (semi auto carbine) is also something I FL size for & use a case gauge. Range pickup I FL size.
I'm not sure what you're looking for in terms of info.
I see it this way:
You full length size when needed. For example when brass won't chamber unless you FL size.
You neck size only when brass will chamber, but neck is too big to hold a bullet. Once fired in a specific rifle I will neck size if it is adequate.
I full length resize when making ammo to fit any rifle (SAAMI spec). AR15 (semi auto carbine) is also something I FL size for & use a case gauge. Range pickup I FL size.
WNTFW, I'm curious as to what people think is how a loaded round is positioned in the chamber; both with just the bolt closed and then when the firing pin strikes its primer to fire the round.
Two examples:
Where's the case head relative to the bolt face?
Where's the case relative to the chamber around it?
Here's some more help regarding my original post.....
Once chambered and the bolt closed, does each type of fired, then resized case rest in the same position?
Is that position such that gravity pulls it down so the case body, shoulder and neck are against the chamber's bottom at those same points with its head against the bolt face?
Are there any dimensional changes to the chambered case before it fires (dimpling of the primer excluded)?
Two examples:
Where's the case head relative to the bolt face?
Where's the case relative to the chamber around it?
Here's some more help regarding my original post.....
Once chambered and the bolt closed, does each type of fired, then resized case rest in the same position?
Is that position such that gravity pulls it down so the case body, shoulder and neck are against the chamber's bottom at those same points with its head against the bolt face?
Are there any dimensional changes to the chambered case before it fires (dimpling of the primer excluded)?
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243winxb
Member
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If you neck size only for a standard chamber with standard ammo (no reaming/neck turning etc), you should consider partial neck sizing (90%) to help align the bullet with the bore.
As 243winxb posted, this is easily done with bushing type dies.
The full potential of bushing type dies will not be realized with out other factors though.
Seating the bullet "into the lands" will help as well, but not feasible in many guns due to long throats and short mags.
As 243winxb posted, this is easily done with bushing type dies.
The full potential of bushing type dies will not be realized with out other factors though.
Seating the bullet "into the lands" will help as well, but not feasible in many guns due to long throats and short mags.
243winxb
Member
Deleted ,
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jfdavis58
Member
Very interesting question! Begs one in reply: do you know the answers?
What you're really asking about is cases that headspace from shoulder to head; no rims or belts. And the answer depends entirely on how you set this headspace dimension for any particular cartridge in this sub-group. Two situations are easy: full length resizing including some shoulder set-back and neck sizing only using one of those specialty dies which works on only the top 40-60% of the neck.
Full length resizing with a shoulder bump places all case dimensions less than chamber dimension so the cartridge sits offset toward the bottom of the chamber. The amount of shoulder bump determines the severity of the out of center angle and hence the run-out of concentricity---this, I would assume would be most like commercial ammunition.
Neck sizing with the fancy die gets you a case that exactly fits the chamber and if other concentricity preserving steps are also taken (neck measurements, bullet sorting, neck turning, etc) gets a nearly perfect alignment of bullet to bore. In this case some of the neck may be undersized to the neck area of the chamber.
One would assume that when fired-at the precise moment the bullet completely exits the case, that in all situations the case exactly fits the chamber dimensions--even if it ruptures doing so.
The interesting scenario is the full length resizing that soft bumps the shoulder or doesn't bump it at all. Here the shoulder partially or completely centers the case neck in the chamber neck. If the neck is well centered with respect to the bore, there should be a correllation between shoulder bump and accuracy.
Now let me take a crack at your questions.
Where's the case head relative to the bolt face?
One would hope that it is in contact or very close <0.005 inches give or take.
Where's the case relative to the chamber around it?
Depends on the shoulder to shoulder position. range is from mostly out of contact and centered in the bottom of the chamber to completely in contact except portions of the neck (possible), completely centered.
Here's some more help regarding my original post.....
Once chambered and the bolt closed, does each type of fired, then resized case rest in the same position?
The concept of fire forming rests entirely on thought that at firing the case dimensions match the chamber dimensions.
