Where does a cartridge sit in the chamber ?

M

Metal god

New member
I've been putting some thought to this and I'm a little confused . I'm asking about bottle neck rifle cartridges .

I've read and or talked to people about this and the answers are all over the place .

I've been told and or read :

It sits on the bottom of the chamber

It sits perfectly centered in the chamber .

It sits with the with the neck touching the the bottom area of where it sits and the the head touching the bottom where it sits making the the whole cartridge sit crooked .

I've been told and or read :

When the firing pin hits the primer it pushes the cartridge forward until the shoulder makes contact with the chamber .

Sooo , I was doing some basic testing as to why one of my bolt action 308s throws the brass 4x farther then the other . I was slowly extracting empty cases from each . I could see how the ejector and extractor were working together to throw the brass free from the firearm .I noticed that the case was pressed against the chamber and receiver wall at about 2 o-clock till it was thrown clear . I also was checking how the cases sit in the bolt face by placing them on the bolt with the bolt out of the gun . I would hold them in the bolt and let the ejector on extractor do there jobs and fling the brass across the room .

Now to the point of this thread .

It would seem to me that the cartridge is sitting in the chamber pressed up against the chamber wall at 2 O-Clock the whole time do to the force of the ejector pushing on the head and the extractor not allowing the ejector to push the case straight forward .

It also would seem that because the way the ejector and extractor are working together the case would not move forward at all when the firing pin hits the cartridge . The case is already being pushed forward by the ejector as far as it will go and because the extractor is holding it , there should be no foreward movement . Even if the extractor is not making contact with the rim until extraction . The ejector would still be pushing the cartridge all the way foreward to where the shoulder would be making contact with the chamber centering the cartridge in the chamber . Therefore there should be no foreward movement of the cartridge when the firing pin hits the primer .

That's my thinking , What say you ?
 
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I'm thinking that if the gun is properly head spaced, the cartridge should be touching the front of the chamber as the bolt locks. And shouldn't more at all when the firing pin hits it.

As for touching the sides of the chamber, and where it contacts the side. My guess is that that would be dependant on whether the firearm has controlled feed or push feed system; where the extractor is located on the bolt face, and the orientation of the rifle as the bolt is being charged.
 
You're talking about a very few thousands of an inch-not like a rock in a bucket. As long as it's consistent, where the contact occurs is irrelevant.
 
There is no universal answer.
It just depends on the rifle in question, and the ammunition in question.

Chamber dimensions vary.
Headspace varies.
Cartridge dimensions vary.
Rim/belt thickness varies.
Extractor location varies.
Extractor clearance varies.
Ejector location varies (if so equipped).
Etcetera, etcetera...


Two examples:
In my Ruger M77 V/T, the cartridge is pinned against the shoulder in the tight chamber, by the spring loaded ejector. It isn't pulled to one side, because the extractor doesn't contact the rim until the bolt is partially opened. In theory, the cartridge is perfectly centered in the chamber. But, I wouldn't be surprised if the case head favors toward the bottom. (It can't go far, though. There's less than 0.001" total clearance at the cartridge base.)

My Ruger M77 Mk II, on the other hand, has a generous chamber and no spring-loaded ejector. Cartridges likely rest on the bottom of the chamber, rattling between the chamber shoulder and the bolt face. With light loads, the cartridges show clear evidence of being pushed forward by the firing pin strike, and allowing primers to back out slightly. With minimum cartridge dimensions, they cases also show evidence of being stopped by the extractor, about 0.003" shy of the shoulder in the chamber.
 
I think it would stay centered for the most part, im no expert but ive seen alot of rounds threw rifles of all actions. I think they stay centered if the action is locked closed with pressure keeping the flat rim of the bullet flush against the chamber. And maybe the hot brass expands a little making it rub somewhere on the chamber, Thats my theory.
 
It isn't normally "centered in the chamber". To achieve that you typically have to neck-size brass that has been fire-formed in your chamber and load it properly. Factory loads aren't custom made for any particular gun, so, by a few thousandths of an inch, they lay in the bottom of most chambers.
 
