Headspace 10-round sample

[Chas1]

Loaded up 10 rounds of 308 Winchester, Went to range, all fired w/o issue, no failure to feed, eject or stuck cases. headspace on 10 cases ranging from .002 above Wilson high step to -.0015 below high step. the average being .0005 above high step. I've been told the high step on the 308 Wilson case gauge is 1.634 if so then measurements range from a high of 1.636 to a low of 1.6325 with the average being 1.6345. And now for what might be the dumb questions but, I'll ask anyway.

1.) Does a .0035 spread on measurements make sense...is that sometimes normal ?
2.) My chamber headspace=1.632 so cases with longer headspace is this due to brass springing back

Any answers and/or suggestions are appreciated.

 

[dmoon]

Mainly tagging in to see the answers here. But I'm sure you are going to be asked how you came up with your chamber headspace. My gut says that number is not accurate, as all of the cases you measured are above the chamber measurement. 3.5 thousandths is not much, but I bet your chamber measurement is actually longer than the 1.636 measurement of your longest case.

 

[Chas1]

Mainly tagging in to see the answers here. But I'm sure you are going to be asked how you came up with your chamber headspace. My gut says that number is not accurate, as all of the cases you measured are above the chamber measurement. 3.5 thousandths is not much, but I bet your chamber measurement is actually longer than the 1.636 measurement of your longest case.

Manufacturer assigns a tag to each firearm produced mine said headspace was set to 1.632

 

[dmoon]

1.632 is the SAAMI center of the MIN/MAX tolerance. There is almost zero chance it actually measures that. If 10 fired cases measure longer than that, your chamber is longer than that.

 

[Chas1]

1.632 is the SAAMI center of the MIN/MAX tolerance. There is almost zero chance it actually measures that. If 10 fired cases measure longer than that, your chamber is longer than that.

Longest case=1.636 then would you agree my chamber is over 308 SAMMI spec ?

 

[Howland]

SAAMI shows 1.630" min and 1.640" max. I'll be wrestling with this soon as I'm getting back into reloading and have 3 .308s to load for and expect 3 different headspace measurements to plan around.

There is a school of thought that says you will not get case stretch to full chamber headspace dimension on the first firing and your instincts are already wondering about springback. My plan was to look for a skewed distribution of case headspace measurements and bumping the shoulders back just before getting to the abnormal slope of the curve.

I'd definitely be interested in hearing from those who've already crossed this bridge.

 

[dmoon]

Longest case=1.636 then would you agree my chamber is over 308 SAMMI spec ?

I do agree, and I would only size the necks until they are tight in the chamber to get a better measurement.

 

[MagnumManiac]

Once fired brass will not give you accurate measurement, neither will a Wilson gauge.

You need to fire the same brass with neck sizing only for at least 3 firings or until a fired case is difficult to chamber on bolt turn down.
Due to brass springback, your measurements could be .002"-.004" below the ACTUAL chamber length.
I used to think once fired brass would be 'close enough', often it isn't. If you take a chamber cast, you will see the difference.

Also, the most accurate tool for measuring fired brass I have ever used is the RCBS Precision Mic. Easy to use, as with the Hornady comparator it is easy to get erroneous results from the case being tipped on the vernier, the Precision Mic simply screws onto the case and is very accurate and easy to read.

Cheers.

 

[bigedp51]

I prefer the Hornady cartridge case headspace gauge over the RCBS Precision Mic.

1. You can spin the case with the Hornady gauge and center the case to get accurate readings.
2. With the Hornady gauge you do not have to buy a separate gauge for each cartridge.

Bottom line to Chas1, buy the Hornady gauge and start using your Wilson type case gauges for pen holders and paper weights. Then measure your new cases and fired cases and see how much shorter the new unfired cases are. Then check each full length resized case for shoulder location uniformity. Meaning brass spring back after sizing.

Then pause at the top of the ram stroke for 4 or 5 seconds with each case that is resized. Pausing will reduce brass spring back and make the shoulder location more uniform.

I never neck size my cases and fire them mutable times to get shoulder location. A new case has soft annealed brass in the neck and shoulder area. Meaning measuring a new case that was fired with close to maximum load is close enough to figure shoulder bump. If you measured a older case that was fired many times and has not been annealed you would have more brass spring back.