Is that position such that gravity pulls it down so the case body, shoulder and neck are against the chamber's bottom at those same points with its head against the bolt face?
Given enough space for movement and a stationary position for sufficiant time, gravity is inescapable.
Are there any dimensional changes to the chambered case before it fires (dimpling of the primer excluded)?
Possibly, there is a school of thought that says resize with no shoulder bump, let the case 'grow'. There is some slop in the breech face to bore/chamber face that naturally exists. Eventually, this case will be longer than the chamber (to shoulder) dimension and during chambering some force will be required to jam the cartridge into place and close the breech. The chamber will then resize the shoulder dimension.
So, How did I do? (BTW-I reasoned and entered this before reading the Brownells article-doing that now. Guess I could grade myself.)
What you're really asking about is cases that headspace from shoulder to head; no rims or belts. And the answer depends entirely on how you set this headspace dimension for any particular cartridge in this sub-group. Two situations are easy: full length resizing including some shoulder set-back and neck sizing only using one of those specialty dies which works on only the top 40-60% of the neck.
Full length resizing with a shoulder bump places all case dimensions less than chamber dimension so the cartridge sits offset toward the bottom of the chamber. The amount of shoulder bump determines the severity of the out of center angle and hence the run-out of concentricity---this, I would assume would be most like commercial ammunition.
Neck sizing with the fancy die gets you a case that exactly fits the chamber and if other concentricity preserving steps are also taken (neck measurements, bullet sorting, neck turning, etc) gets a nearly perfect alignment of bullet to bore. In this case some of the neck may be undersized to the neck area of the chamber.
One would assume that when fired-at the precise moment the bullet completely exits the case, that in all situations the case exactly fits the chamber dimensions--even if it ruptures doing so.
The interesting scenario is the full length resizing that soft bumps the shoulder or doesn't bump it at all. Here the shoulder partially or completely centers the case neck in the chamber neck. If the neck is well centered with respect to the bore, there should be a correllation between shoulder bump and accuracy.
Now let me take a crack at your questions.
Where's the case head relative to the bolt face?
One would hope that it is in contact or very close <0.005 inches give or take.
Where's the case relative to the chamber around it?
Depends on the shoulder to shoulder position. range is from mostly out of contact and centered in the bottom of the chamber to completely in contact except portions of the neck (possible), completely centered.
Here's some more help regarding my original post.....
Once chambered and the bolt closed, does each type of fired, then resized case rest in the same position?
The concept of fire forming rests entirely on thought that at firing the case dimensions match the chamber dimensions.
Is that position such that gravity pulls it down so the case body, shoulder and neck are against the chamber's bottom at those same points with its head against the bolt face?
Given enough space for movement and a stationary position for sufficiant time, gravity is inescapable.
Are there any dimensional changes to the chambered case before it fires (dimpling of the primer excluded)?
Possibly, there is a school of thought that says resize with no shoulder bump, let the case 'grow'. There is some slop in the breech face to bore/chamber face that naturally exists. Eventually, this case will be longer than the chamber (to shoulder) dimension and during chambering some force will be required to jam the cartridge into place and close the breech. The chamber will then resize the shoulder dimension.
So, How did I do? (BTW-I reasoned and entered this before reading the Brownells article-doing that now. Guess I could grade myself.)
USSR
Member
Would Bart B. be Bart Bobbitt?
Don
Don
It is like waking up in a new world every morning, PSI is equal in all directions, the extractor has more influence on the static lay of the ammo in the chamber than gravity, partial neck sizing is the best of all worlds, the unsized portion of the the neck will center the case in the chamber, adjusting the die, press and shell holder with a feeler gage would make all issues 'non-issues'.
Then there was the man that asked the questions "when I check the target am I looking at 'groups' or 'patterns'? His thinking? If he did everything correctly, he should have one hole groups and if there was a variation he should get a pattern of one hole groups.
F. Guffey
Then there was the man that asked the questions "when I check the target am I looking at 'groups' or 'patterns'? His thinking? If he did everything correctly, he should have one hole groups and if there was a variation he should get a pattern of one hole groups.
