Headspace measurements all over..confused

[BrokenWheel]

I am going to start reloading for the 7.62x39, so I'm building up my supply of brass by shooting Winchester and Remington ammo that I buy at walmart. I started measuring the headspace (Hornady Headspace comparator)
on my used brass that I shot from my rifle. Heres the problem - not only do the measurements differ between the brands, but also within each brand. Since I want to set up my resizing die properly I don't know what numbers to use.

I thought the brass was suppose to fire form to the chamber..

 

[918v]

But the rifle violently extracted the case from the chamber, thereby altering its external dimensions. It would be nice if you could turn off the gas. I recommend you size the cases down incrementally until you are happy with the way the bolt closes. Take the top cover off and close the bolt on a sized case under hand pressure without the recoil spring.

 

[fguffey]

“Headspace measurements all over..confused”

You are not the only one, there is a political correctness about the (word) HEAD SPACE that runs most to the curb, reminds me of the elephant and the curious 6 from India.

You are not using ‘a head space gage’ you are using a comparator, a comparator is used to compare, again, I am the only fan of standards, I am the only fan of transfers and I am the only fan of verifying I do not measure head space with a comparator, then there is “Head space is etc..” Memory work is memory work, something like ‘repeat after me’, In the real world I have boring conversations with reloaders when head space is introduced into the conversation.

ME? I measure the length of the chamber from the face of the bolt to the shoulder of the chamber, nothing about the length of the chamber that says head space, then I measure the length of the case from the head of the case to its shoulder. THEN? I subtract the length of the case from (when measured from the head of the case to the shoulder of the case) from the length of the chamber, something like B subtracted from A = C. AGAIN, I have an Eddystone M1917 with a chamber that is longer than a minimum length case (full length sized) by .016”, I know that confuses everyone, back to the fan of standards and transfers, I could chamber a go-gage, knowing the go-gage will chamber in everything but a rifle with a short chamber tells the user nothing! The gage allows the bolt to close, a go-gage chambered in my Eddystone M1917 will still leave a .011” difference between the length of the chamber and length of the gage.

Most difficult part, a minimum length case is a standard, a go-gage length case IS .005” LONGER THAN A MINIMUM MINIMUM LENGTH CASE, IT IS is a standard, again, I do not shoot gages, I shoot ammo, by knowing the length of the chamber in thousandths, if I had a go gage for the chamber I am reloading for I would adjust the comparator to .000” then measure the length of the cases from the head of the case to the shoulder (I could day datum but that is more memory work) to determine if the case length is longer and or shorter from the usual places, in thousandths.

Because I know the distance from the deck of the shell holder to the shoulder of the die with the ram up and locked! I can use the head space gage to adjust the die and verify the die and shell holder’s ability to restore the case back to minimum length. For example: I install a go-gage into the shell holder, with the primer punch/sizer ball assemble removed, raise the ram then measure the gap between the top of the shell holder and bottom of the die, the gap will indicate the difference between the length of the go-gage and die from the deck of the shell holder and to the shoulder of the die. I recommend the the ram be raised first then the die adjusted down to the shoulder of the gage carefully and slowly.

And that is the reason I use a feeler gage, the gap created when adjusted the die off the shell holder with a go-gage results in cases that are go-gage length cases and have no difference in length or, the case is minimum length +.005” if the chamber, case and die are 30/06.

Length of the chamber, I can measure the length of the chamber (30/06) from the bolt face to the shoulder of the chamber with a 280 Remington CASE THAT IS MINIMUM LENGTH/FULL LENGTH SIZED, NOT A PROBLEM FOR ME, I CAN MEASURE THE LENGTH A 280 REMINGTON CASE WITH A COMPARATOR, I FIND IT EASIER AND FASTER TO USE HOME MADE TOOLS.

