Head space questions

[grubbylabs]

So I just got these head space gauges and I am getting mixed readings.

If I understand the process right you measure a case you just shot. Then set the sizing die so that it bumps the shoulder back .001 or so for a bolt rifle and about .003 or so for a semi auto rifle.

Well it seems that all the cases from my new AR vary by about .003 or so. These were all new Winchester cases, this is the first time I have used them.

I did not size or trim before I loaded.

Do I have a problem or is this normal?

[jepp2]

Well it seems that all the cases from my new AR vary by about .003 or so

That would be on the high side of what I would expect you to get. But keep in mind that:

- You are probably getting at least 0.001" variation due to your measuring technique. It might even be closer to 0.002". Do a blind study to determine how much of it is you.

- I would expect between 0.001 and 0.002" variation in the brass from the same lot after firing in your rifle. If you have mixed headstamps or # of firings it could be higher.

So it isn't totally out of line. Since I load for several AR's, I resize to a given headspace that I know will chamber in all. So when I set up, I adjust my die for that headspace and work my way through. I use my Wilson gage for setting that headspace.

For my bolt guns, I use the Hornady Lock-N-Load like you are using and normally only see about 0.001" variation.

[grubbylabs]

I have been putting the case in and turning it as I close the calipers so that the case centers and the measurement comes up consistent. They are all from one new bag of Winchester brass.

[jepp2]

Why don't you size about 5, check them for proper chambering in your AR (the bolt should close with finger pressure) and then measure those 5 and see how consistent they are? Inconsistent lube will result in inconsistent headspace from sizing.

[grubbylabs]

They come out consistent from the sizer, but not after firing from the gun.

[jepp2]

I'll measure some I recently fired in my AR and post the variation of fired bases I see from mine. I'll post the results tomorrow.

[mehavey]

They will not come out of the (gas gun)/rifle consistent. Because the action cycles while still under (considerable) pressure, as likely as not your shoulders are being blown slightly forward (or not) as the case clears. It's a crap shoot and will vary from case-to-case/manufacturer-to manufacturer/lot-to-lot.

(Ain't this fun?)

You need to graaaaaaaadually size a fired case while continually trying to close the (fully-stripped bolt) bolt on THAT case. As you squeeze the sides in, the case will elongate and (hopefully) keep the bolt from closing at some point. Keep sizing (1/16 turn at a time) until the case just fits again and the bolt closes freely.

Measure THAT case, and then set to size just a few thou short of that measurement.

[grubbylabs]

Thanks for the tip, I was wondering if it was not something like that.

[Sport45]

The rifle may be bending the rims a little on extraction. That would affect your measurements as well.

[Bart B.]

Regarding case headspace as measured by the Hornady (and other makes) gauges, note the following:

First off, rimless bottleneck cases are not "blown" forward when fired. The firing pin drives the round forward in the chamber and its shoulder is hard pressed against the chamber shoulder when it's fired. The front half of the case presses hard against the chamber walls while the back half gets stretched back until the case head stops against the bolt face. The pressure expanding the case also sucks the neck back such that the fired case is a bit shorter after firing than before. And case headspace (head to shoulder) is typically longer after firing than before with new cases.

Second, both new and fired cases will have a few thousandths spread in case headspace. Depending on how much lube's on fired cases when full length sized as well as how much the press springs sizing the fired cases, case headspace on sized cases will also have a few thousandths spread.

Third, a few thousandths spread in case heaspace ain't gonna make any difference in accuracy that's noticable unless one shoots their stuff no worse than 1/10th MOA.

And fourth, gas guns do not have "considerable" pressure when the fired case ejects. If they did, there would be "considerable" comments made about that gas blowing back and bothering folks shooting such firearms. "Considerable" means different amounts in actual numbers for different folks.

[Slamfire]

That inset in the pressure curve of a 7.62 Nato round is of great importance to a gun designer. All gas operated semiautomatic mechanisms unlock before barrel pressure is zero. This is called the “residual blowback effect” by LTC Chinn. The mechanism unlocks at a pressure lower than the burst strength of the case sidewall. I believe this is to length the time useful energy is available to function the mechanism.

I don't know if you have fired a HK91 but that gun opens up so early in the pressure cycle that the cases are ejected 20 to 30 feet away. They are really humming.



Even those these cases were fired lubricated this residual pressure is enough that the shoulders move during case extraction. These 23 times fired cases were small base sized to 0.003” below chamber (more or less) and were always fired in my M1a either coated in Johnson paste wax or I left the RCBS water soluble lube on them.