F. Guffey
I'll cover what happens when the round's chambered. The following is based on my observations, tests and measurements (OTM's). If your OTM's differ from mine, please post and explain them.
Oh, and yes, USSR, that's me.
But first, note that there's no such thing as a perfectly round chamber or fired case. They are all out of round by a thousandth or so, cases more so than chambers. If you have good tools to measure out of round (dial indicators on V blocks for cases and star gages for chambers, not outside micrometers nor hole mics) you can see what they really are. So there's no such thing as a "perfect fit" between case and chamber.
And second, fired cases from bolt action rifles are always smaller in outside dimensions than the chamber they were fired in. Not much smaller, but a thousandth or two is about all for max, safe loads. I've measured proof loads (blue pills) and they're smaller, too, but not as much.
Once the bolt's closed on a rimless bottle neck round, the bolt's extractor pushes the back of the case off center opposite the extractor's place on the bolt head. The back of the case doesn't rest on the chamber bottom but instead gets pushed against the chamber wall at the case pressure ring's location. Mauser style claw extractors push the case straight to the side. Sliding ones in the bolt face, such as the post-'64 Model 70's, push the case up. You can easily see where your bolt's extractor pushes the back of a chambered round.
Spring loaded plunger ejectors in bolt faces push the round forward until its shoulder centers in the chamber shoulder. So the front of the case body is typically clear of the bottom of the chamber. Centering the case shoulder in the chamber shoulder also centers the case neck in the chamber neck with a couple thousandths clearance around it. Even partial neck sized cases have clearance; go measure the chamber neck and case neck diameters and you'll see how much there is. If the action has a Mauser style ejector external from the bolt, it won't touch the case at all and the round could be anywhere between the bolt face and chamber shoulder, but it's back end is still pressed off center by the extractor and contacts the chamber wall opposite the extractor.
Depending on where the egg-shaped case fits the egg-shaped chamber, the clearance around the case will vary. Full-length sized cases will have a few thousandths more clearance than neck-only sized ones. If the bolt face has been squared with the chamber axis, it will have about the same clearance all around the fairly square case head; if not squared, the clearance will vary depending on how the round's oriented in the chamber and how much out of square the case head and bolt face is..
So here the chambered round fits; back end's against the chamber wall at the case pressure ring. The front's centered in the chamber at the shoulder if pushed there by the in-line ejector or just sitting there a bit below center if an external ejector's used.
Now the trigger sear releases the firing pin and it headed for the primer. It'll travel a fourth to a third of an inch in a few thousandths of a second and strike the primer with 25 to 30 pounds of force. Rimless bottle neck round such as the .30-06 weigh about 1 ounce.
My next post will explain what happens when the firing pin smacks the primer. If you well understand what's above, you can probably figure this out yourselves.
Oh, and yes, USSR, that's me.
But first, note that there's no such thing as a perfectly round chamber or fired case. They are all out of round by a thousandth or so, cases more so than chambers. If you have good tools to measure out of round (dial indicators on V blocks for cases and star gages for chambers, not outside micrometers nor hole mics) you can see what they really are. So there's no such thing as a "perfect fit" between case and chamber.
And second, fired cases from bolt action rifles are always smaller in outside dimensions than the chamber they were fired in. Not much smaller, but a thousandth or two is about all for max, safe loads. I've measured proof loads (blue pills) and they're smaller, too, but not as much.
Once the bolt's closed on a rimless bottle neck round, the bolt's extractor pushes the back of the case off center opposite the extractor's place on the bolt head. The back of the case doesn't rest on the chamber bottom but instead gets pushed against the chamber wall at the case pressure ring's location. Mauser style claw extractors push the case straight to the side. Sliding ones in the bolt face, such as the post-'64 Model 70's, push the case up. You can easily see where your bolt's extractor pushes the back of a chambered round.