You could put some dykem blue (or soot, carefully) on a cartridge or empty sized brass and see where it rubs off to find out, I imagine.
 
I think Metal god has it right, I've given this some thought before. How much this effects accuracy is I'm sure variable from gun to gun though. An old friend of mine use to orient each case in his 700 BDL Varmint Special 7mm-08 with the head stamp in the same location, he shot very, very well. Makes a case for neck sizing, if your after that last bit of accuracy.
 
Thanks guys , I was expecting a little more debate on the issue . I was pretty much thinking like FrankenMauser has shown . It will depend on the rifle your using and how the action operates . I can see the cartridge sitting in the chamber many different ways depending on the rifle being used and how the brass is sized .
 
Gravity still applies inside a firearms chamber so unless there's a reason that it is forced not to, a cartridge first of all sits in the bottom of the chamber.

In addition, any pressure from an ejector will slightly cant the cartridge if there is any space to move it.

A "properly headspaced" cartridge will have 1 or at most 2 thousandths of an inch of space (length) in a bolt gun or 3 thousandths or so in a semi.

If the dies are made for the gun, the entire body will have 1 thou or so space around the sides.

If the dies are generic (which almost all are unless they're custom), you'll have whatever space is the difference. No way to know. The die could be SAAMI minimum and the chamber could be SAAMI max or anywhere in between.

In any case, gravity applies, so unless the cartridge is (improperly, BTW) "crush fit" in the chamber, it will sit on the bottom and be pressed slightly to the side by the ejector.
 
The other thing that got me thinking about this was when I was trying to find my max OAL . Before I had my OAL gauge I would load dummy rounds until I would be jaming the bullet into the lands . If the bullet was just barely touching I would only see marks on one side of the bullet . I was thinking either the cartridge is not sized straight and square or there was a force pushing it to one side . I did not feel that gravity would cause enough pressure to cause a mark . It's a non issue now but I do like to know why things are happening . I'll load a pic of the mark when I get home .
 
The universal answer. . . .at least where it's at when fired and most of the time before that. . . .

Bottleneck cases headspacing on their shoulders are pushed forward by the firing pin's impact centering their shoulder perfectly in the chamber shoulder a few microseconds before the primer's crushed enough to fire. Their back end's pushed against the chamber wall by the extractor's force against the groove in the case head; see where the extractor is with the bolt closed and the point opposite it is where the case's back end is pressed. There's enough force from the firing pin to set the case shoulder back a thousandth or more before the round fires. So there's typically a few thousandths "head clearance" between the breech face and case head when it's fired.

With bolts having an inline spring-loaded ejector at the edge of the bolt face, that pushes chambered rimless bottleneck cases all the way forward centering their shoulder in the chamber shoulder when the bolt's closed before the firing pin falls. M1 and M14/M1A ejector springs are strong and very well center the cartridge shoulder in the chamber shoulder before firing it.

When fired, no rimless bottleneck case is resting on the bottom of the chamber; external forces move it such that its shoulder's hard into the chamber shoulder centering the case's front part there with the case neck floating in space near the center of the chamber neck. It's one of the oldest myths regarding firearms that all cases rest in the bottom of the chamber when they're fired. Check out the external forces applied to them when they're chambered; it's easy to figure out where those forces position the case before it's fired. Especially when one observes the extractor in most push feed bolt actions pushes the head of the case up and Mauser style controlled feed actions' extractors push the case head sideways.

Use your Wilson rimless bottleneck case gauge to see the case mouth position in the gauge's mouth when it's slowly pushed all the way forward in it centering perfectly as the case shoulder stops against the gauge shoulder. Move the back end of the case hard against the gauge's side at different places and see how little it effects the centering of the case mouth in the gauge mouth. That gauge is just like a rifle barrel chamber; cases fit it the same way. Chamber a .243 Win case in a Wilson gauge made for the .308 Win and notice how perfectly centered its case neck and mouth is in that .308 gauge.

Better yet, cut the barrel off just in front of the chamber mouth then square it up even with the case mouth. Chamber some dud-primed new, fired, full-lenght and neck-only sized cases in it then see where their mouths center in the chamber neck as the bolt's slowly closed while holding the trigger back and the firing pin and in-line ejectors push those cases full forward in the chamber.