As you can see, the shoulder has been moved so the length is above gage max.



Incidentally, this lubrication prevented case head stretch. The FAL cases came apart in a couple of loads, a bud fired them dry in his FAL. My M1a cases I used them till the primer pockets got too large. I sectioned cases early on that had case neck cracks or body splits, hence the "R5" for reloaded 5, "R18" for reloaded 18.




This is what a bud of mine got in his M1a after five firings with dry cases.

[jepp2]

I just measured 10 cases I recently fired in my AR. 9 of the 10 cases were within 0.001" (actually slightly less) and 1 was just under 0.002" longer. So the variation I am getting is less than yours.

[Unclenick]

Jepp2,

That can be due to how the exact load times itself with the gas system. Also, have you set your fired cases in a straight line on a flat surfaced and sighted down them? If the case mouths and necks don't line up perfectly, there's a good chance some rim bending or burring has occurred that can mess with readings.

People who shoot bolt guns and neck size-only, find that after a number of load cycles that depends on how hot the load is, the case starts to get too snug to chamber freely and they have to go through one cycle of full-length resizing before starting to neck size again so the case will fit. What this tells you is that brass springback after firing is significant. If you use a light load, in particular, the case may come out already about as short as you'd want to resize it. This is why some people have gotten away with neck sizing-only even in some gas guns (though I recommend against it because of the added slamfire risk).

To know the length of the chamber itself, you'd want to measure a fired case, neck size it and shoot it again, and measure again and keep repeating until you felt it start to get snug, at which point it is actually slightly bigger than the chamber. This is possible because the chamber itself expands during firing and the steel is more elastic than the brass, so that when the brass comes right up to chamber size it thereafter is expanding with the steel, but not springing back quite as much. You then look back a measurement or two and have the actual chamber length. Plotting the past measurements in Excel to find a low R² trendline would be a good way to smooth the measurement errors out.

For the gas gun, for which I think neck sizing-only is risky, and to also to save range time, Mehavey's method is actually easier, though 1/16 of a turn on a standard die is 0.0044" inches, so I would use about a quarter of that per adjustment. I've attached a scale for this and for seating stems. The upper left one is what you want. Cut out the center and slip it over the die and onto the lock ring or the press (if you're not using the lock ring), then use a pencil or Sharpie mark on the die body threads as an index to see how many graduations you are turning it.

I have also found that not adjusting the die at all, but just running the brass back in and out of the sizing die can knock a thousandth off. Running it back up into the die and letting it rest a few seconds can sometimes get you up to a couple of thousands. So I would measure and test then re-resize and measure again before and try it in the chamber again before adjusting the die down at all.


Slamfire,

Beeeutiful pix! Thanks for sharing those.

A note to beginners: If you don't know what an incipient head separation looks like, look at the third photo, fifth case from the left (next to the beheaded case) and look at the line just above the head. That case will come apart along that line if you try to use it again (assuming it even survives resizing).

Originally Posted by Slamfire

Incidentally, this lubrication prevented case head stretch.

If you've looked through Varmint Al's pages, his FEA of the .243 shows the same thing will happen with dry cases if you have a smooth polished chamber. The stretch is not actually eliminated, but rather is then spread out over half an inch to an inch of the case instead of being localized at a pressure ring just in front of the head web. It would take an awful lot of reloads to thin brass significantly over that great a length.

Al also showed the resulting increase in bolt thrust from polishing the chamber was insignificant. Same with using petroleum-based lubes. You have to go to very, very low coefficient of friction lube, like molybdenum disulfide or hex form boron nitride (about 1/10 the COF of petroleum based lubes) to increase bolt thrust about 50%. Lots of old wives tales are out there on this subject.

[Bart B.]

Slamfire, that's a good looking chart on pressure, distance and time for a 7.62 round's bullet in the barrel. It stops at 22 inches so was it based on a 24 inch barrel and the bullet starting out with a 2 inch head start in the case?

Also, where did it come from?

My only issue with it is where the bullet's movement starts. MIL SPEC release force for 7.62 NATO bullet's between 40 to 60 pounds of force. At about 530 to 800 psi of pressure in the case producing that force on the back of the bullet, it'll start to move out of the case neck. Then several more pounds or so of force to push it fully into the rifling. Whereas the chart shows the bullet doesn't start moving until there's about 8000 psi (596 pounds of force).

This is an area of internal ballistics I'm not totally understanding based on my knowledge of physics. But the following helps:

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA431357

[Slamfire]

Slamfire, that's a good looking chart on pressure, distance and time for a 7.62 round's bullet in the barrel. It stops at 22 inches so was it based on a 24 inch barrel and the bullet starting out with a 2 inch head start in the case?

