Relationship between headspace and chasing the lands?

[divingin]

Below is the same load, one a fire forming with zero headspace, the other with .003”. The results show clearly on the primer and it’s probably not what most people think.

I think the shorter case is allowing the primer to back out of the pocket on firing, and getting reseated as the case's hold on the chamber wall releases and the case gets pushed back against the bolt face. I also think this is why the primer condition is a not-so-great indicator of pressure (at least when judging someone else's loads.)

 

[DLT]

This may be unpopular but imho a zero set back might not be the best way to keep a rifle cycling after ignition, nothing good comes from fighting a bolt open during a relay or hunting when you need to send another round.
Additionally, .002 -.0025 is only the thickness of a human hair, that I can’t imagine being considered excess movement on brass causing shorten life span. I dont recall ever wearing out brass unless it’s primer pockets getting loose.

I have to agree with your statements

 

[dellet]

I think the shorter case is allowing the primer to back out of the pocket on firing, and getting reseated as the case's hold on the chamber wall releases and the case gets pushed back against the bolt face. I also think this is why the primer condition is a not-so-great indicator of pressure (at least when judging someone else's loads.)

Sort of. Here’s a side view.



The fiat primer is from the case shot with zero headspace, and when I mean zero it’s probably better described as negative. The bullet is seated in the barrel ahead of the case, the case is intentionally left long so as not to be short after fire forming. Without a bullet in the case, the neck actually extends into the freebore. It’s a crush fit. The primer is exposed to full peak pressure as is the case.

The second case, the primer is almost not flattened at all. It protrudes from the case almost the distance of the head space. This is because it’s an extremely low pressure load. Quickload estimate of 18,500 psi. Enough pressure to expand the case and stick it to the chamber wall, not enough to force it back and reset the primer.

Interestingly enough, this is a single shot rifle and the extracter/ejector basically pushes the cartridge against the breech block. So to get any room for the primer to pop out, it has to force the case forward when ignited, then when the case then grabs the chamber, then the primer backs out.

So that’s a large rifle magnum primer showing excessive pressure below 20,000 psi. And also extremely low pressure with the same load. It’s not always pressure.

The case is a 303 British that has been shortened and is more less a straight taper from .454” at the head to 30 caliber. Think 30-40 Krag as a straight taper.


32-40 Remington, preceded the 32-40 Ballard/Winchester and used a .308 bullet.

 

[RegionRat]

Would it be fair to say that regardless of all the different views on how the brass and chamber dynamics work during the firing cycle, that the OP will need to test his changes and then decide which way to go?

How one "calls" their shoulder bump size and their seating depth has more noise in the discussion than the size of the change the OP is discussing.

If we gave ten different people his gun, tools, and materials, we would likely get a spread of bump and depth answers that are as wide as the change he is discussing.

If we accept his observations at face value, and assume we are in a discussion of a system that shoots in the threes or less, there are only 50/50 odds that just turning the seater one way or the other will "put him back" where he was.

Brass sizing affects ignition, and pressure, not just the seating value.

That bump change may or may not follow baseline by a simple seating depth adjustment if we are talking about very high level shooting.

Closing a bolt on essentially a zero fit case and assuming seating depth is closer to a one-to-one with seating depth, but once we introduce clearance, things can get muddy in terms of which way to turn the seater to get back to some baseline depth, and even then this would not guarantee a return to a baseline performance.

If we are talking about a rig that aggs in the ones or twos, nothing short of a test will answer which way the seating depth will want to go, and that is assuming the performance was optimized before.

We have not heard much from the OP since his initial post. I would be interested in what the context and performance level of the rig and ammo is for a baseline, and what it does after things are settled.

In any case, in for the range reports.

 

[dellet]

Would it be fair to say that regardless of all the different views on how the brass and chamber dynamics work during the firing cycle, that the OP will need to test his changes and then decide which way to go?

How one "calls" their shoulder bump size and their seating depth has more noise in the discussion than the size of the change the OP is discussing.

If we gave ten different people his gun, tools, and materials, we would likely get a spread of bump and depth answers that are as wide as the change he is discussing.

If we accept his observations at face value, and assume we are in a discussion of a system that shoots in the threes or less, there are only 50/50 odds that just turning the seater one way or the other will "put him back" where he was.

Brass sizing affects ignition, and pressure, not just the seating value.

That bump change may or may not follow baseline by a simple seating depth adjustment if we are talking about very high level shooting.

Closing a bolt on essentially a zero fit case and assuming seating depth is closer to a one-to-one with seating depth, but once we introduce clearance, things can get muddy in terms of which way to turn the seater to get back to some baseline depth, and even then this would not guarantee a return to a baseline performance.