Spring loaded plunger ejectors in bolt faces push the round forward until its shoulder centers in the chamber shoulder. So the front of the case body is typically clear of the bottom of the chamber. Centering the case shoulder in the chamber shoulder also centers the case neck in the chamber neck with a couple thousandths clearance around it. Even partial neck sized cases have clearance; go measure the chamber neck and case neck diameters and you'll see how much there is. If the action has a Mauser style ejector external from the bolt, it won't touch the case at all and the round could be anywhere between the bolt face and chamber shoulder, but it's back end is still pressed off center by the extractor and contacts the chamber wall opposite the extractor.
Depending on where the egg-shaped case fits the egg-shaped chamber, the clearance around the case will vary. Full-length sized cases will have a few thousandths more clearance than neck-only sized ones. If the bolt face has been squared with the chamber axis, it will have about the same clearance all around the fairly square case head; if not squared, the clearance will vary depending on how the round's oriented in the chamber and how much out of square the case head and bolt face is..
So here the chambered round fits; back end's against the chamber wall at the case pressure ring. The front's centered in the chamber at the shoulder if pushed there by the in-line ejector or just sitting there a bit below center if an external ejector's used.
Now the trigger sear releases the firing pin and it headed for the primer. It'll travel a fourth to a third of an inch in a few thousandths of a second and strike the primer with 25 to 30 pounds of force. Rimless bottle neck round such as the .30-06 weigh about 1 ounce.
My next post will explain what happens when the firing pin smacks the primer. If you well understand what's above, you can probably figure this out yourselves.
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USSR
Member
For those of you who don't know, Mr. Bobbitt has written extensively over the years about the care and feeding of high power rifles. His knowledge is extensive, and it is backed up by research, competitive shooting and hands-on experimentation.
Don
Don
Kelbro, yes, sort of, one better than the other, control feed (It's all in the claw) the bolt face is open on the bottom when moving forward then rotated clock wise to lock, then the open portion of the bolt is to the left opposite the extractor, again, I do not want to talk it to death, the extractor has more influence on the case, static position, than any other factor, I needed a few hundred cases but had wait for someone to separate all the 'good ones' from the ones that had dents on the left side, he would put them down, I would pick them up, he wanted to know why I picked the cases with dents, "Cheaper, with dents they are cheaper".
Influence of the extractor? if there was no influence (could be perceived) the go-gage checkers would not instruct others to remove it (extractor) when checking head space with a go-gage, and some extractors have more influence than others. some are designed to 'allow' others do not 'allow ?'.
Check influence of the extractor: Turn your Mauser upside down, open and close the bolt to allow a round to be feed into the chamber, How does it do that?
F. Guffey
Influence of the extractor? if there was no influence (could be perceived) the go-gage checkers would not instruct others to remove it (extractor) when checking head space with a go-gage, and some extractors have more influence than others. some are designed to 'allow' others do not 'allow ?'.
Check influence of the extractor: Turn your Mauser upside down, open and close the bolt to allow a round to be feed into the chamber, How does it do that?
F. Guffey
kelbro asks:
Partial neck sizing doesn't support one end, Fired case necks aren't as big as the chamber's neck. They spring back a few thousandths. Measure your chamber neck diameter then compare it to a fired case neck's diameter. Typically 2 to 4 thousandths smaller.
As the bolt face recess is several thouandths larger than the case rim; it won't center the case.
Besides, there's several thousandths slop in positioning the bolt head in the receiver ring. If case headspace is any longer than chamber headspace (common when using a full length sizing die to partially size a fired case neck), the bolt will close harder than it does on an empty chamber. This happens as the case slightly binds between chamber shoulder and bolt face. If the bolt face ain't squared with the chamber axis, then it's high point can contact the case head anywhere and if it's the high point on the case, more binding will occur. All of this can change the normal position of the bolt head in the receiver and this can cause accuracy problems. The bolt head should close and stop at the same position relative to the case and receiver ring every time. Best example of this is with an M1 or M14 rifle; after single loading a round for long range matches then tripping the op rod to close and seat the bolt, if you hit the op rod handle with your hand (often done by folks who don't trust their rifle's bolt closing and locking abilities), that round will shoot away from where the others have gone.
