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Thread: Headspace measurement
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27-07-2020, 11:39 #1
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Headspace measurement
How much headspace is too much?
My 7x64 is not new. We can chamber brass fireformed in a friends (new) 7x64 in this rifle, but not vice versa.
New brass fired from my rifle seems to extend the case shoulder (on datum line) from 53.36 to 53.9mm after springback, an increase in the region of .54mm.
Once fired, this new brass grows from 63.86 to 64.32mm OAL, whilst chambering easily after so shot. However, I can get 4 patches of scotch tape extentions between the base of this specific case and the boltface before experiencing chambering with slight stiffness. This extended case measures 64.46mm OAL / 54.02 at the datum line, but could be subject to compression due to torque on closing.
Sami specs suggest the datum line (in the middle of the shoulder) of the chamber to be around 53.43mm. The scotch tape case is at 54.02, a differance of .585mm, which is consistent with the amount the case' OAL grows upon firing.
A new case datum line to chamber's datum line seems to have a difference of .66mm. Is this too much?
*** Measurements are via dial caliper. Datum line measurements are made with the aid of a resized, cut, deburred .357 case inverted "on top" of the 7x64 shoulder, that seems to find the datum line.
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27-07-2020, 12:51 #2
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Re: Headspace measurement
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27-07-2020, 13:03 #3
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Re: Headspace measurement
The difference between the SAAMI go and no-go gage is 0.010" (0.254mm). 53.231mm to 53.485mm. This also agrees with the min and max dimensions on the SAAMI drawing. If your chamber does not fall in that range, the chamber is out of spec.
So, either the headspace is way out, or your measuring equipment. A competent gunsmith can give you a definitive answer.
"A new case datum line to chamber's datum line seems to have a difference of .66mm. Is this too much?"
Yes. According to the drawing, it should not be more than 0.433mm. Although, sometimes new cases are slightly smaller than SAAMI spec.
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27-07-2020, 13:10 #4
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Attached is the SAAMI chamber and cartridge drawing
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27-07-2020, 13:25 #5
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Re: Headspace measurement
Have you had a look at the CIP data, as the 7x64 is an European caliber and they would have supplied first data.
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27-07-2020, 13:51 #6
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Re: Headspace measurement
Yes; from drawings I checked the Saami & CIP specifications are not identical, but very close to one another.
I'm pretty sure the headspace is out of specification.
Is it worth it to ream the chamber and set the barrel back? Almost cheaper to buy a Howa...
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27-07-2020, 13:56 #7
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27-07-2020, 13:57 #8
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Re: Headspace measurement
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27-07-2020, 14:03 #9
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Re: Headspace measurement
As the base line is not clearly defined on the shoulder, I thought that dims I posted might be easier to measure.
If you do have a headspace problem then just don't push the shoulder back as in a normal full length size.
How long have you been reloading for this rifle and have you had case separations?
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27-07-2020, 14:39 #10
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Re: Headspace measurement
Yes: I have had a case separation on a fresh case. However, with this process of measuring the shoulder I've realized that I've been resizing incorrectly for some time, seating the dies too deep. I like to neck size fresh brass to iron out the dents inthe case mouth, but I might have been too eager. This rifle has seen some fair use over the past 5 years.
I'm hoping to maintain the fireformed cases's shoulder with a next batch, to see what the result might be. If I am able to maintain a "longer" case in the chamber within a lower end of the spectrum pressure, I might get some further use of the barrel. Driepoot, when does excessive headspace become too dangerous?
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