Headspace question.

[mrmuffy]

I am having a hard time wrapping my mind around the headspace sizing concept.
I understand that headspace is the distance between the closed bolt face and the datum line for a rimless cartridge like my 270.
I know that headspace is a specified distance determined by SAAMI that is applied by rifle manufacturers when machining chamber dimensions.
I guess I don't follow that there could be a variance in headspace that would require sizing cases different from SAAMI specs.

[Doodlebugger45]

It's disconcerting of course. But those steel chambers are cut by tools made out of steel themselves. The tools wear at various rates, so none of them are perfect after the first few times in use. After thousands and thousands of chambers are cut, the tools are wearing down in various places so the resulting measurements are just slightly different over time. It only takes a thousandth here and a thousandth there and you have some significant differences. Probably not anything your eye could see, but it's there.

It is a basic principle used by hotrodders to really get a high performance engine in their car. It's called blue printing. You get pistons, bearings, valves, everything to measure exactly the specified amount. A thousandth here or there and it all becomes many horsepower.

[HiBC]

As a handloader,you have some control over how much you reset the brass when you size.Assuming the brass has been fired in the rifle you will reload forfor the sake of discussion,the brass is fireformed to the chamber.We could say,if you necksize only,you will have zero headspace.

If you set your dies by screwing them to a tight toggle-over on the shellholder,the shellholder and die tolerances will combine to set your shoulder back how much? Probably someplace in SAAMI spec,but yoU may have Hornady dies and a Lee shellholder.Maybe they work well together.
RCBS makes the "precision Mic",and Stony Point makes some dial caliper attachments so you can measure the shortening of the case,and define your own headspace clearance.In a bolt gun,you might want .002.
Suppose you are loading for an old lever 30-30 that has some extra headspace using the rim.Don't set the shoulder back,and use it,rather than the rim to headspace.

It is a work-around you can use.Just remember there will be problems if your brother in law needs to borrow a few rounds on a hunt.

An old,messy method I used to use was to candle smoke the shoulder,and sneak the die down to just flaw the smoke .

It can be useful to use a feeler gage if yopu decide you need .004,you can repeat it.Redding sells sets of shellholders to step +/- on the headspace.

There is the SAAMI headspace,which is the spec for a firearm to be proper.
Then there is a headspace which is the actual difference between the cartridge case and the chamber.Similar to the way a wildcatter reloads very precise ammo to a non-standard chamber,a handloader can control the clearance for a specific rifle through reloading techniques.

[Scorch]

Like any machine operation, the reaming and cutting of firearms chambers have a specified length and a tolerance range (commonly expressed as +/- a certain dimension). SAAMI (Small Arms and Ammunition Manufacturers Institute) specifications for a given cartridge or chamber state the maximum dimensions that small arms and ammunition manufacturers should allow. For SAAMI specifications that headspace dimensions will be expressed in inches/mm with a stated limit to allowable variance, for example + .000", -.005" (your chamber should be no larger than specified, but it can be just a little smaller).

In factories, barrels are reamed to finished dimensions on machines that can operate within those tolerances. Ammunition is manufacturered to fit into those chambers without excessive unfilled space. Due to variations in chamber cutting, ammunition has to be made to the minimum chamber dimension. When fired, the case expands to fill the chamber, then rebounds a little. In order to get the best accuracy out of your individual firearm, you custom form the ammunition to fit your individual firearm's chamber.

[brickeyee]

your chamber should be no larger than specified, but it can be just a little smaller

The method for firearms is to set a minimum size for chambers with a positive tolerance, and a maximum size for ammunition with a negative tolerance.

This allows any correct chamber to accept any correct ammunition.

The chamber dimension is then XX, +limit, -0.
The ammunition is XX +0, -limit.
By setting the XX dimensions just barely apart you can be sure a minimum chamber will accept maximum ammunition.

[TEDDY]

YOU FULLY UNDERSTAND THAT RIGHT.

[Slamfire]

Headspace is based on how much stretch the cartridge case can have before it ruptures. As a general rule, cartridges will break if they have to stretch more than .006".

I have stretched case more, and I have stretched cases less. I have gotten case head separations when everything was right, I have gotten case head separations when everything was wrong.