I got the image from

Pg 4-18 AMCP 706-260 AUTOMATIC WEAPONS

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=AD0868578

If you cannot find any more explanation of the test conditions of that chart from the pamphlet, as I did not, then your questions cannot be answered.

You know what totally buggers me, I spent $100.00 for a paper copy of the AMCP, and now, it is free on the web!. At least my scan image is better .

Lots of old wives tales are out there on this subject.

As I have expressed many times, it all starts with Hatcher’s cover up of the true problems with the 1921 NM tin can ammunition. Hatcher completely discounts any problems due to the tin coating on the bullet. He does acknowledge that the coating cold welded itself to the case neck, but totally ignores the bore obstruction this creates. Instead, he totally blame shifts any problems to civilians and the practice at the time of greasing bullets. Over the 90 years since this has morphed into a virtual religion, but all of it is based on an Army cover up.

[Unclenick]

Slamfire,

Yep.


Bart,

You're thinking static rather than dynamic conditions. When I first joined this board a fellow who had participated in the HP White tests in the 70's said they'd found bullets didn't start moving until about 10,000 psi. QuickLOAD thinks it's more like 3600 psi, corresponding pretty well to the flat spot in Slamfire's pressure curve. The flat spot is due to the pressure increase partly stalling as the volume expands as the bullet slips from case neck to throat. There's at least one plot in the 1965 Lloyd Brownell study showing flats at around 12,000 psi. So this can vary.

In the dynamic system, even though bullet pull is only 60 lbs slipping from the neck under the kinetic friction coefficient, given the short length of time the bullet has to start moving, its inertial resistance is a significant addition to that. The result is that instead of graciously popping forward, the neck is actually expanded from the rear by pressure, rolling forward almost to the case mouth before the bullet is fully released. This is the reason the mouths of fired cases are usually curled inward a little bit as compared to the rest of the neck. The rest of the neck expanded, but as the expansion neared the mouth, gas started leaking past it and equalizing pressure on the other side, so the mouth ceased expanding. We know this begins before the bullet has obturated the bore because super high speed photos show gas and powder particles preceding the bullet at the muzzle. Dr. Brownell attributed the effect of seating distance off the throat to the amount of greater gas bypass the deeper seating allowed.

Below is a photo of some cases I sectioned to show a well-used verses newer case, but the mouth curling inward is shown on both.

Once you get to the rifling, Harold Vaughn showed 6mm bullets (or maybe it was .270; I'll have to check) need around 1200 lbs of force to swage them into the rifling. That was about 20,000 psi static pressure equivalent, but assuming the kinetic coefficient of friction is about half the static coefficient of friction, about 10,000 psi would be right under dynamic conditions and with the bullet already moving. It made me wonder if that's the 10,000 psi HP White actually measured as the bullet wasn't really getting into the bore until then. Without looking at their experimental setup in detail, I can't really guess accurately.

[Slamfire]

The result is that instead of graciously popping forward, the neck is actually expanded from the rear by pressure, rolling forward almost to the case mouth before the bullet is fully released. This is the reason the mouths of fired cases are usually curled inward a little bit as compared to the rest of the neck. The rest of the neck expanded, but as the expansion neared the mouth, gas started leaking past it and equalizing pressure on the other side, so the mouth ceased expanding. We know this begins before the bullet has obturated the bore because super high speed photos show gas and powder particles preceding the bullet at the muzzle. Dr. Brownell attributed the effect of seating distance off the throat to the amount of greater gas bypass the deeper seating allowed.

Unclenick: I learned something new today. Thanks!

[reynolds357]

I only work on bolt action rifles, but I do not technically
"measure" headspace. On a bolt action competition rifle, I set the headspace so that the go gauge is pretty sticky; The no go will not even come close to going. On hunting rifles, I try to hit dead between go and no go.

[Bart B.]

Unclenick, thanks for the clarification on pressure point pushing bullets out. I'm now better informed. I hate ignorance. When d'ya think powder residue get on case necks and shoulders of first fired new cases; right when the bullet exits the case or after the bullet leaves the barrel? I've read both, but I sometimes think it's peak pressure dependent.

reynolds357, I know lots of 'smiths chamber barrels for a snug fit on a GO gauge. As long as the bolt closes without any effort on loaded rounds, best accuracy is assured. Any binding of the bolt on a loaded round guarantees accuracy will suffer.