Once one understands all of this, they'll probably understand why it's important to get fired case necks well centered on case shoulders when sized down. That's the key to precision alignment of the case neck, and therefore, the bullet, on the case shoulder. It's the case shoulder that centers the bottleneck case's bullet in the bore. There can be one to several thousandths inch clearance around the case body near its shoulder to the chamber body and the bullet's still perfectly centered in the bore when the round fired.

New belted cases get pushed forward until their belt stops against the chamber belt ridge. And their shoulder's typically a few thousandths short of touching the chamber shoulder. When fired, their shoulder gets blown forward against the chamber shoulder and back end's pushed hard against the bolt face; they shrink back a thousandth or so after the bullet leaves. When full length sized and their shoulder's set back a couple thousandths, they'll headspace on their shoulders and their belt's a couple thousandths back from the chamber belt ridge. So they're like rimless bottleneck cases thereafter unless their shoulder's set back to near new case location.

Rimmed cases behave the same way as belted ones except the front of their rim is the reference point for headspacing.
 
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Bart B. said:
The universal answer. . . .at least where it's at when fired and most of the time before that. . . .

Bottleneck cases headspacing on their shoulders are pushed forward by the firing pin's impact centering their shoulder perfectly in the chamber shoulder a few microseconds before the primer's crushed enough to fire. Their back end's pushed against the chamber wall by the extractor's force against the groove in the case head; see where the extractor is with the bolt closed and the point opposite it is where the case's back end is pressed. There's enough force from the firing pin to set the case shoulder back a thousandth or more before the round fires. So there's typically a few thousandths "head clearance" between the breech face and case head when it's fired.

With bolts having an inline spring-loaded ejector at the edge of the bolt face, that pushes chambered rimless bottleneck cases all the way forward centering their shoulder in the chamber shoulder when the bolt's closed before the firing pin falls. M1 and M14/M1A ejector springs are strong and very well center the cartridge shoulder in the chamber shoulder before firing it.

When fired, no rimless bottleneck case is resting on the bottom of the chamber; external forces move it such that its shoulder's hard into the chamber shoulder centering the case's front part there with the case neck floating in space near the center of the chamber neck. It's one of the oldest myths regarding firearms that all cases rest in the bottom of the chamber when they're fired. Check out the external forces applied to them when they're chambered; it's easy to figure out where those forces position the case before it's fired. Especially when one observes the extractor in most push feed bolt actions pushes the head of the case up and Mauser style controlled feed actions' extractors push the case head sideways.

Use your Wilson rimless bottleneck case gauge to see the case mouth position in the gauge's mouth when it's slowly pushed all the way forward in it centering perfectly as the case shoulder stops against the gauge shoulder. Move the back end of the case hard against the gauge's side at different places and see how little it effects the centering of the case mouth in the gauge mouth. That gauge is just like a rifle barrel chamber; cases fit it the same way. Chamber a .243 Win case in a Wilson gauge made for the .308 Win and notice how perfectly centered its case neck and mouth is in that .308 gauge.

Better yet, cut the barrel off just in front of the chamber mouth then square it up even with the case mouth. Chamber some dud-primed new, fired, full-lenght and neck-only sized cases in it then see where their mouths center in the chamber neck as the bolt's slowly closed while holding the trigger back and the firing pin and in-line ejectors push those cases full forward in the chamber.

Once one understands all of this, they'll probably understand why it's important to get fired case necks well centered on case shoulders when sized down. That's the key to precision alignment of the case neck, and therefore, the bullet, on the case shoulder. It's the case shoulder that centers the bottleneck case's bullet in the bore. There can be one to several thousandths inch clearance around the case body near its shoulder to the chamber body and the bullet's still perfectly centered in the bore when the round fired.

New belted cases get pushed forward until their belt stops against the chamber belt ridge. And their shoulder's typically a few thousandths short of touching the chamber shoulder. When fired, their shoulder gets blown forward against the chamber shoulder and back end's pushed hard against the bolt face; they shrink back a thousandth or so after the bullet leaves. When full length sized and their shoulder's set back a couple thousandths, they'll headspace on their shoulders and their belt's a couple thousandths back from the chamber belt ridge. So they're like rimless bottleneck cases thereafter unless their shoulder's set back to near new case location.