Standardization Committees try to make standards that are easy to live with. Standards have to be agreed on, but standards don’t have to be uniform or consistent. Headspacing on rimless cartridges/belted cartridges/rimmed cartridges are all different.

Your 270 Win is a bottle neck cartridge. It has a shoulder. Some one decided to measure headspace off that shoulder and everyone had to agree where. Once that was decided on gage makers were able to make gages. Headspace is measured from that point on the shoulder to the base, for rimless cartridges.

Profiles on chambering reamers can be quite different in-between the shoulder and the base. So while the rifle may correctly headspace, some chambers are “fat”, others “lean”.

This is also true for sizing dies.

I use a Wilson cartridge headspace gage for every cartridge that I can buy one, and I use that as my guide for sizing my cases. Typically I push the shoulder back .003”, or I just size to gage minimum. That works well for 99% of the rifles I own.

[Bullet94]

As a practical matter, some amount of head clearance must be present to assure reliable operation and chambering. In most modern rifle chambers using SAAMI spec ammunition, Head Clearance will be something on the order of four to six thousandths (.004" to .006") of an inch.
As you can see, head clearance is actually the difference between the headspace measurement and the case/cartridge (for rimless bottleneck cases -as measured from the case head to the datum on the case shoulder). As such, it is entirely possible to have Head Clearance problems in a firearm that has correct headspace, if the ammunition is improperly resized. It is also possible to experience these same problems with ammunition having the proper SAAMI specs, if the headspace of the firearm is out of specifications. Head Clearance problems can frequently be corrected by proper die adjustment, but any suspect ammunition or firearms should be checked by a competent gunsmith. While the condition may be corrected or compensated for, cases that have developed signs of incipient head separations must be destroyed and scrapped at once.

Here are some quotes from SAAMIs glossary -

"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.

HEAD CLEARANCE
The distance between the head of a fully seated cartridge or shell and the
face of the breech bolt when the action is in the closed position. Commonly
confused with headspace."

Referring to headspace as anything to do with a case I believe is incorrect because Headspace is a characteristic of a chamber (not a case, or ammunition).
Headspace is specified on the SAAMI chamber drawing (and not on the cartridge drawing).

There are minimum and maximum chamber dimensions... (the rifle makers use) - Headspace.

There are minimum and maximum cartridge dimensions... ( the die makers and ammo makers use) - Which effect Head Clearance - (Commonly confused with headspace).

[gendoc]

QUOTE>

"When fired, the case expands to fill the chamber, then rebounds a little. In order to get the best accuracy out of your individual firearm, you custom form the ammunition to fit your individual firearm's chamber."



well put!!!!!!!!!

[Alleykat]

Rather than making a mountain out of a molehill, or writing 1000 words when a few might do just as well, suffice it to say that headspace chances when a bottleneck cartridge is fired. The shoulder will require bumping for proper headspace. A case gage, like the Wilson, can be used to set up the sizing die for proper shoulder-bumping.

[steve4102]

Setting up your FL die for PFL sizing.


1. Put the Shell Holder in the press and raise the ram.

2. Screw the Full Length(FL) Die into the press so it is about a "nickle's thickness" above the Shell Holder.

3. Lube a "Fired" case (walls and inside the neck) and squash it.

4. Remove the Lube and try closing the bolt on it in the chamber.

5. If the bolt closes with no resistance, screw the FL Die into the press about 1/8-1/4 turn and repeat steps 3 & 4.

6. As you feel the resistance begin, slow down how much you screw the FL Die into the press so you are at about 1/16 of a turn, or "Fine Tuning". At some point you will not be able to close the bolt and you are extremely close to having the FL Die in the proper position.

NOTE: The reason for this is because the FL Die has begun Resizing the Case-walls down to the Pressure Ring. As it does so, the Case-body lengthens slightly which in turn moves the Case-shoulder slightly forward. Then as the "Fine Tuning" continues the Case-shoulder makes contact with the FL Die and is moved slightly reward(or slightly shortens the Case-head to Case-shoulder dimension).

7. Stop when there is a slight bit of resistance when closing the bolt on the empty case. You now have a "slight crush fit" for the case in that specific chamber, or Zero Headspace.