If we are talking about a rig that aggs in the ones or twos, nothing short of a test will answer which way the seating depth will want to go, and that is assuming the performance was optimized before.

We have not heard much from the OP since his initial post. I would be interested in what the context and performance level of the rig and ammo is for a baseline, and what it does after things are settled.

In any case, in for the range reports.

That pretty much nails it. The simple answer caught in the noise.

Headspace is determined by bolt face to case head clearance caused by whatever contacts in the chamber. It’s not shoulder space or rim space or bullet space. But those as touch points will determine headspace.

Jump and jam on a shouldered cartridge is determined by shoulder and how far the bullet sticks out. Or the belt, rim or case mouth,

So the hidden secret is that adding headspace, moving the shoulder back, moves the bullet forward at the cost of moving the primer away from the firing pin, and the case neck towards the taper at the end of the case at the end of the chamber where it transitions from case neck diameter to groove diameter. Shoulder back, neck forward.

So if you’re chasing the lands by setting the shoulder back it’s a limited amount of potential movement. At some point you either pinch the bullet in the case neck and get a huge pressure spike, or the firing pin can’t set off the primer. Even with low pressure loads.

High pressure loads and repeated resize will eventually cause a case head seperation. But that really has nothing to do with jump or jam.

 

[JFrank]

I’m pretty sure most rifles have more than a few thousandths of clearance on the extractor given the potential rim thickness variations that are possible.

Not being an expert, I suppose that’s a matter of how well they are fit. My no and go gauges are only .004 difference , each are held in with no ejector only extractor, for whatever that worth.

 

[Evan]

Not being an expert, I suppose that’s a matter of how well they are fit. My no and go gauges are only .004 difference , each are held in with no ejector only extractor, for whatever that worth.

No, i agree. The extractor holds it all the way into the chamber, and the ejector pushes it against the extractor, but I think there is more than 0.004” of slop between the case and the bolt face when it is under the extractor, so that when you close the bolt, the chamber shoulder pushes the case back toward the boltface.

 

[JFrank]

No, i agree. The extractor holds it all the way into the chamber, and the ejector pushes it against the extractor, but I think there is more than 0.004” of slop between the case and the bolt face when it is under the extractor, so that when you close the bolt, the chamber shoulder pushes the case back toward the boltface.

Worthy conversation, we’ll have to continue this sometime.
Thx
Jim

 

[dellet]

No, i agree. The extractor holds it all the way into the chamber, and the ejector pushes it against the extractor, but I think there is more than 0.004” of slop between the case and the bolt face when it is under the extractor, so that when you close the bolt, the chamber shoulder pushes the case back toward the boltface.

Worthy conversation, we’ll have to continue this sometime.
Thx
Jim

The case head snaps over the extractor when it meets enough resistance to pushing the cartridge forward. Generally shoulder contacting the chamber when lowering the bolt handle into battery.

Again the base to shoulder distance comes into play. Many cartridge specs allow up to .010” of headspace adjustment. The chamber allows the same. So a minimum cartridge in a maximum chamber, vs maximum cartridge in minimum chamber may require the extractor to be at least .020” more than rim thickness off the case head. Considering how short some new brass is probably need to add another .010”.

So the smaller the headspace clearance, distance between bolt face and case head, the more clearance or space you have between the rim and extractor when the bolt is closed.
Misfires due to excessive headspace are a combination of the extractor not allowing the case to brace against the chamber for firm primer strike, and or lack of enough firing pin protrusion.

All this is based on most Saami specs. I’m sure there are hundreds of exceptions.

 

[Wild Bill IV]

my opinion is that if the shoulder is bumped further and bullet seating die is unchanged then it will result in the bullet being closer to the lands (or jammed further if jamming) when the round is chambered.

You are so correct!!! Bump the shoulder back 0.00x", the bullet will move forward in the chamber 0.00x" without a change in seating die!!!!

The dies are independent of each other!!! Both dies (form and bullet seating) are based relative to the shell holder case base surface area or jaw pressure plate (Forster and Bonanza presses)!!!! Even, the spring sleeve seating dies are will seat off the case base surface of the shell holder or jaw pressure plate!!! However, if damage to the sleeve, sleeve spring, and/or die body occurs from setting the die too deep in the press, variances in bullet seating can occur!!!!!! Causing irregular bullet seating depth of the jammed shoulder of the sleeve!!!!!

