So i picked up a new tikka 223 remington. I measured a factory round with my hornady headspace tool and fired it. After firing the "headspace" was .001" shorter than the unfired case. They chambered just fine and shot great. Im just a bit confused why it would be shorter after firing? Thanks.
Headspace question.
- Thread starter Mtstalker
- Start date
Geometry shifts. Shoulder angle changes, diameter variances...that’s why headspace gauges are relieved in certain locations.
The case expands like a balloon and gets shorter the first time or two. Neck size and shoot the same case several times and it will get closer to the truth.
I prefer to use a case with a too long shoulder location. Then I just push the shoulder back until i can barely close the stripped bolt with light friction.
I prefer to use a case with a too long shoulder location. Then I just push the shoulder back until i can barely close the stripped bolt with light friction.
6MMsteve
Gold $$ Contributor
it will grow back don't worry, I was stumped same as you
With out knowing all the details , I'd say that's a combination of two things . 1) a generous chamber diameter 2) a lower pressure round . If your chamber is a little loose ( not much ) and the round is a lower pressure round like some factory 223 rounds such as some PMC loads . The case will expand filling the diameter of the chamber but wont have enough pressure to stretch the case head back up against the bolt face . This will not only cause your head to datum point ( case headspace) to be shorter . The actual case length will be shorter as well .
That's my first thoughts anyways . What ammo are you shooting ?
That's my first thoughts anyways . What ammo are you shooting ?
Where the hell is fguffey? He's got a real good "handle" on headspace!
My guess would be that after you fired the new factory loaded round it expanded to fit the chamber of the rifle; the chamber of the rifle was not identical as the inside of the cylinder shaped comparator thingy the Hornady headspace measuring tool uses. A subsequent measurement of a fired case, like "datum", measured a changed engagement of the brass shoulder to inside of the comparator and came up with .001 less.
I think .001 is of no practical significance when considering head-space. I would guess that measuring a whole bunch of different ammo that worked real good for any number of rifles would show a variability somewhat greater than .001. I like to adjust my ammo so I can slightly feel resistance upon closing the bolt and this is with rifles not having a spring loaded plunger type ejector.
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I forgot -- Merry CHRISTMAS --
I think .001 is of no practical significance when considering head-space. I would guess that measuring a whole bunch of different ammo that worked real good for any number of rifles would show a variability somewhat greater than .001. I like to adjust my ammo so I can slightly feel resistance upon closing the bolt and this is with rifles not having a spring loaded plunger type ejector.
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I forgot -- Merry CHRISTMAS --
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I don't see how the case could get shorter as expansion upon firing would occur in every direction.
I also adjust my F/L sizing using a stripped bolt and that is real easy to do with my Ruger M77's MKII's (no plunger ejector).
Factory ammo is made small to fit all types of firearms. They want it to feed in Joe Blasters AR as well as ur target rifle.
So, when the round is fired it expands first in the direction where the brass is the thinnest which is around its circumfrence. That is going to cause the case to shorten its OAL first.
This is also what causes the banana curve in a fired case. The thin side of the case wall expands faster than the thick side n causes the " curve" in the case.
As I pointed out above . The case does not always expand in all directions do to insufficient pressure to stretch the head back to the bolt face after the firing pin has pushed the cartridge fully forward . Although IMO the ammo he used and the numbers he got would seem to indicate there should have been enough pressure to expand the case in all directions . Think of it as the opposite of FL sizing . To squeeze down , the case must get longer because the brass must go somewhere . To make the case fatter the brass must come from somewhere . Put another way if you could expand the case to be a perfect circle/ball would the case get longer or shorter ? Shorter right because as the sides get fatter the neck and head move towards the center . Remember we are only talking .001 to .002 , that's not a whole lot of movement or difference .
Are you removing the primer before measuring? I get that same result if I measure the spent brass with the primer still in the pocket. Removing the primer without disturbing the shoulder is one way, another is to use a LNL comparator gage on one blade of your calipers and the headspace gage on the other. That way any primer cratering will be taken out of the measurement.
Merry Christmas
Merry Christmas
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@ up to 60 K psi and more, I don't buy this -- raging massive shock of sufficient duration, inflicted on brass of varying hardness and thickness but still relatively weak & soft followed by limited spring-back at work-hardened portions - equal pressure (psi) exerted everywhere . Circumference (radial tension) & longitudinal tension (headspace) pressure forces would be spent on brass consisting different degrees of hardness resulting to annealing; examination of unpolished annealed cases indicates annealing including shoulder areas. Brass bases including primer pockets are thicker and harder to resist the up to 60K plus psi.
F/L sized cases must be periodically trimmed because they grow in length; this presents the question - did the case from base to shoulder (datum) grow requiring F/L resizing to make the bolt close real easy in a chamber having consistent radii (yes?) and did the neck portion grow because the brass just did not evaporate making periodic trim events necessary (yes?).
Thank-you for your thought-full replies.
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Err wait what ??? I wish I was smart enough to know if what you just said was BS or not .
FWIW . During 308 load development while shooting the lower charges of IMR 4895 . I was using brass with a shoulder bump of .004 from fire formed . When the cases were ejected the primers were siting .004 above flush from a .003 below flush unfired resting point . This meant the firing pin forced the cartridge forward until it stopped on the chamber shoulder . The gases expanded the case enough to seal the chamber and hold it in place but did not create enough overall pressure to stretch the head of the case back up against the bolt face . This left that .004 of head clearance between the bolt face and case head . This allowed the primer to back out of the primer pocket that same .004 . .It takes much more pressure to stretch the case back to the bolt face then it does to simply expand the case enough to seal the chamber . So if all you case does is balloon it's self out to seal the chamber it very well could be shorter after firing then before it was fired if the pressure was low . What that pressure would be I have no idea but would assume it to be below 50k .
Another example would be case head separation .
How does this happen ? The front 2/3 of the case has expanded and sealed the chamber locking it's self to the chamber walls . While the lower third of the brass is to thick to expand to the size of the chamber . This allows the lower 1/3 of the case to move rearward towards the bolt face while the upper 2/3 stays stuck to the chamber walls . When this happens the case must give somewhere and that's at that pressure ring area where the brass is actually stretched longer making that material in that area thinner then before . Do this repeatedly with poorly sized cases and the case will completely separate in that area .
Measure a few NEW UNFIRED CASES before loading them. Then load them and shoot them. Then measure the length after firing but BEFORE resizing. You will find the cases are SHORTER.
The cases are stretched diametrally. When you FL resize them the stretched brass is forced back close to its original diameter. The stretched material cannot be compressed back to original thickness. The stretched material goes the only place that is not constrained. That is up the neck.
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This is for sure a tight headspaced gun
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