Partial neck sizing doesn't support one end, Fired case necks aren't as big as the chamber's neck. They spring back a few thousandths. Measure your chamber neck diameter then compare it to a fired case neck's diameter. Typically 2 to 4 thousandths smaller.
As the bolt face recess is several thouandths larger than the case rim; it won't center the case.
Besides, there's several thousandths slop in positioning the bolt head in the receiver ring. If case headspace is any longer than chamber headspace (common when using a full length sizing die to partially size a fired case neck), the bolt will close harder than it does on an empty chamber. This happens as the case slightly binds between chamber shoulder and bolt face. If the bolt face ain't squared with the chamber axis, then it's high point can contact the case head anywhere and if it's the high point on the case, more binding will occur. All of this can change the normal position of the bolt head in the receiver and this can cause accuracy problems. The bolt head should close and stop at the same position relative to the case and receiver ring every time. Best example of this is with an M1 or M14 rifle; after single loading a round for long range matches then tripping the op rod to close and seat the bolt, if you hit the op rod handle with your hand (often done by folks who don't trust their rifle's bolt closing and locking abilities), that round will shoot away from where the others have gone.
Quoting USSR,
I figured Bart B. was a total newb thinking too much before getting hands on (like I do) or some sort of expert who was asking a question he knew that answer to (like I'm NOT!) . . .
Bart B.
Looking for the next installment!
Thanks for sharing your knowledge,
WNTFW
I figured Bart B. was a total newb thinking too much before getting hands on (like I do) or some sort of expert who was asking a question he knew that answer to (like I'm NOT!) . . .
Bart B.
Looking for the next installment!
Thanks for sharing your knowledge,
WNTFW
Chapter 2....
Along comes the firing pin driven hard against the primer with 20 to 30 pounds of near instant force. If the loaded rimless bottleneck round’s shoulder isn’t already seated fully in the chamber shoulder, the firing pin’s force will certainly do that. There’s enough clearance on the extractor to allow this to happen. If everything from the shoulder forward is very straight, this also centers the case neck in the chamber neck with the bullet in that centered neck well centered on the bore axis with a couple thousandths clearance between the case neck and chamber neck. Even if there’s a slight difference between case shoulder angle and that of the chamber shoulder; both are cone-shaped and centering is assured because the chamber shoulder is larger in diameter. This much energy striking the back end of the cartridge may well move it off contact with the chamber wall where the extractor’s pushing it. Or, it may make the case contact the chamber wall harder. I suspect it depends on where the bolt head’s positioned in the receiver ring and its reaction to extractor forces pressing the case off center.
Firing pin force sets the case shoulder back a few thousandths of an inch as it dimples the primer cup crushing the priming compound detonating it like a hammer on a match head. And the neck gets a tiny bit longer, too. Now the case head is further away from the bolt face by the same amount the shoulder was set back. This is the only time the loaded round is in the most critical position for firing. Any variable in how the bullet’s positioned at this time effects accuracy. It doesn’t matter how much clearance there will always be around the case neck or body relative to the chamber; as long as it’s always about the same amount, that’s all that’s needed. The case head will be a few thousandths off the bolt face. There will be some small, very small angle between the chamber axis and the cartridge axis because of how much the extractor pushes the back of the case off center. But it’s very repeatable from shot to shot so it’s not important. Both neck-only and full-length sized cases will be, for all practical purposes, fit the chamber the same way. The exception is full-length sized ones will have their back end a tiny bit further off center and the neck-only sized ones may not have their case necks holding the bullet as perfectly aligned with the chamber. If bullets are seated out enough to set back a few thousandths when pushed into the lands when chambered, this will help a tiny bit with their alignment to bore center providing the case necks are well aligned and centered with the shoulder.