Standards: Reloadrs do not have standards, you are measuring case length of a large assortment of cases, no where in reloading will you find sizing cases that have been fired 10 time is the same as full length sizing cases that have never been fired, then there is once fired, etc., then there is the hammering a case gets when full loads are used, accumulative? Sometimes my press wins, other times the case wins, sometimes I have to adjust my press die and shell holder to overcome resistance to reloading, and part of that is the reason I do not carry my ammo in a baggy.

F. Guffey

 

[Jaxondog]

My head is hurting again

 

[Smokey Joe]

Hmmm...

@F. Guffey--I'm more confused now than when I started.

@Broken Wheel--If you're shooting an AK or an SKS, IMHO, precision headspacing doesn't matter. Get all the cases FL resized, short enough to chamber, and let it go at that. These firearms aren't built for precision accuracy, they are built to go bang every single time you pull the trigger. And being gas-operated semi-autos, they are hard on cases, as 918 V has pointed out. If your unmodified AK or SKS does better than minute-of-pie-plate at 100 yd, call it good. You'll have to FL size the cases every time you reload them, and plan on a relatively short case life.

If you're shooting a bolt action 7.62x39, then that is a whole different ball game, and there you can start getting fussy about accuracy. If this is the case (no pun intended) buy yourself 100 or 200 high-quality brass cases, e. g. Norma or Lapua, fire-form them in the rifle in question, and keep them for that rifle exclusively. Only neck-size them after fire-forming, and they will fit that rifle's chamber perfectly, as long as you keep the pressure down to normal, with no further futzing about headspace.

Good luck in your pursuit, and please keep us posted!

 

[MEHavey]

...recommend you size the cases down incrementally until you are happy with the way the bolt closes.
Take the top cover off and close the bolt on a sized case under hand pressure without the recoil spring.

This +1

If you can, strip the bolt of the extractor/ejector so there is no extraneous pressure on the case at all, then determine (using Hornady H/S kit) the Case* Headspace dimension when the case will just allow the bolt to close with no hesitation at all. (That's important). ** This is your MAX case headspace dimension.

Take 2-3 thousands off that number as your desired Size-To Dimension and set your dies up from there.




* Truth-in-definition: This is the headspace dimension of the cartridge CASE, which is different (slightly) from the true CHAMBER headspace dimension.
The Chamber is the round hole. The Case is the round peg. You get the picture. Don't worry about it.
Just remember this number is associated with the case, and should always be a few thousandths smaller than whatever the Chamber headspace is.

**The bolt MUST close without any resistance in a gas gun. None.

 

[Slamfire]

The basic problem is that you are trying to estimate chamber headspace by using brass fired out of a semi automatic mechanism.

All gas guns unlock while there is still residual pressure in the barrel, this pressure is less than the rupture strength of the case, this is done to increase the amount of time there is useful energy to activate the mechanism.

Notice the inset curve in this pressure diagram. The time period at or below 650 psia is very important to firearms designers. They design their mechanisms so it does not unlock above 650 psia (about the pressure it takes to rupture a case sidewall) .



What this results in are stretched cases. Cases that are longer than the chamber it was fired in.

As you can see in this picture of some of my 23 times fired M1a brass. Even though I rubbed on Johnson paste wax, to break the friction between the chamber and the case, preventing case head separations, the brass still elongated each time it was fired. Because the case was lubricated the elongation is only due to the shoulder expanding during extraction, if the case was dry the front end of the case would have stuck to the chamber and the case head would have been stretched before extraction.



With a gas gun, you have to use chamber headspace gages to determine the true headspace of the rifle. You are not going to be able to determine it by an examination of fired brass. This is not true for bolt rifles. The brass is extracted when chamber pressure is zero and thus is very close to a correct chamber length.

Without chamber headspace gages you have to assume that the rifle is correctly headspaced, maybe it is better said that you just have to hope. But unless you are having casehead separations, it is a reasonable assumption.

In a correctly headspaced gas gun, size 0.003" below chamber dimensions, or failing that data point, size to cartridge case gage minimum.

My recommendation is to size your brass to gage minimum using a Wilson type cartridge headspace gage. Wilson gages are correct, GO corresponds to a GO chamber headspace gage, and NO GO corresponds to a NO GO chamber headspace gage.