IMO, ALWAYS find max pressure for the chamber!!! Working with 2 cases each, in 0.2 grain powder charge increments (0.1 grains for cases less than 30 grains capacity)!!!! There is a point, when using the bump comparitor at the range, when you find max formed pressure when the bump stops increasing and has hit a plateau (consistent bump measurements)!! However, the plateau may not show itself in some cartridges!!! You might exceed max pressure where the bolt opens really hard!!! Most of the times, an extruder imprint will be present!!! DEMILITARIZED the rest of the test loads at that point!!!!

Set aside a couple of cases that were just below the excessive pressure point for future references such as checkering for headspace growth and chasing the lands (keeping zero bump, but short stroking, stopping the press short of the start of cam over, on form sizing the neck only)!!!!!! NOW, you don't have to remove the extractor to find bullet touch (Also called 'RIFLE SEATING DEPTH' in the old days)!!!

NOW, if you are going to bump back, say 0.002", and want to maintain bullet touch, turn the seating stem down 0.002"!!!

Use those set aside and marked max pressure cases!!! I never purchase the more expensive Go/No Go gauges except for changing out barrels!!! With factory built rifles, the much cheaper max pressure reference cases will suffice for setting bump and periodic checking of headspace vs bump distance!!!!

 

[M-61]

Headspace is determined by bolt face to case head clearance caused by whatever contacts in the chamber.

That’s the datum line.
Headspace is is from the closed face of the bolt to the datum line.
There should be a full understanding of headspace before bumping a shoulder back. The headspace is set when the barrel is screwed into the action. Other than screwing the barrel in or out, the headspace cannot be changed. The cartridge case ‘fills’ this area loosely. The case’s dimensions are from the cartridge base to the datum line on the shoulder.
Lots happen here when the trigger is pulled. I’ll stop here with this one statement. Not enough headspace is harmless. You won’t close the bolt. Conversely excessive headspace can be dangerous. Many an action has been blown apart because of excessive headspace. Bumping shoulders has to be a carefully measured process. You are not changing the headspace, rather shortening the cartridge dimensions.
I’m talking about bolt actions

 

[dellet]

That’s the datum line.
Headspace is is from the closed face of the bolt to the datum line.
There should be a full understanding of headspace before bumping a shoulder back. The headspace is set when the barrel is screwed into the action. Other than screwing the barrel in or out, the headspace cannot be changed. The cartridge case ‘fills’ this area loosely. The case’s dimensions are from the cartridge base to the datum line on the shoulder.
Lots happen here when the trigger is pulled. I’ll stop here with this one statement. Not enough headspace is harmless. You won’t close the bolt. Conversely excessive headspace can be dangerous. Many an action has been blown apart because of excessive headspace. Bumping shoulders has to be a carefully measured process. You are not changing the headspace, rather shortening the cartridge dimensions.
I’m talking about bolt actions

This is not exactly correct, and I worded that very carefully for a reason. Headspace should not confused with head clearance(I should have made that clear). The actual definition of headspace:

HEADSPACE​

The distance from the face of the closed breech of a firearm to the surface in the chamber on which the cartridge case seats. 1. BELT: A type of chamber design in which the cartridge seats in the chamber on an enlarged band ahead of the extractor groove of the cartridge body. 2. MOUTH: A type of chamber design in which the cartridge seats in the chamber on the mouth of the cartridge case. 3. RIMLESS: A type of chamber design in which the cartridge seats in the chamber on the shoulder of the cartridge case. 4. RIMMED: A type of chamber design in which the cartridge seats in the chamber on the rim or flange of the cartridge case.

The only way to adjust headclearance with a cartridge is to change the contact point. Most cartridges the only way is to seat the bullet so it contacts instead of case mouth, rim or belt. But a cartridge with a shoulder opens up the option of changing head clearance without setting the bullet at touch.

Datum lines are basically a reference number pulled out of thin air to provide reference points to argue over on the internet. As are definitions vs common use. Headspace in the chamber is a measurement of bolt face to datum line, while head clearance is measured from case head to a datum line of the same diameter, but different length. Where you set that length on the case will determine how much head clearance you will have between the case head and bolt face. Just be careful not to label that clearance (the gap between the casehead and bolt face) as space. It might confuse someone into using a space gauge with a clearance gauge and get a wrong reading that causes them to blow something up.

All in fun, but yes I should have been more clear

 

[M-61]

This is not exactly correct, and I worded that very carefully for a reason. Headspace should not confused with head clearance(I should have made that clear). The actual definition of headspace:
I tried very hard with my wording. I don’t see where I was wrong with the exception of not making clear enough he’s not changing the headspace. I should have added that I was not only talking about bolt actions BUT rimless cartridges as well.
I fully agree with your being careful to choose the correct nomenclature as to what you are writing about. I wasn’t going to reply at all except I wanted to stress the possible danger in just ‘bumping’ shoulders back for the heck of it. The datum line problems were started by fguffy on this very site years ago.