When the primer finally makes enough fire, the powder starts burning, building pressure, and expanding the case against the chamber walls. First the neck and shoulder expand as they’ve got the thinnest walls, then the front of the case body and while pushing the case head back stretching the body along with it and pulling some of the case neck out of the chamber neck. The case head finally stops against the bolt face with considerable pressure in PSI which makes the case head about the same angle of the bolt face. Meanwhile, the body stops expanding against the chamber body walls with the same pressure. As both the case and chamber are not perfectly round, each round fired will have its case body first start pressing against the chamber walls at different places for each shot. Pressure starts to drop after the bullet’s about a third of the way down the barrel and goes to zero quickly after its left the muzzle. As cartridge brass has elasticity, it now shrinks the case away from the chamber walls a couple thousandths and there it sits until the bolt’s opened. It probably wasn’t oriented the same as it was on its first firing and its out of round points may well be at different places than after its first firing.
(Note: This case head space shortening can be seen by firing a 10 to 15 percent reduced load then noting how far the primer’s backed out stopping against the bolt face or using a case head space gage before and after the firing. There wasn’t enough pressure to push the back half of the case against the bolt face. But enough pressure existed to push the primer part way out of its pocket.)
From these situations, it should be easy to see than the only significant physical difference between neck-only and full-length sized cases when they’re fired is back at their pressure ring. Neck-only sized cases will have a body diameter a couple thousandths of an inch larger than a full-length sized one. Either one has their case shoulders’ centered just at the chamber shoulder when fired. Their back ends will be pushed off center by the extractor about half their difference in diameters at the pressure ring. Full-length sized case with its smaller diameter will be off center by only a thousandth of an inch; maybe two at the most. At the bullet’s tip, it’ll be tilted off center by about one-third that amount at the most; not enough to make a difference which doesn’t matter much any because whatever angle it is at will typically repeated for each shot.
I’ve noticed one interesting difference between neck-only and full-length sized cases in one very important area. Remember how the case shoulder centers the front of the round in the chamber, especially the neck holding the bullet? Measuring sized cases for neck runout showed me that full-length sized cases can have straighter necks than neck-only sized ones. It’s best measured and seen if you support the case only at its pressure ring and mid point on the shoulder with the dial indicator 1/16th inch back from the case mouth. Which makes sense to me as neck-only sizes the neck on a case whose body is not supported by anything to keep it in alignment with the neck during the sizing operation.
Lapping a full-length die’s neck out so it’s 2 to 3 thousandths smaller than a loaded round’s neck diameter eliminates the use of an expander ball (IMHO, more correctly called a neck-bending ball). Decap your fired cases in a separate die then clean them before they’re lubed and sized. Setting the fired case shoulder back only a couple thousandths with the case body held tight in alignment with its neck tends to keep the neck straighter and better oriented with the shoulder. I’m convinced that when the full-length sized round fires with its shoulder hard and perfectly seated against the chamber shoulder, its straighter neck lets the bullet enter the rifling with less distortion therefore shooting more accurate. I’m also convinced this is why fired cases properly full-length sized in dies with lapped out necks typically shoot more accurate than neck-only or partial-neck sized fired cases.
Along comes the firing pin driven hard against the primer with 20 to 30 pounds of near instant force. If the loaded rimless bottleneck round’s shoulder isn’t already seated fully in the chamber shoulder, the firing pin’s force will certainly do that. There’s enough clearance on the extractor to allow this to happen. If everything from the shoulder forward is very straight, this also centers the case neck in the chamber neck with the bullet in that centered neck well centered on the bore axis with a couple thousandths clearance between the case neck and chamber neck. Even if there’s a slight difference between case shoulder angle and that of the chamber shoulder; both are cone-shaped and centering is assured because the chamber shoulder is larger in diameter. This much energy striking the back end of the cartridge may well move it off contact with the chamber wall where the extractor’s pushing it. Or, it may make the case contact the chamber wall harder. I suspect it depends on where the bolt head’s positioned in the receiver ring and its reaction to extractor forces pressing the case off center.