8. Once you get it where you want it, take some masking tape or a black marker and put a Witness Mark where the FL Die Lock Ring is positioned to hold the FL Die in this position. Loosen the FL Die and return the Lock Ring to align with the Witness Mark and sung up the Set-Screw.

Screw the FL Die back into the press and try squashing another case. It should have the same resistance as the previous one. If not, you need to re-adjust the Lock Ring so it does. But you are extremely close to where you want it, so make very small changes at this point.

Occasionally check the fit of the cases in the chamber, say every 5th firing just to make sure nothing has slipped. Checking more often won't hurt anything and give you confidence that the Set-Up is still proper.

You have now Set-Up the FL Die to P-FLR and will have the very best opportunity at having long case life and the very best accuracy possible.

[wncchester]

"I don't follow that there could be a variance in headspace that would require sizing cases different from SAAMI specs."

Well, sorta different from the "specs" anyway. First, understand that Brickeye is exactly right on his description of cartridge (die) and chamber tolerances. With "normal" machining tolerances there would be a possiblity of overlap but with SAAMI tolerances that simply won't happen. Any dimensions falling within the limits, max to min, is as accurate as need be for commercial ammo. But, we reloadera aren't usually loading for the masses, we are loading for outselves.

Any case or chamber that is made within the specifications are "normal", or dead on so far as the tolerance specs. Normal chamber headspace - length - variance is something like a 6-8 thou range, ditto the cartridge size. Thus, there's a potential of up to 16 thou of cartridge slop that can exist and still allow both the case and chamber to be within SAAMI tolerance! So, there is rarely a "perfect fit" unless we do something while resizing to make it so.

Another fact, a perfect die and rifle chamber isn't quite what many seem to think. Part of the "problem" is new reamers get ground to the large size of their range. Then as they get worn and resharpened they get smaller until they are tossed when below the right size. Best fit occurs with a size die cut from a new reamer while the chamber is cut with an old, minimum size reamer.

Now, forget all the technical definitions of headspace. that's mostly useful to gunsmiths who cut chambers. For reloaders, the practical meaning of "headspace" is that the cartridges properly fit our chamber. If our case is too long we don't have enough chamber headspace and we won't be able to close the action. If the case is too short there will be too much chamber headspace and the cartridge will be a rattle fit in that chamber. Rattle fits allow case stretching and eventual head seperations. For both safety and long case life, reloaders can, and should, make their cases fit their chambers as snugly as possible and still allow easy chambering.

Full length resizing simply means the length of the finished cartridge will allow it to load into any SAAMI chamber. I don't care about any other chambers. it's my ammo, it will be fired in my rifle and that's what I want it to fit! To achieve the best fit many of us do what some call "partial FL sizing" but, IMHO, it means correctly sizing for a specfic firearm so, in that sense, there's nothing "partial" about it.

[Bullet94]

It’s funny to me how some people don’t want to know the correct terms when talking about head clearance and headspace but head clearance is commonly called headspace. I agree when resizing cases for only one chamber you try to minimize head clearance.


From Sierra (I don’t agree with Sierra’s use of headspace) but here it is -

Full Length Sizing

Full length sizing is the most commonly used form of resizing. Full length sizing dies reduce the neck and body dimensions of a fired case to allow free and easy chambering. In most instances, full length sizing will reduce the case dimensions enough to allow it to be used interchangeably in several different rifles chambered for the same cartridge. Contrary to popular opinion, full length resizing is commonly used by the vast majority of competitive shooters, especially in those disciplines where rapid-fire is involved. It should be clearly understood that full length sizing does not reduce a fired case to its original, unfired dimensions. The goal here is to bring the case dimensions down far enough to allow the reloaded case to be chambered without difficulty.

Adjustment of the full length die calls for the die body to be screwed down in the press until it contacts the shell holder at the top of the ram’s stroke. If the ammunition is to be reused in the same gun the cases were originally fired in, back the die off 1/2 to 3/4 of a turn and size a lightly lubricated case. Wipe the case dry, and chamber it in the gun. If any resistance is felt, lower the die body another 1/8 of a turn (or less), and repeat the process with another fired case. This is repeated until the action will just close without resistance. When this has been accomplished, set the lock ring to secure the die in place. This method will ensure that the fired cases are resized with a minimum amount of headspace. This case has now been “custom fitted” to that particular chamber.

