Use of Hornaday LnL Bump gauge

[scratcherky]

I recently got a bump gauge to check the head space length of 270 win cases against SAAMI specs. The 270 Win spec is 2.0526-.0070 inches from base of case to the .375 datum plane on the case shoulder. Using the gauge, my readings are consistently .010 to .011 below the minimum spec even on cases that have not been resized and on factory ammo. I have not been able to understand this and a Hornaday rep I spoke with could not explain it. Can any of you shed some light on this. I sure would appreciate it.

I know that I can still use the gauge to check how much I am bumping the shoulder back while resizing, but do not understand why I get the below minimum head space length readings.

[wncchester]

"... I have not been able to understand this .."
 
Well, it appears you misunderstand what that "bump gage" is supposed to do.  It's NOT a 'head space gage', it only gives a modestly accurate measure of that.  If it were made to be truly accurate for headspace it would cost MUCH more!  All it's supposed to do is let us find where the relative positions of fired and resized shoulders are; that's really all reloaders need.

[scratcherky]

Even though I referred to the gauges as bump gauges, Hornaday sells these as headspace gauge bushings. The only critical dimension on the gauge is the .375 inch dimension that determines the shoulder reference datum plane of the case. I verified this dimension. In order to measure the headspace length, it is a simple measurement with a dial caliper.
So even though, I agree that it can be used to get relative shoulder positions, I do not agree that I should not be able obtain accurate headspace lengths of cases. I still do not understand.

[necchi]

Where did you get the specs for the datum line?
Are you sure it's a .375 hole in that insert?

[scratcherky]

I got the specs from the SAAMI website and I measured the gauge hole and it is .375 inch. Here is the spec link:
 http://www.saami.org/PubResources/CC_Drawings/Rifle/270%20Winchester.pdf

[necchi]

Interesting, Thank You for the link.
 
I think it's a matter of interpretation. The listed measurement your looking at doesn't say Minimum or Max,,
 It's simply listed as you typed it, I think it means at that point it should measure between
2.0526 through 2.0070,,
 So that leaves an allowable variable of 0.0456 which is well within the range you are reading as you measure your cases,,
 
 When really dissecting SAMMI we have to keep in mind what it's meant to do,, and that's keep a "standard" where all ammo made for the caliber will fit in all guns chambered for that caliber. Nothing is carved in stone for what the over the counter components have to be exactly.
 They just have to be within a given range.
  Now on the same page is SAMMI chamber spec's and we see the same datum location actually lists Min and Max, where the Minimum is tighter than the max listed for the case makeing a 0.0039 crush at that particular datum location, 
 
I'm just guessing mind ya about all this, maybe someone else has a better explaination.
 
 What I've done is take cerrosafe castings of my chambers so I know what the reality is, then physically check brass in the camber for fit. I do know that it takes more than one firing to blow a new case out to the chamber dimentions then I can think about shoulder bump, at least in my guns.
 
 Trouble I've got isn't shoulder bump issues it's body size. My chamber body size is big, for me to get brass to fit useing an FL die I have too bump the shoulder a bit just to get the body down to where it fit's the chamber.

[scratcherky]

necchi, thanks for the reply.
The 2.0526-.0070 is really not open to interpretation IMO. That is just a method of putting a dimension and tolerance on an engineering drawing. Another method would be 2.0526+.0000/-.0070. Given that this particular dimension would be 2.0456 to 2.0526 inches with 2.0456 being the minimum. My measurements are all below minimum by .010 to .011 inch.
I have never seen a rifle where one firing of a full load would not blow the brass out to chamber dimensions. In my rifles I can just neck size fired brass bumping the shoulder back .001 to .002 inch as long as I fire the reloads in the same rifle.

[wncchester]

"I agree that it can be used to get relative shoulder positions, I do not agree that I should not be able obtain accurate headspace lengths of cases. I still do not understand."
 
First, headspace is in the chamber, not the cartridge, so no matter what Hornady calls it, it's only a case shoulder gage (and that not highly precise). 
 
We cannot measure our fired cases and obtain an accurate reading of the chamber because a fired case is a very poor transfer standard.  If you haven't noticed it, individual cases from the same box will springback differently enough to vary as much as 3-4 thou differece in shoulder position during both firing and resizing.   As reloaders, we don't need to agonise over headspace; the best we can do is make our reloads match whatever the chamber is.  At least for bolt guns, there is no valid reason to bump a shoulder back further than the fired position because the fired shoulder length is already 1 or 2 thou shorter than the chamber.
 
Boring a .375", etc, datum line is easy enough.  But to achieve what you seek for length would require that each part of the gage be precisely machined for length, then precisely assembled and precisely mounted on the jaws of your caliper; that's not going to happen very often.  And even if they were that precise it would demand we be capable of precisely repeating the pressure we place on the case being measured; that's not a common skill.
 
Bottom line, forget trying to match a SAAMI dimension and use your excellant little gage to make ammo that fits your chamber, it's quite precise for doing that.