Why such varying headspace measurement on fired cases

Chas1

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Feb 15, 2009
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Cases in question are 308 FC factory loaded ammo once fired in same bolt action rifle. Using a Wilson case gauge the 67 once fired cases measure as follows:

14 cases @ 1.630
27 cases @ 1.631
17 cases @ 1.632
2 cases @ 1.633
3 cases @ 1.634
2 cases @ 1.635
1 case @ 1.636
1 case @ 1.638


That's a .008 headspace variance on cases fired and extracted from the same rifle. Can anyone offer any explanation on why such a widespread variance...is this normal ? I do see that most cases (86%) fall pretty close (1.630 - 1.632)...any thoughts ? Lastly if I were to bump shoulders back by typical .002 then I'd have cases that would span 1.628 - 1.636 that doesn't seem to make sense given the chamber is a fixed measurement and I would think that one would want all cases to have same headspacing...so am I supposed to just come up with some average headspace number and hope for the best ? Any input would be greatly appreciated, thanks
 

bigedp51

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Many times you have to fire a case several times before the case becomes a snug fit and shoulder needs to be bumped back. This varies with chamber pressure and brass spring back after firing.

Take one of your 1.630 length cases with the primer removed and and measure from the base to the case mouth and write it down.

Now just using your fingers start a fired spent primer into the primer pocket.







Now chamber the case, close the bolt and let the bolt face seat the primer.

Now measure the case again from the base of the primer to the case mouth.

Now subtract the first case measurement from the second and this is the primer protrusion.

The amount of primer protrusion is also your head clearance or the air space between the case and the bolt face. Normally on a bolt action you want .001 to .002 shoulder bump that should also be your head clearance.



The primer protrusion will tell you how much shorter the case is than the chamber at the shoulder. This will give you a idea of actual chamber headspace.


When you full length resize you squeeze the case and it will become longer. And if the die is adjusted to high off the shell holder you can actually make the case headspace longer than the chamber.



Place a .010 feeler gauge on top of the shell holder and screw your die down until the die contacts the feeler gauge.



Now resize a case and try chambering the case and see if you can close the bolt. If it does not chamber then use a .008, .006, .004 etc feeler gauge until the case chambers without resistance.
 

Chas1

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Do you know what they were before you fired them? ;)
Like said above sometime it can take 2 or 3 firings to get them all to the chamber size.
In hindsight I wish I knew what they were before, but don't.
 

Chas1

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Feb 15, 2009
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3,557
Many times you have to fire a case several times before the case becomes a snug fit and shoulder needs to be bumped back. This varies with chamber pressure and brass spring back after firing.

Take one of your 1.630 length cases with the primer removed and and measure from the base to the case mouth and write it down.

Now just using your fingers start a fired spent primer into the primer pocket.







Now chamber the case, close the bolt and let the bolt face seat the primer.

Now measure the case again from the base of the primer to the case mouth.

Now subtract the first case measurement from the second and this is the primer protrusion.

The amount of primer protrusion is also your head clearance or the air space between the case and the bolt face. Normally on a bolt action you want .001 to .002 shoulder bump that should also be your head clearance.



The primer protrusion will tell you how much shorter the case is than the chamber at the shoulder. This will give you a idea of actual chamber headspace.


When you full length resize you squeeze the case and it will become longer. And if the die is adjusted to high off the shell holder you can actually make the case headspace longer than the chamber.



Place a .010 feeler gauge on top of the shell holder and screw your die down until the die contacts the feeler gauge.



Now resize a case and try chambering the case and see if you can close the bolt. If it does not chamber then use a .008, .006, .004 etc feeler gauge until the case chambers without resistance.
Sounds like a great suggestion for measuring actual rifle headspace, I'm going to try that and see what I come up with for a measurement...and then maybe bump .002 down from that. Thanks, I'll post measurement.
 

Chas1

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Feb 15, 2009
Messages
3,557
Sounds like a great suggestion for measuring actual rifle headspace, I'm going to try that and see what I come up with for a measurement...and then maybe bump .002 down from that. Thanks, I'll post measurement.
Closed bolt on 3 cases with primers extended out and once ejected each measured 1.634. So looks like chamber has headspace of 1.634.
 

djtjr

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Oct 28, 2008
Messages
278
Location
New York, NY
Many times you have to fire a case several times before the case becomes a snug fit and shoulder needs to be bumped back. This varies with chamber pressure and brass spring back after firing.

Take one of your 1.630 length cases with the primer removed and and measure from the base to the case mouth and write it down.

Now just using your fingers start a fired spent primer into the primer pocket.







Now chamber the case, close the bolt and let the bolt face seat the primer.

Now measure the case again from the base of the primer to the case mouth.

Now subtract the first case measurement from the second and this is the primer protrusion.

The amount of primer protrusion is also your head clearance or the air space between the case and the bolt face. Normally on a bolt action you want .001 to .002 shoulder bump that should also be your head clearance.



The primer protrusion will tell you how much shorter the case is than the chamber at the shoulder. This will give you a idea of actual chamber headspace.


When you full length resize you squeeze the case and it will become longer. And if the die is adjusted to high off the shell holder you can actually make the case headspace longer than the chamber.



Place a .010 feeler gauge on top of the shell holder and screw your die down until the die contacts the feeler gauge.



Now resize a case and try chambering the case and see if you can close the bolt. If it does not chamber then use a .008, .006, .004 etc feeler gauge until the case chambers without resistance.

This is really helpful thanks
d
 

Mikecr

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5,241
Location
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bigedp51 provided method will give you headspace, while using a flat enough primer, I like it but you shouldn't bump from that. You should fully fire-form cases, with no body sizing, before 1st bumping/die setup -from the interference fit with your chamber at that point. This, because new shoulders move both forward AND change shoulder angle (affecting headspace tool datum and case fit). A lot of times necks will pull back from chamber end due to shoulder angle forming(messing with your trim length).
There is just no substitution for the local forming to your particular chamber.

It's also unlikely to fully form with a single shot. And there will be some HS variance from case to case for the life of the brass. With this we should check HS every case, every reload. No assumptions.
 

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