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Old April 4, 2014, 09:40 AM   #1
Bart B.
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Measuring Rimless Bottleneck Chamber Headspace

Some years ago, I wondered what my .308 Win barrels' chambers were actually at. Borrowed an adjustable headspace gauge with a micrometer type adjustment easily interpreted to .0002" accuracy. Finding the settings where one longer by .001" didn't let the bolt close and another .001" less did, the actual chamber headspace ended up being 1.632"

Fired cases from that chamber had a case headspace of about .001" less; 1.631". Two other .308 Win rifles had the same difference between actual chamber headspace and fired case headspace. Same thing with a .30-06 and .270 Win chambered rifle with an adjustable headspace gauge for them.

Therefore, it's my opinion that one can measure their rifle's chamber headspace quite accurately by measuring a fired case (one that's been shot with normal, max loads or a factory round) then adding .001".

Last edited by Bart B.; April 4, 2014 at 11:25 AM.
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Old April 4, 2014, 09:48 AM   #2
Jimro
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Bart B.,

Your logic is sound for bolt action rifles, I'd take measurements from multiple once fired cases though. I don't think it would work for semi-auto rifles though.

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Old April 4, 2014, 11:20 AM   #3
Bart B.
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I need to mention that if you rifle's bolt face is a bit out of square with the chamber axis, fired cases from it will also have their heads a bit out of square. As they're indexed differently in the chamber, a given case will let the bolt close easily at one orientation but 180 degrees out the bolt will bind.

As always, chamber headspace measurements are best taken with a stripped bolt. Interference from springs and other parts can cause false, inaccurate readings. Best way to close the bolt on anything used to "gauge" the chamber is with the rifle upright and horizontal, push the bolt closed then tip its handle over and let it fall naturally into the closed position. Using any force, especialy with bottleneck cases, can cause the case brass to give a bit and end up with its shoulder pushed back a bit.
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Old April 4, 2014, 12:25 PM   #4
Unclenick
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Bart,

It has to be somewhere in that ballpark just because the malleability of brass doesn't let it stretch too far before it yields. Did you get to try this at different load pressures to see how much difference that makes? Full pressure neck sized-only cases will gradually grow too snug to chamber when they've been through enough load cycles, requiring a cycle with full length resizing before they can start to be loaded by neck sizing-only again. But if the loads are on the lighter side, they seem to go on forever (well, until the neck splits, if you don't anneal them).

That suggests a pressure sign. If you know what size brass is coming out of your chamber at normal pressures, if it starts coming out longer, the steel must be stretching. I know M.L. McPherson uses case growth as a pressure sign in Marlin 1895's, but there the locking lug is so far behind the boltface and the length of receiver walls being stretched is longer than in a front lug bolt gun, so I just assumed it wouldn't work in the latter. But maybe dragging a Precision Mic or similar tool to the range to monitor growth from head to shoulder datum in bottleneck cases during load workup isn't a bad idea for a second opinion.
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Old April 4, 2014, 12:26 PM   #5
Slamfire
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Quote:
Some years ago, I wondered what my .308 Win barrels' chambers were actually at. Borrowed an adjustable headspace gauge with a micrometer type adjustment easily interpreted to .0002" accuracy. Finding the settings where one longer by .001" didn't let the bolt close and another .001" less did, the actual chamber headspace ended up being 1.632"

Fired cases from that chamber had a case headspace of about .001" less; 1.631". Two other .308 Win rifles had the same difference between actual chamber headspace and fired case headspace. Same thing with a .30-06 and .270 Win chambered rifle with an adjustable headspace gauge for them.

Therefore, it's my opinion that one can measure their rifle's chamber headspace quite accurately by measuring a fired case (one that's been shot with normal, max loads or a factory round) then adding .001".
I hope someone with a material background could pipe in and give us an idea of just how accurate a technique measuring headspace from fired cases is, given that brass relaxes after the pressure is gone.
I measure fired brass all the time, but not in the chamber. I use Wilson type chamber gages, because I don’t trust the chamber drop technique.



Once I purchased gages I went and checked the headspace of cartridges I sized, and these cases were sized in dies that were set up according to the chamber drop technique. Case headspace was all over the place. I believe this is due to the loss of sensitivity that happens when you are, essentially crushing a brass tube, at the end of another steel tube, that has a handle, and you are doing performing this with cammed surfaces. The worst was the 7.5 Swiss. That rifle is a straight pull and there is an inner sleeve to the bolt, and after you take the firing pin mechanism out, the empty bolt still has to be cammed shut with the bolt handle.

I drop fired cases in the headspace gages and I am confident that the measured results are “good enough for Government work”.

It is also instructive to buy a set of calibrated gages. I have pin gages and starrett block gages, http://www.starrett.com/metrology/me...ck-information. Just take a Starrett block gage, made to a millionth of an inch, and measure it a couple of times with whatever caliper you have.

{Edit: Sorry, you can't link to copyrighted materials. Have to take your own picture of the gages in this instance. See the board copyrighted materials policy, here.}


Maybe you are the lucky type and can repeatedly measure to a ten thousandth’s, but I know, I am not.
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Last edited by Unclenick; April 7, 2014 at 02:58 PM. Reason: Removing copyrighted materials
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Old April 4, 2014, 03:27 PM   #6
Bart B.
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Unclenick, I only used normal max loads as well as commercial and MIL-SPEC ammo doing those tests. Didn't use any reduced loads as I already knew they'll sometimes end up with less case headspace after firing than when new.

Nor did I do any tests with neck only sized cases. But I've been told they grow in case headspace a tiny bit every reload and shoot cycle until the bolt starts binding. I believe that some of that binding is caused by out of square bolt faces and case heads as case headspace grows to the limits of the chamber and/or locking lugs' surfaces are at a slight angle relative to the bolt face (like airplane propellers) and the last 15 to 20 degrees of rotating the bolt closed pushes the case forward against the chamber shoulder.

And dragging a case headspace gauge to the range for measuring fired cases used in load workup is a good idea.

Slamfire, when I measured a given case after firing, I got repeatable readings across a .0002" spread. It took a while to just barely grip the thimble on my case headspace gauge so the same amount of torque was put on each fired case. The next case would have about the same spread but centered several ten-thousandths away. There was a bit over .001" spread across all cases The adjustment thimble on my old A.R. Martin headspace gauge thimble had 40 tpi and its diameter was enough (almost 7/8ths inch) that the .001" graduations were just over 1/10th inch apart. I'd zeroed it with my Forster .308 Win. GO headspace gauge.

When I resized cases such that they had headspace about .002" longer than when fired, they started binding when I closed a stripped bolt on them. This was my basis for claiming fired max load cases' headspace is about .001" less than actual chamber headspace. That aside, someone else doing the same tests with their stuff may get slightly different readings and beliefs. My way was probably not an exact science, but close enough for my interests.

Last edited by Bart B.; April 4, 2014 at 04:57 PM.
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