Firing pin force sets the case shoulder back a few thousandths of an inch as it dimples the primer cup crushing the priming compound detonating it like a hammer on a match head. And the neck gets a tiny bit longer, too. Now the case head is further away from the bolt face by the same amount the shoulder was set back. This is the only time the loaded round is in the most critical position for firing. Any variable in how the bullet’s positioned at this time effects accuracy. It doesn’t matter how much clearance there will always be around the case neck or body relative to the chamber; as long as it’s always about the same amount, that’s all that’s needed. The case head will be a few thousandths off the bolt face. There will be some small, very small angle between the chamber axis and the cartridge axis because of how much the extractor pushes the back of the case off center. But it’s very repeatable from shot to shot so it’s not important. Both neck-only and full-length sized cases will be, for all practical purposes, fit the chamber the same way. The exception is full-length sized ones will have their back end a tiny bit further off center and the neck-only sized ones may not have their case necks holding the bullet as perfectly aligned with the chamber. If bullets are seated out enough to set back a few thousandths when pushed into the lands when chambered, this will help a tiny bit with their alignment to bore center providing the case necks are well aligned and centered with the shoulder.
When the primer finally makes enough fire, the powder starts burning, building pressure, and expanding the case against the chamber walls. First the neck and shoulder expand as they’ve got the thinnest walls, then the front of the case body and while pushing the case head back stretching the body along with it and pulling some of the case neck out of the chamber neck. The case head finally stops against the bolt face with considerable pressure in PSI which makes the case head about the same angle of the bolt face. Meanwhile, the body stops expanding against the chamber body walls with the same pressure. As both the case and chamber are not perfectly round, each round fired will have its case body first start pressing against the chamber walls at different places for each shot. Pressure starts to drop after the bullet’s about a third of the way down the barrel and goes to zero quickly after its left the muzzle. As cartridge brass has elasticity, it now shrinks the case away from the chamber walls a couple thousandths and there it sits until the bolt’s opened. It probably wasn’t oriented the same as it was on its first firing and its out of round points may well be at different places than after its first firing.
(Note: This case head space shortening can be seen by firing a 10 to 15 percent reduced load then noting how far the primer’s backed out stopping against the bolt face or using a case head space gage before and after the firing. There wasn’t enough pressure to push the back half of the case against the bolt face. But enough pressure existed to push the primer part way out of its pocket.)
From these situations, it should be easy to see than the only significant physical difference between neck-only and full-length sized cases when they’re fired is back at their pressure ring. Neck-only sized cases will have a body diameter a couple thousandths of an inch larger than a full-length sized one. Either one has their case shoulders’ centered just at the chamber shoulder when fired. Their back ends will be pushed off center by the extractor about half their difference in diameters at the pressure ring. Full-length sized case with its smaller diameter will be off center by only a thousandth of an inch; maybe two at the most. At the bullet’s tip, it’ll be tilted off center by about one-third that amount at the most; not enough to make a difference which doesn’t matter much any because whatever angle it is at will typically repeated for each shot.
I’ve noticed one interesting difference between neck-only and full-length sized cases in one very important area. Remember how the case shoulder centers the front of the round in the chamber, especially the neck holding the bullet? Measuring sized cases for neck runout showed me that full-length sized cases can have straighter necks than neck-only sized ones. It’s best measured and seen if you support the case only at its pressure ring and mid point on the shoulder with the dial indicator 1/16th inch back from the case mouth. Which makes sense to me as neck-only sizes the neck on a case whose body is not supported by anything to keep it in alignment with the neck during the sizing operation.
Lapping a full-length die’s neck out so it’s 2 to 3 thousandths smaller than a loaded round’s neck diameter eliminates the use of an expander ball (IMHO, more correctly called a neck-bending ball). Decap your fired cases in a separate die then clean them before they’re lubed and sized. Setting the fired case shoulder back only a couple thousandths with the case body held tight in alignment with its neck tends to keep the neck straighter and better oriented with the shoulder. I’m convinced that when the full-length sized round fires with its shoulder hard and perfectly seated against the chamber shoulder, its straighter neck lets the bullet enter the rifling with less distortion therefore shooting more accurate. I’m also convinced this is why fired cases properly full-length sized in dies with lapped out necks typically shoot more accurate than neck-only or partial-neck sized fired cases.
Bart B. , I'm really enjoying your posts and certainly appreciate your expertise. I'm sure we're all going to learn a great deal from you and I look forward to numerous interesting and educational discussions. I have some comments though.