The only way to adjust headclearance with a cartridge is to change the contact point. Most cartridges the only way is to seat the bullet so it contacts instead of case mouth, rim or belt. But a cartridge with a shoulder opens up the option of changing head clearance without setting the bullet at touch.

Datum lines are basically a reference number pulled out of thin air to provide reference points to argue over on the internet. As are definitions vs common use. Headspace in the chamber is a measurement of bolt face to datum line, while head clearance is measured from case head to a datum line of the same diameter, but different length. Where you set that length on the case will determine how much head clearance you will have between the case head and bolt face. Just be careful not to label that clearance (the gap between the casehead and bolt face) as space. It might confuse someone into using a space gauge with a clearance gauge and get a wrong reading that causes them to blow something up.

All in fun, but yes I should have been more clear

 

[Wild Bill IV]

dellet and M-61 have now explained the difference between HEADSPACE and DATUM POINT with their arguments!!!

How do we know that we are is getting a true, measurable shoulder bump???
I have explained it by my method of finding max headspace with full fire forming of Virgin brass!!!

For the OP, if he chooses to opt out of my method, there is another method of confirming the desired shoulder bump!!! I'm not not talking about the crude (caveman approach) of using Scotch Tape!!! Mechanics sometime use Plastigauge for measuring main bearing clearance and shiming rack and pinion gears for proper loading (lash/backlash)!!! The Plastigauge comes in blue, red, or green colors (depending on the clearance range)!!!! With the muzzle of the rifle point straight down, drop the formed case into the camber!!! Place two strips of Plastigauge across the degreased base on the flat surface with tweezers!! Slowly drop the bolt face down and carefully lock the bolt!!! Turn the rifle muzzle up, unlock the bolt and carefully open it!!! Try to keep the brass from ejecting or rattling!!! Now, the gauge plastic is smashed to a different width!!! It is stuck on the bolt face and/or base of the case!!! Measure the width with scaled ruler included in the Plastigauge package, and that is your shoulder bump back distance from true headspace!!!!

Plastigauge can be purchased at automotive part stores or warehouses!!!!

EDIT NOTE: MEASURE THE THINNEST PART OF THE SMASHED STRIPS!! THE THICKER WIDTH MAY BE DUE TO THE EJECTOR PLUNGER!!!

 

[Kracken McGraw]

was wondering why one might want to push the shoulder back any further than light contact,

Because everyone at one time probably read that Sierra’s Tech comment about a “rat turd in a violin case” comment. lol!

 

[Kracken McGraw]

my opinion is that if the shoulder is bumped further and bullet seating die is unchanged then it will result in the bullet being closer to the lands (or jammed further if jamming) when the round is chambered.

I can’t wrap my mind around that one.

 

[Wild Bill IV]

I can’t wrap my mind around that one.

It makes sense!!! Lets pick a reference point of the say the case mouth!!! Adjusting the form die down increases the shoulder bump, pushing the case mouth down too (in theory) when properly lubed inside and out of the case neck!!! Now, the bullet will be seated shallower into the neck if the seating die is not changed! Now, when chambered, the shoulder is in contact with the chamber, and the setback in the bump moves the bullet forward (the same distance as the set back or bump adjustment) in the chamber!!

 

[Kracken McGraw]

It makes sense!!! Lets pick a reference point of the say the case mouth!!! Adjusting the form die down increases the shoulder bump, pushing the case mouth down too (in theory) when properly lubed inside and out of the case neck!!! Now, the bullet will be seated shallower into the neck if the seating die is not changed! Now, when chambered, the shoulder is in contact with the chamber, and the setback in the bump moves the bullet forward in the chamber!!

But the rifle bolt is holding the case rim in the same spot as well as the shell holder is on your seating die.
But believe me, I am very often wrong.

 

[JFrank]

There use to be a method talked about of partially seating a primer ( spent I assume ) then closing the bolt . The extended primer will indicate the head space once the case is extracted.

 

[M-61]

There use to be a method talked about of partially seating a primer ( spent I assume ) then closing the bolt . The extended primer will indicate the head space once the case is extracted.

Yes. And we are talking about, or at least I am, about bolt actions.
It is to check your headspace in your rifle with a cartridge case fired in again, your rifle.
Take a fired case, take a fired primer, and just get it started in the primer pocket. Insert in the chamber or at least get it started. Close the bolt.
Withdraw the cartridge. You will see the primer is not fully seated. Measure the height the primer is sticking out. That’s the headspace.