Plus when you full length resize the case can be resized and made longer than the chamber. Meaning starting with the die not touching the shell holder and working downward and chambering the sized case you can go by feel when closing the bolt. And keep adjust the die downward until the case chambers freely.

 

[parshal]

The most accurate way to determine headspace is to get a go gauge and do it like your smith. Add shims/tape/etc. to the head of the go gauge until you have resistance when closing bolt. Measure the thickness of your shims and that's your headspace.

M... starting with the die not touching the shell holder and working downward and chambering the sized case you can go by feel when closing the bolt. And keep adjust the die downward until the case chambers freely.

This is the best way to set your die for your chamber and the actual headspace dimension doesn't really matter.

 

[bigedp51]

Chas1
You can not used a Wilson type case gauge to check a "fired" case. This is because the "fired" case diameter can prevent the case from dropping all the way into the gauge.

The Wilson type drop in case gauges are used to check full length resized cases. And if the case falls between the low and high steps in the gauge the case is "approximately" the correct size to fit and chamber.

Below a Colt 5.56 Field gauge at 1.4736.

Below the same Colt headspace gauge in my "adjusted" Hornady cartridge case headspace gauge. And is reading as close as I can get it to the Colt headspace gauge. Meaning the vernier caliper is only accurate to .0005 so the reading is very close.

Below a "fired" case from my AR15 at 1.4675

I then used Redding competition shell holders and adjusted the die to bump the case shoulder back .003. If I had adjusted the die per the dies instructions with the die making hard contact and press cam over the case shoulder would have been pushed back .007. Meaning shorter case life and possible case head separations.

Below the cases were fired in a brand new Savage .308 rifle and full length resized with the die making hard contact with the shell holder. Meaning the die was "not" set up for minimum shoulder bump of .001 to .002.

The difference in case life was due to case wall thickness and the hardness and quality of the brass.

Bottom line, you need a Hornady gauge to measure your fired cases and then set your die up for minimum shoulder bump. And once you get the Hornady gauge you can use the Wilson case gauge as a pen holder and paper weight.

 

[dmoon]

Biged, straighten me out on this please. You show a gauge, that I am assuming is a go-gauge (which would be approximately .003 short of the maximum chamber dimension), and a case that is .0055 short of your gauge. You say you are bumping the fired case back another .003. In the end you are 0.0091 short of your go-gauge right?

 

[bigedp51]

Biged, straighten me out on this please. You show a gauge, that I am assuming is a go-gauge (which would be approximately .003 short of the maximum chamber dimension), and a case that is .0055 short of your gauge. You say you are bumping the fired case back another .003. In the end you are 0.0091 short of your go-gauge right?

The Colt Field gauge pictured is maximum military headspace and not a GO gauge. I just grabbed the closest .223/5.56 headspace gauge to adjust the Hornady gauge and it happened to be the Field gauge.

If you look at a SAAMI cartridge and chamber drawing the headspace is listed as Min and Max with .010 between the two gauges. The GO and NO-GO gauges are used for checking and adjust headspace on "new" rifles. And the field gauge is for checking older worn rifles for maximum allowable headspace.

I adjusted the Hornady cartridge case headspace gauge by placing a .011 feeler gauge between the red and silver sections of the gauge. Meaning the unadjusted Hornady gauge was reading .011 shorter than the Field gauge. Yes the Hornady gauge is just a comparator gauge "BUT" if adjusted it will give very close measurements to the actual chamber length.

Your head clearance is the "air space" between the rear of the case and the bolt face. And how far the case must stretch to meet the bolt face when fired. And your shoulder bump is the same approximate distance as you head clearance, "IF" the sizing die is adjusted correctly.

 

[MagnumManiac]

Biged,
As I said, I find the RCBS Precision Mic to be the MOST accurate to use.
I also have the Hornady comparator, it is not as accurate, even when spinning the case.

The Precision Mic is also very good if you have several cartridges based on the same case design, like a 243, 30-06 or short belted case like a 338WM.
I can use the same mic for 25-06, 270 & 338-06, no need to buy another.
Same is true for 264WM & 338WM.
I also use the same one for 300 Weatherby, 340 Weatherby & 375 Weatherby. The only one I have that is unique is the one for 300WM.
They give me far more accurate readings than the hornady gauge does, which is more of a comparator than an actual measuring gauge.

Cheers.