 

[MEHavey]

Slamfire's is probably the safest route.
Full length resize/gauge and be done.

 

[fguffey]

“I thought the brass was suppose to fire form to the chamber..”

And I said “You are not the only one” and then came the head aches, not my intent,

When I fire a case it becomes a once fired case, others after firing a case they become fire fire formers, I form first then fire, I have one rifle that has a long chamber, it is .016” longer than a minimum length case, I form 280 Remington cases to 30/06 cases, while forming the shoulder I adjust the die to avoid oversizing the case. I adjust the die off the shell holder .014” it makes no since to anyone here but my die has threads, it is adjustable, all I have to know is the length of the chamber, I could fire a case to fire form, I prefer to form first then fire, I am not the fan of working the brass, then depending on the action the brass could stretch between the case head and case body, that is a bad habit, others have a problem with the case, bullet and powder out running the firing pin, again, I avoid that phenomena by cutting down on all that case travel.

I use the same method and or technique when forming 8mm57, 7mm57 etc. moving the shoulder back is easier than blowing them forward or driving the shoulder forward and then forcing the case to stretch between the case body and case head.

 

[Walkalong]

Slamfire's is probably the safest route.
Full length resize/gauge and be done.

Agreed. Get a case gauge and set your sizer with it.

 

[918v]

And if his rifle is out of spec then what?

 

[Walkalong]

Then he has bigger problems, which will show up soon. If he is concerned about his rifle having excessive headspace, he needs to get it checked. If it does, then he can come back and ask how to load for a rifle with excessive headspace.

 

[918v]

Your way is unsafe. You are advocating the use of an American gauge to set up a sizing die for a chi-com rifle. Incremental sizing is a safer procedure.

 

[Walkalong]

Interesting theory. SAMMI has specs for 7.62X39. Would a case gauge not be built to such specs? Are you saying you don't trust the rifle to be head spaced properly because of where it is built?

 

[Slamfire]

Your way is unsafe. You are advocating the use of an American gauge to set up a sizing die for a chi-com rifle. Incremental sizing is a safer procedure.

That was the way I used to set up my dies before I found Wilson cartridge headspace gages. Later I was able to measure some of those cartridges once I got gages and cartridge headspace was all over the place. Still, it was better than nothing.

As to the OP having a ChiCom rifle, he never said. It is a could be.

Some of the surplus importers checked rifle headspace but I can't swear that they all did that.

If the headspace is off it will show up in time...

 

[hentown]

OP: Just get a case gage and use that to set up your sizing die. Walkalong: I think you're beating your head against a brick wall!

Why would anybody with half-a-brain think that factory 7.62x39 ammo would be safe to shoot, but reloaded ammo, using a SAAMI-spec case gage wouldn't?? Is that lunacy, or what?

 

[918v]

People who understand the concept of headspace would not make that statement. You can fire a case one, twice, but it may let go on the third try if the headspace is excessive. I think it is lunacy to trust a gauge when the sizer die can so easily adjust to the chamber, especially on an AK. Why spend the money on some gauge when the bolt is so easily manipulated? The chamber is right there, readily accessible. But then some people would rather not think about details.

 

[fguffey]

I determine the length of the chamber first, then adjust the die, I can measure the length of the chamber, I can measure the length of a case from the head of the case to the shoulder, incrementally, then there is the companion tool to the press,THE FEELER GAGE!, No L. Willis Comparator stand, no Sinclair/Hornady comparator called a head space gage? I am not the commercial oriented reloader, I am more about learning everything there is to know about the tools that are available and should be within reach when reloading, nothing like standards, transfers and verifying, I am the fan of verifying.

F. Guffey

 

[243winxb]

Heres the problem - not only do the measurements differ between the brands, but also within each brand.

The head to datum line measurement can get shorter on firing. (M16A1) It takes at least 3 firings or more, for brass to fully form to the chamber. To allow this to happen, back the FL die off the shell holder, or neck size only if a bolt action.