Boared at home....with .308 case & caliper

yomomma

yomomma

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Ontario - Hamilton area
So what do you do when your bored?

Well today I decided o look further into the M305 head space issue.

I had sent three casings to Hungry, unfortunately they were not fired with the gas system off.

So here I sit with a hand full of casings which were. First thing I did was to make myself a cartridge watchamacallit thingy. Took a piece of 3/4 aluminum round stock and drilled a 0.4000" hole through the center. OAL of the watchamacallit thingy is 1.292".

I measured 6 empty cases with the primer removed, done by hand. I removed any burrs that I found on the bottom of the cases. Measurements were as follows.
Measured DTWTHS
2.9330 1.6405
2.9345 1.6420
2.9360 1.6435
2.9345 1.6420
2.9240 1.6315
2.9335 1.641

What was interesting is that I decided to measure two loaded rounds and my Lee re-sized brass.

Measured DTWTHS
S&B
2.9155 1.623
PPU
2.921 1.6285
LEE
2.923 1.6305

*note DTWTHS = Derived Theoretical Watchamacallit Thingy Head Space

Lets assume all numbers being what they are,that they are valid.

I found it interesting that the S&B .308 is below the Minimum head space value for Win .308

7.62x51mm NATO Go 1.6355"
7.62x51mm NATO Field Reject 1.6455"

.308 Winchester Go 1.630"
.308 Winchester No Go 1.634"
.308 Winchester Field Reject 1.638"

7.62x51mm NATO minimum cartridge headspace 1.630
7.62x51mm NATO maximum cartridge headspace 1.633
7.62x51mm NATO average cartridge headspace 1.6315

.308 Winchester minimum cartridge headspace 1.627
.308 Winchester maximum cartridge headspace 1.633
.308 Winchester average cartridge headspace 1.630

The set up.

The watchamacallit thingy





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"...look further into the M305 head space issue..." Most of which is the result of early Polytech's imported, Stateside.
Headspace has nothing whatever to do with the cartridge. It's a rifle manufacturing tolerance only.
"...Field Reject..." No such thing. It's a 'Field' guage. Tells you if the headspace is within the max tolerance.
 
Seems like a good use of a slow afternoon, the cartridges will be below minimum headspace often as headspace is a chamber parameter, not cartridge. If your cartridge is over minimum headspace, such as with your Lee sized brass and the PPU, you would have a slight crush on chambering in a minimum headspace chamber. Some see this as desirable in bolt actions, M14s it can be dangerous, but that's another thread.

So in summary, cartridges likely should be at or below minimum headspace for .308 if they are marked as such.
 
sunray said:
"
Headspace has nothing whatever to do with the cartridge. It's a rifle manufacturing tolerance only.
.
Click to expand...

Isn't the internet great....so many opinions...

Headspace is term for the dimensions of a firearm's chamber & a cartridge. Most people assume headspace only refers to the internal dimensions of a chamber, but this is only half correct. Think of it as being similar to a shoe size measurement: size 10 gives you an idea if your foot will fit into a shoe or not. But the measurement itself is simply a way of determining if the cavity is too big, too small, or just right for your foot to fit into. Knowing that a shoe is a size 10 will do you no good unless you know that your foot is a size ten as well. The measurement is commonly used in reference to one object, but it's purpose is to determine compatibility of two objects.
Click to expand...
 
Much as I hate to give sunray any credit, in this one instance, his opinion holds water. Your quote there is a bit of a sideways way of explaining it, sure, the purpose of headspace in a chamber is to accommodate appropriate ammunition, appropriately. It doesn't mean ammunition will have the dimensions of a headspace gauge, or even that it will be over minimum headspace for the chamber, only that a minimum spec and sub-maximum spec chamber will safely accommodate the cartridge. There is certainly a relationship between the two, that's the whole point as everyone here is aware, but it doesn't mean they match or even run the same numbers. Gauge wise, those numbers apply to chambers, which will have a close but not number to number matched relationship with commercial ammunition.
 
The whole point of this is to generate discussion...so were doing good.

It is interesting non the less that the S&B is that much "under sized" in relation to the PPU and the resized brass. That sucker must rattle around those long norc chambers
 
I have a quantity of 7.62 brass that was accidently made with 5 thou too long to the shoulder. You can feel a lot of bolt compression in a bolt gun. It is great for my m14s that are typically about 8 thou deeper than minimum.
 
image.axd


Setting the headspace on a resized case is important to ensure reliable functioning of the rifle, avoid damage to the rifle, maximize case life and achieve consistent accuracy. The objective of this article is to show you a method of setting up a full length resizing die that allows you to quickly adjust its setting to achieve the desired amount of shoulder set-back (headspace) for a number of rifles chambered for the same cartridge or for different lots of brass.

"Headspace" in the context of the rifle chambered for a rimless cartridge is actually the distance from the bolt face to the point in the chamber that is halfway up the shoulder. In the reloading context we often use the term to indicate how much clearance have we built into the brass case as compared to that distance. Simply stated, if the headspace measurement on a resized piece of brass is 0.002" less than the chamber headspace of the rifle in which it was fired, we often say the brass has 0.002" headspace. That's not a technically correct use of the term, but it will do for most reloading purposes
Click to expand...
 
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