This will normally provide the best case life and accuracy, but does require that the ammunition only be used in the gun in which it was originally fired. If the ammunition is to be used in a different gun of the same chambering, of course!, the sizing die should be adjusted down until it just contacts the shell holder at the top of the ram’s stroke.

This results in a slightly greater amount of headspace, but will allow the ammunition to be used in several different guns. Whichever adjustment style is used, full length sizing is generally the best sizing method for most shooting situations.

I use these for making the final adjustment -

http://www.sinclairintl.com/product/6170/s

Also makes adjustment easy for more than one chamber.

I like these for measuring cases -

http://www.sinclairintl.com/product/...eadspace-Tools

And these for measuring headspace -

http://www.midwayusa.com/viewproduct...tnumber=561125

[mrmuffy]

Thanks much to everybody!
Your responses have really helped me understand how and why headspace / head clearance can vary and the importance of proper measurement.
I will make sure to invest in some quality tools.

[T. O'Heir]

Headspace is a rifle manufacturing tolerance only. Cartridges don't have headspace. There's no adjusting headspace by changing how a case is sized.

[Bullet94]

Thanks much to everybody!
Your responses have really helped me understand how and why headspace / head clearance can vary and the importance of proper measurement.
I will make sure to invest in some quality tools.

You really don’t need any tools except your full-length sizing die. Just resize like Sierra lists above. I wouldn’t buy anything to measure your chamber. The RCBS Precision Mic is useful when resizing for a semi-auto but is not needed for a bolt action rifle.

[mrmuffy]

I didn't understand that cases are sort of "elastic" in the sense that they expand to the chamber wall when the round is fired and then bounce back a bit.
It was also the "custom fit" that was confusing to me.
I have seen the term used before and I didn't follow why a case would be sized to any dimension other than SAAMI specs.
Now I understand that the aim is to custom form the ammo to fit an individual rifles chamber - since they are not all the same.
Thanks for the detailed responses.

I'll be looking at some of the recommended tools to check my work.
- Hornady LNL headspace kit.
- Wilson cartridge headspace guage.

[k Squared]

Ladies and Gentlemen,

Lots of good information here, but I’ve got a question about my own situatuion.

I recently purchased an RCBS Precision Mic so I could adjust my sizing die for proper headspace. I pulled out 5, once fired, cases and had a significant variance in the case measurement. The reading varied from -.001 inches to + .004 inches. This seems to be a wide range if I’m trying to get a consistent .002 inch pushback during resizing.

I am measuring once fired, 30-06 Lake City, brass that appears to be from the same production lot. It was fired from my M1 Garand.

I noticed the instructions for the Precision Mic state it may not be helpful for use with semi-auto or lever-action rifles.

Should I get rid of the Mic and size the cases just enough to allow consistent chambering or should I be doing something differently with the Mic?

[wncchester]

"I recently purchased an RCBS Precision Mic so I could adjust my sizing die for proper headspace. I pulled out 5, once fired, cases and had a significant variance in the case measurement. The reading varied from -.001 inches to + .004 inches. This seems to be a wide range if I’m trying to get a consistent .002 inch pushback during resizing."

You are experiencing the real world effects of varing springback. You will NEVER get your sized casess to a consistant anything. Nor do you need to.

For consistant chambering, size enough that all cases are at least .001" smaller in length than the chamber. That will be, or should be, compaired to the largest of your fired cases.

The Case Mic is as good a tool to measure that as any on the market and it's quite simple to use. Forget trying to follow the +/- calibration marks, all you need to is know what the fired and sized lengths are.

[k Squared]

Wncchester,

Thanks for the advise. That's basically what I've been doing for the last several years (sizing just enough to get consistent chambering).

I won't bother getting mics for my AR-15 and M-1A.

[GeauxTide]

I've used the "nickel thickness sizing" for 40 years with Magnum and Bottlenose cases and had great success and case life.