Head Space Measurement Mysteries

[BoydAllen]

When we bump shoulders(usually in the range of .0005 to .002), the case body IS supported . The issue with sizing the case far enough down has a lot to do with the "coverage" of the chamber, vs. that of the functional part of the interior of the die. If there is a line at the bottom of the case that is just inside of the chamber that the die cannot reach, and the die is such a close match to the chamber that it barely reduces the diameter of the body of the case, there will be problems if hot loads are used. One of the things that can contribute to this is how dies are commonly chamfered at their bottom opening. Anything that is done here, beyond just breaking the corner, and smoothing it, reduces the die's coverage. The problem is that like all machined parts, there are tolerances for the depth that chambers are reamed to as well as the same thing for dies. For many, the easiest way to resolve these issues, which can work very well, is to simply remove some metal from the top of the shell holder. Just because you find this solution inelegant, is irrelevant, it has worked very well for a lot of benchrest shooters, to solve a problem with the least amount of effort, and no negative consequences. We can measure sized cases, and how they fit in their chambers, by a combination of comparing measurements before and after sizing, and the feel as they are chambered.

 

[fguffey]

When we bump shoulders(usually in the range of .0005 to .002), the case body IS supported .

I agree, again, I can not get a die to bump the shoulder before the neck and die body contacts the die, that makes it difficult for me to place shoulder bump in front of full length sizing and case forming, meaning I control full length sizing in thousandths.

F. Guffey

 

[BoydAllen]

By using custom dies we end up with the desired amount of shoulder bump at the same die position that gives us the amount of body diameter reduction that we want. For instance, I have a Harrell die that only reduces the base of my cases about .0005 (from the diameter of a tight one) and I cannot measure any reduction in diameter at the shoulder. This is with the die set to bump the shoulder back .001 from the largest measurement obtainable after repeated (of warm loads) firings with neck sized brass. We can have our cake, and eat it too. When I chamber a case that has been sized with this die, the bolt closes as it should. The whole key to being able to do this is close coordination between chamber and die dimensions. In short range benchrest, warm to hot loads are pretty normal, and for those we cannot get away with neck, or neck and shoulder bump sizing. We need to FL size, while at the same time moving the brass as little as possible, keeping the case as straight as we can.

 

[fguffey]

Quick background...been building and chambering all my rifles for25+ yrs. Primarily a HP/SR XC & LR shooter and have done very well over the years. I have always used complete sets of HS gauges when chambering and building. No issue there. For simplicity sake - let me limit this discussion to .308 Win. I normally chamber/ HS to 1.631" in a bolt gun and 1.634" in a gas gun. Staright forward stuff.
What starting me thinking and ended up resulting in insomnia was the transferability of measurements between gauges, measuring tools, dies, brass and everything else connected to HS.
I checked the HS of a 40X .308 rifle with Forster HS gauges recently - 1.631" - right where it was supposed to be. I moved to check the brass that I thought I had fired in this rifle - not having a baseline and using a Stoney Point (Hornandy) insert that was stamped .400" (datum line for the .308) I decided to "calibrate" the insert gauge on the actual Headspace gauge.....WOW. I zeroed the dial calipers (8" Mitutoyo Digitals to .0005), inserted the HS gauge and I got a HS reading that was .011" short of what the gauge was stamped. (1.630")
I checked every gauge - each one was .011 short. This lead me to check the Stoney Point insert - the diameter of that hole (datum line) should be .400" - it was...so where is this massive error coming from?
I decided to check a different method - using a Redding Instant Indicator set up for HS and the proper .400 Redding insert - I measured all the gauges again - they were correctly sized relative to one another; .001 steps from 1.630 - 1.638. This set up will not give a direct reading - just a comparative one. So - I measure the brass with the Stoney Point - 1.631". I put the Forster HS Gauges back in the caliper / Stoney Point - .011" short. Check the brass in the Redding Instant Indicator: +.001" right on. So, now I am wondering - what is going on? What am I missing?
Any understand this or have experience on this? I get there is tolerance stacking and variability between mfr's - but this is just down right bizarre. Is it related to the shoulder angles of the gauge or the contact edge of the Stoney Point insert? I have considered making my own inserts gauged exactly to datum line dimensions.
Any help would be appreciated.

How can this be? What am I missing?

Sounds great Mr. Allen, back to the original question, he has gages, he has dies, presses and cases, then there are chambers, sounds to me like he wants all pieces, parts and attachments to agree, I start with standards, transfers and verifying. Again, I start with .000”, There are tons of space used up on reloading forms covering ‘HEAD SPACE’, the length of the chamber from the datum/shoulder to the bolt face, to me? head space is fill in the blank_________________., meaning head space is the length of the chamber. Once I have determined the length of the chamber I off set the length of the chamber with the the length of the case from the shoulder/datum to the head of the case. When I determine the length of the chamber I am forced to use one standard, .000”. I can not have a different standard for every tool I use.