1. You say that rifle chambers are not "perfectly" round and that they're typically off by a 0.001" or so. I would think that the concentricity of the bore and chamber are equally if not more important so what are your thoughts on that?
2. What about full-length resizing dies? What sort of errors show up in them in terms of roundness, concentricity of the body and neck etc. I would suspect that it's more than the errors found in high quality barrels such as those made by Krieger.
3. Is neck-sizing only (with a bushing) followed by body sizing only going to produce comparable results to full-length sizing with a "lapped" sizing die?
ANSWERED HERE: http://www.thehighroad.org/showpost.php?p=5511714&postcount=21
4. I'm getting excellent results using Redding neck-sizing dies on cases fired in my Krieger barrels. As I mentioned in another thread, neck runout is only +/- 0.0005".
5. Can you elaborate on how to accurately lap the neck of a full-length sizing die?
ANSWERED HERE: http://www.thehighroad.org/showpost.php?p=5511714&postcount=21
1. You say that rifle chambers are not "perfectly" round and that they're typically off by a 0.001" or so. I would think that the concentricity of the bore and chamber are equally if not more important so what are your thoughts on that?
2. What about full-length resizing dies? What sort of errors show up in them in terms of roundness, concentricity of the body and neck etc. I would suspect that it's more than the errors found in high quality barrels such as those made by Krieger.
3. Is neck-sizing only (with a bushing) followed by body sizing only going to produce comparable results to full-length sizing with a "lapped" sizing die?
ANSWERED HERE: http://www.thehighroad.org/showpost.php?p=5511714&postcount=21
4. I'm getting excellent results using Redding neck-sizing dies on cases fired in my Krieger barrels. As I mentioned in another thread, neck runout is only +/- 0.0005".
5. Can you elaborate on how to accurately lap the neck of a full-length sizing die?
ANSWERED HERE: http://www.thehighroad.org/showpost.php?p=5511714&postcount=21
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1858 asks:
There's nothing in the chamber behind its shoulder that's critical to accuracy. They have nothing to do with getting the bullet straight into the bore without distorting it. The chamber just keeps the case from blowing apart when the powder in it burns. But the bolt face that's behind the chamber has to be square with the chamber axis else case heads will never be square and that'll ruin best accuracy.
Yes, their out of round errors are more than the best bores have. They're the same as dies as far as what's critical. All they do is shape brass cases that are not even in wall thickness all the way around, so they don't have to be perfect like bores do.
Sound good to me. Decent ammo will shoot very well indeed with up to .003-in. bullet runout if the bullets start pretty straight into the throat. New cases with that much runout with metered (not weighed) powder charges will easily shoot well under 4 inches at 600 yards in semiautomatic M1 and M14 military rifles with fat mil spec chambers; they've shot under 3 inches in bolt guns with SAAMI chambers.
Kelbro, when it is not 'all in the claw' it is about the 'ring of steel' recess/counter bore, this is the push feed system, the case is 'free to fly' between bolt pick up and chambering, center and support is any one's guess, as to fit, measure the diameter of the rim and compare to the inside diameter of the recess/counter bore, at best the fit is loose, that is good (for me), I do not live in the world of perfect, I live in the other one. The perfect dimention of a 30/06 case rim is .473?, the web starts at .470 and the thickness of the web can be as thin as .200 up to .270, the point? If the case dimensions were not measured before firing no value can be gained from measurements made after firing but, if the rim and head of the case are upset (compressed) by firing and if the recess/counter bore starts with a tight fit chambering a case from the non-perfect world could cause difficulty in chambering. by design, the bolt face of the push feed will accommodate cases that are not perfect, remember, like the shell holder with a deck height of .125 + or - a (less than) .001, the bottom .125 of the case plus the radius on the bottom of the die is not sized.
I have 30/06 cases that have rim diameter from .468 to .475, some of them must have been fire in a machine gun.
F. Guffey
I have 30/06 cases that have rim diameter from .468 to .475, some of them must have been fire in a machine gun.
F. Guffey
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