Head Space Gauge Tutorial

[Lapua40X]

Just discovered a head scratching data conflict that, hopefully, some of you more experienced reloaders can explain.
The 6BR Norma data on http://www.6mmbr.com/6mmbr.html sets the neck/shoulder intersection at 1.2378. A brand spankin' new Laupa case measures 1.1630 on my head space gauge, and my full length resized cases (Wilson 6BR Norma die) measure 1.1540.
Makes a guy wonder where to aim.

 

[jonbearman]

How are you measuring your case's?

 

[jelrod1]

I just measured a new Lapua 6br and by eyeball with a caliper it is 1.236. What kind of headspace gage do you have that checks the neck to shoulder intersection?

 

[fdshuster]

My PTG reamer print drawings (3) all show 1.2371". Crude measuring with the Starrett dial caliper puts it in the same ballpark.

 

[jelrod1]

After reading the OP over again I'm going to assume a few things and try to give a better answer than I did before. I will assume you are using a Hornady style caliper attachment gage. Also assuming you are subtracting the gage length from your measurement to come up with your numbers. If so there is a hole in it that probably does not match the SAAMI diameter that measures how far down the shoulder "headspace" is.

The 1.2378 dimension means nothing for what you are trying to accomplish. It is the neck, shoulder juction not where the gage is measuring.

For reloading purposes you are using the gage as a comparator to before and after dimensions. I use a once or twice fired case as my "zero" on the gage and aim to either be at, or .001 below that on sized cases depending on the caliber. Keeps all the brass the same, works it less, ect.. I use a different style gage but the application is the same.

Based on the OP it looks like you set the shoulder back .009 from new. I don't size new cases only run a mandrel through them. Pushing the shoulder back from new does'nt accomplish anything but overworking the brass right from the start. You may not have done this, but I'm assuming some things since it wasn't stated whether anything was fired or not.

 

[Lapua40X]

Thanks guys. So far, so good. Using jelrod's outline as a reference:


I will assume you are using a Hornady style caliper attachment gage.
Yes sir, that's correct
Also assuming you are subtracting the gage length from your measurement to come up with your numbers.
That too is correct
If so there is a hole in it that probably does not match the SAAMI diameter that measures how far down the shoulder "headspace" is.
That's where I get confused I know that "headspace" is calculated at the datum line and that my measurements don't actually reflect that measurement, but it seems to me that publishing SAMMI specs. for "headspace" that can't be verified using available reloading instruments is useless information.
For reloading purposes you are using the gage as a comparator to before and after dimensions.
That helps a lot. At least it makes me feel more comfortable with the readings I'm getting. When I look at a spec. and can't get results that match them I tend to get anxious ... "safety obsession" I suppose. :

 

[bigedp51]

The only way to get an accurate "zero" reference point with the gauges below is with a GO headspace gauge, then you can take off your shoes and do the higher math.

 

[jsn]

Which bushing are you using? According to the drawing fdshuster provided you should be using a bushing for a 30 degree neck, if you aren't you will get a number that is way off.

Otherwise you Hornady gauges should be within .010 if everyone hit the specification when producing whatever they produced.

 

[BoydAllen]

The Hornady tool is mislabeled. It is a comparitor, not a headspace gauge. It is made for comparing head to shoulder measurements, not giving an absolute number. If you want a standard to look at things with, get a GO gauge and measure it and your various cases with your Hornady tool, and note the differences, not the absolute numbers. If you do some checking, I think that you will find that all new, unfired brass is short from head to shoulder. That is the whole story.

 

[gotcha]

Not to be contentious but I've had chambers where I set HS tight to the gauge. Then on second firing had brass HS .001" to .0015" longer than gauge. ??? A good friend & BR Gunsmith told me this was not uncommon due to the tremendous stress put on bolt during ignition. Bolt lift was heavy & re-chambering the fired cartridge was akin to shoulder bumping and actually shortened (.001") the case HS. Has anyone else experienced this phenomenon? I'm guessing that flex in an action could also cause this? This was a SA cartridge in a LA. Not trying to muddle this post, but rather, offer an observation that might be pertinent. Dale

 

[mikecr]

Gotcha, that case bodies grip chamber walls on firing, and stretch a bit to allow casehead to reach boltface, is normal.
It can and should be managed, and setting HS by a book is NOT what I mean by managing it.

Reloaders here have implied that HS is 'relative' to each chamber & tool, and this is true.
This is not SAAMI standard values from SAMMI set shoulder datums. It's a value your chamber tells you, having fireformed brass in it, and while consistently using pretty much any tool that provides any datum on a linearly angled shoulder.
Not sure it's so simple with radiused shoulders..

 

[BoydAllen]

Gotcha,
What you described is the reason that FL sizing for every reloading(with good die to chamber fit, and precise bump) has become so common in short range benchrest. I have not measured the interference, but it can definitely be there based on bolt feel. The other area that tightens is at the base of the case. One thing that will increase the problem is if locking lugs do not bear properly on their abutments, which can be very common in factory actions.
Boyd

 

[jelrod1]

The Hornady tool is mislabeled. It is a comparitor, not a headspace gauge.

Yeah that bothers me too. The same person probably named it that named the concentricity gage. Concentricity is included in the measurement, but you are actually measuring runout. Concentricity is measuring how the centers of two circular features are related and assumes the diameters are "perfect". Runout measured on rollers and an indicator accounts for concentricity but also form factors like roundness and profile. Measured in one spot it's called runout and if measured the entire specified length it is total runout. Doesn't really matter for the purpose but I think about it every time I see it.

 

[gotcha]

Mikecr & Boyd, Thanks for the input. I think you've both confirmed the mechanics of the phenomenon I've observed. I've checked before & after measurements when sizing. But, hadn't given thought to the importance of checking the cartridge base/web. You can bet I won't miss it next time BTW lugs were hand lapped as opposed to machine cut to match the lug recess. Could this be a factor?? Thanks, Dale