I understand head space gages are coveted, a mistake is made when the user assumes it is correct. The head space can be used as a standard if the user understands the head space gage can be used to check the accuracy of a Wilson case gage, the head space gage can be used to check the accuracy of a full length sizer die. Back to one standard, one standard can determine if all the tools being used agree, again, I have one standard, I start at .000”.

F. Guffey

 

[butchlambert]

Quick background...been building and chambering all my rifles for25+ yrs. Primarily a HP/SR XC & LR shooter and have done very well over the years. I have always used complete sets of HS gauges when chambering and building. No issue there. For simplicity sake - let me limit this discussion to .308 Win. I normally chamber/ HS to 1.631" in a bolt gun and 1.634" in a gas gun. Staright forward stuff.
What starting me thinking and ended up resulting in insomnia was the transferability of measurements between gauges, measuring tools, dies, brass and everything else connected to HS.
I checked the HS of a 40X .308 rifle with Forster HS gauges recently - 1.631" - right where it was supposed to be. I moved to check the brass that I thought I had fired in this rifle - not having a baseline and using a Stoney Point (Hornandy) insert that was stamped .400" (datum line for the .308) I decided to "calibrate" the insert gauge on the actual Headspace gauge.....WOW. I zeroed the dial calipers (8" Mitutoyo Digitals to .0005), inserted the HS gauge and I got a HS reading that was .011" short of what the gauge was stamped. (1.630")
I checked every gauge - each one was .011 short. This lead me to check the Stoney Point insert - the diameter of that hole (datum line) should be .400" - it was...so where is this massive error coming from?
I decided to check a different method - using a Redding Instant Indicator set up for HS and the proper .400 Redding insert - I measured all the gauges again - they were correctly sized relative to one another; .001 steps from 1.630 - 1.638. This set up will not give a direct reading - just a comparative one. So - I measure the brass with the Stoney Point - 1.631". I put the Forster HS Gauges back in the caliper / Stoney Point - .011" short. Check the brass in the Redding Instant Indicator: +.001" right on. So, now I am wondering - what is going on? What am I missing?
Any understand this or have experience on this? I get there is tolerance stacking and variability between mfr's - but this is just down right bizarre. Is it related to the shoulder angles of the gauge or the contact edge of the Stoney Point insert? I have considered making my own inserts gauged exactly to datum line dimensions.
Any help would be appreciated.

How can this be? What am I missing?

Sounds great Mr. Allen, back to the original question, he has gages, he has dies, presses and cases, then there are chambers, sounds to me like he wants all pieces, parts and attachments to agree, I start with standards, transfers and verifying. Again, I start with .000”, There are tons of space used up on reloading forms covering ‘HEAD SPACE’, the length of the chamber from the datum/shoulder to the bolt face, to me? head space is fill in the blank_________________., meaning head space is the length of the chamber. Once I have determined the length of the chamber I off set the length of the chamber with the the length of the case from the shoulder/datum to the head of the case. When I determine the length of the chamber I am forced to use one standard, .000”. I can not have a different standard for every tool I use.

I understand head space gages are coveted, a mistake is made when the user assumes it is correct. The head space can be used as a standard if the user understands the head space gage can be used to check the accuracy of a Wilson case gage, the head space gage can be used to check the accuracy of a full length sizer die. Back to one standard, one standard can determine if all the tools being used agree, again, I have one standard, I start at .000”.

F. Guffey

Give it up old man. You're just wanting to see yourself in print.

 

[okedoke]

thanks Butch & Boyd, I have never seen some one so void of basic logic (stop this asap) --- good day

 

[fguffey]

Give it up old man. You're just wanting to see yourself in print.

before that:


Methanks that you just want to argue for argument's sake. I will leave it at this. I make my own full length dies with a reamer that Dave Kiff made based on my chambering reamer. It works, I have no click raising my bolt to extract and It does not over work my brass.

before that:

In the "olden days" I made a die that would size the body only all the way down. I had to trim the shell holder down to get the ram that low. I have bump dies, and I no longer have any neck dies, except for a couple hunting rounds. If you make a custom die to size your brass body to your desired dimension and make a .001 bump if necessary, you are not overworking your brass. Take a chamber cast and compare it to your sized case.

And still, I ask, all the way down to what or where, again I am told to make a chamber case, not my intent but I promise, we will disagree with methods and techniques used on making chamber cast.

F. Guffey

 

[fguffey]

I never assume I have the right to disagree, If I disagree I simply disagree, If Mr. Allen and or butchlambert has a question or there is something they do not understand let them ask.

F. Guffey

 

[BoydAllen]

Back at you ;D

 

[fguffey]

Boyd Allen, Thank you for the display of class.

F. Guffey

 

[therifler]

opsoff1 i hope this answers your question . T.R.