Rimmed cases behave the same way as belted ones except the front of their rim is the reference point for headspacing.
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Bart,

This is a little off topic (sorry Metal God), but what is the best way for belted cases to headspace for accuracy? New cases will headspace off of the belt and fired and resized cases headspace off of the shoulder, so which way is better? Is it best to resize so the case headspaces off the belt once more, or is it better to bump back slightly and leave it to headspace on the shoulder?
 
Five In A Dime, belted cases can shoot with amazing accuracy either way as long as the bullet's pushed back a bit in the case neck when the round's chambered. When very accurate bullets were finally available for belted magnums used in long range matches, folks got equal accuracy with new cases or those full length sized bumping the shoulder back a thousandth or two so they headspaced on the shoulder. But the case body has to be sized down all the way to the belt; not stopping a few thousandths in front of it as most full length sizing dies do. Sub 3/4 MOA accuracy at 1000 yards was normal doing this back in the 1960's and it's not any better these days.

I prefer to have belted cases headspace on their shoulder as that ensures their necks are better centered in the chamber neck. Especially when the bullet's seated so it doesn't touch the rifling when chambered. Full length sizing them is the best way to do that. Neck only sizing does not always make the case neck well centered on the case shoulder as nothing aligns both when the neck's sized down.

Many rifle teams shooting new belted cases in their 30 caliber magnums got excellent accuracy with the bullets seated just short of the lands, but bullet runout was typically less than .002". They won a lot of matches setting records along the way doing this. A proper barrel correctly chambered and well fitted to a receiver correctly bedded to the stock is a lot more important to best accuracy than "perfect" ammo.
 
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FWIW

I got to thinking why don't I see those marks any more . Really It's two reasons 1) I don't load the rounds that long ( duh ) 2) I have not seen those marks since I changed from a standard die to a FL bushing die . Which I think confirms the bushing die helps center the neck on the case . Maybe my standard die sizes the neck a little off center and that is what caused those marks on one side . I should add the barrel was heavily fouled at the time I most noticed those marks .

It's not a good picture but you can see the scuff mark that only goes around the bullet about 1/6 of the diameter
8nyh.jpg


No prob on the belted mag ? I'm sure I'll own one some day ( 7mm mag )
 
Metal god, neither RCBS or Redding full bushing dies center sized case necks perfectly on the shoulder. Their bushings have a couple thousandths clearance to the bushing chamber walls. Nor do they size the neck all the way to the shoulder; they stop several thousandths short. So there can be a tiny bit of misalignment between sized case neck axis and sized case shoulder axis. But they do produce much straighter cases than standard full length sizing dies and excellent accuracy is at hand. Both Redding and RCBS claim that unsized part of the case neck at the shoulder helps align the case neck in the chamber neck. Neither company understands that's not reality (or if they do, they don't mention it); the unsized part of the case neck floats free in the chamber neck when its shoulder centers in the chamber shoulder.

Best sized case neck alignment to shoulder is with standard full length die's necks honed out to a couple thousandths smaller than a loaded round's neck diameter. The difference is small but benchresters see the difference in groups down in the ones and zeros. You can see the difference if you measure neck runout relative to the case body axis at the shoulder datum relative to the pressure ring at the back end of the case, not on the outside of the case body behind the shoulder.

That scuff mark may be caused by the bullet rubbing against the chamber throat as it's pulled out of the chamber by the bolt while an in-line ejector is pushing on one side of the case head. Open the rifle's bolt, point the muzzle straight down, drop a loaded round into the chamber, then raise the muzzle up so the round drops out. See if the scuff marks are on the bullet when it's chambered that way. Is that mark always at the same place relative to round orientation in the chamber?
 
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Bart,

Would a FL die prepared the way you instruct leave any work to do after sizing? Would you have to ream the neck for consistent thickness? Why do the companies not make dies this way and sell them? What is the point of oversizing and expanding to the correct diameter? Why have expander buttons at all?
 
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