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Author Topic: Head Space Measurement Mysteries  (Read 2660 times)

Offline opsoff1

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Head Space Measurement Mysteries
« on: 01:29 PM, 03/04/13 »
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  • 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? 


    Offline BillPa

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    Head Space Measurement Mysteries
    « Reply #1 on: 05:11 PM, 03/04/13 »
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  • 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?

     This set up will not give a direct reading - just a comparative one.   

    How can this be?  What am I missing?

     Well, if your inserts were like mine they have a slight chamfer on the edge of the bore allowing them to ride further up the shoulder resulting in a short measurements.   To make matters worse, if your a "twister" it tends to wear the aluminum over time with use. 

     I've pretty much junked all the aluminum inserts and made new ones from steel or brass . After boring them the last thing I do is take a facing cut to make sure the very edge of the bore in sharp and measures exactly the diameter I need.

    Bill

    Online BoydAllen

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    « Reply #2 on: 05:38 PM, 03/04/13 »
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  • What you are probably missing is the use for which the Stoney Point gauge was designed. I think that its intended purpose was to take before and after sizing case measurements to facilitate adjusting FL or shoulder bump dies for an exact amount of shoulder bump. I also use mine to track shoulder position through a cases first few firings to determine the maximum value (since it takes more than one firing for a case's "headspace" to max out), from which I set bump. When used for its intended purpose, mine works very well. As I do with all of my caliper attachments, I leave  the dial caliper at jaw to jaw zero. 


    Offline gotcha

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    Head Space Measurement Mysteries
    « Reply #3 on: 10:34 PM, 03/04/13 »
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  •  oppsoff, Had the same questions as you. Checked w/ Hornady techs & they stated their inserts for HS should NOT be used for exact meas.------ Comparative only for the reasons stated by BillPA.  I believe BillPA's solution would be appropriate. I opted to use the Sinclair insert for the Hornady HS tool because it makes contact w/ the entire shoulder angle & duplicated the measurement of the HS gauge which was stamped on my gauge. Not unusual for fired brass to show slightly longer HS than HS gauge due to massive pressure exerted during F/F.

    Offline opsoff1

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    Head Space Measurement Mysteries
    « Reply #4 on: 10:30 AM, 03/05/13 »
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  • All these answers are great, thank you all for the feedback - re: BillPa - I am going to make my own inserts from brass, ream to the exact dimension & face.  I did reinspect last night and with a 7x lens, there is a chamfer on the edge and it is not concentric.  The inserts are going in the scrap heap.
    Gotcha & BoydAllen - I pretty much figured they were comparative tools - appreciate the confirmation - it just irks me to no end that if a mfr puts something like that on the market - at least do it right. 
    I like to use these tools as a quick sorting culling mechanisim when going through new or fired brass - i have a lot of new brass in 223 & 308 that I like to separate into lots by weight - as a matter of course I check HS - I do find some oddball dimensioned pieces (>1%) either way under or way over.  These are culled.
    Thanks again for the feedback.

    Online CatShooter

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    « Reply #5 on: 07:08 AM, 06/08/13 »
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  • What you are probably missing is the use for which the Stoney Point gauge was designed. I think that its intended purpose was to take before and after sizing case measurements to facilitate adjusting FL or shoulder bump dies for an exact amount of shoulder bump. I also use mine to track shoulder position through a cases first few firings to determine the maximum value (since it takes more than one firing for a case's "headspace" to max out), from which I set bump. When used for its intended purpose, mine works very well. As I do with all of my caliper attachments, I leave  the dial caliper at jaw to jaw zero.

    Good morning Boyd.  I came across this in a search for something else.

    I found the same thing with mine - some dingle-berry at Hornady used a countersink to break all the edges of the inserts.


    I would respectively disagree with you about your thoughts regarding the "intended use" of the gauge.

    Any measuring tool should be expected to accurately measure what it is intended to measure, unless it is stated in the instructions... "This gauge is intended to measure comparative/relative headspace, and not absolute headspace, and is not intended to be used for determining true measurements."

    IF... the tool was intended for comparative/relative measurements, then there would be no need for using bore holes that are accurate, and to the SAAMI standard... any ol' hole would do (that doesn't sound right, but you know what I mean).

    It is more likely that at the manufacturing stage, the manager of the final assembly folks (at a whoppin' minimum wage + 20%) thought they should finish the inserts so they looked "nice", and take the rough edges off the inserts.

    I would gladly pay 3 times the price if the inserts were made of stainless steel and properly faced and lapped as a last step.

    But, there is a light at the end of the tunnel.

    If you take a 12" square piece of plate glass, and a #600 (or finer) silicone carbide "Wet-n-dry" paper, and some light oil, you can lap the inserts to remove the counter sunk section, and the gauge is 98% more accurate.

    CatShooter.
    Spring has sprung, da' creek has riz, I wonder where dem kitties is?
    Here Kitty, kitty, kitty.


    Offline rayjay

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    « Reply #6 on: 10:10 AM, 06/08/13 »
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  • Since discovering the little brass bushings that come with Harrell's dies I quit using Stoney Point stuff. I have made several of these bushings for cartridges that I didn't have Harrell's dies for. I can pick up any piece of brass and measure it and tell you how it's going to fit in my chamber.
    Ask lots of questions but question all the replies.

    Offline WayneShaw

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    « Reply #7 on: 11:11 AM, 06/08/13 »
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  • The only gauge I use is the go-gauge when it is chambered. From that point on, the fit and/or bump from sizing is checked in the chamber with a stripped bolt.

    I don't work on gas guns, so I don't understand the different headspace, but then again, I suppose a gas gun needs a bit more room to slam shut.

    Offline butchlambert

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    « Reply #8 on: 09:55 AM, 06/09/13 »
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  • I agree with you Wayne. I still have some Stony Point cartridge cases if anybody wants them.  $7.50 shipping 243, 6ppc,6.5X55, and 6mmbr.
    I just get too many different measurements.

    Online BoydAllen

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    « Reply #9 on: 10:55 AM, 06/09/13 »
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  • I have seen situations where friends had custom chambers and by the time that they had the right feel, the shoulders of their cases were bumped back excessively. That is why, particularly in print, or on the internet, where readers of different levels of skill and experience will probably be reading, I think that the best approach is to measure bump with some sort of gauge. Once it has been established that a particular FL die is correct for a given chamber then of course feel can be used, and as far as that goes, if the shoulder bump is at a maximum, and the feel is not right, that disqualifies that die for that chamber. Ultimately, we have no way of knowing about how a readers die fits his chamber, and if the die is too large, doing a perfect job of following directions for doing it by feel will have his shoulders bumped back too far.

    Offline WayneShaw

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    « Reply #10 on: 05:11 PM, 06/10/13 »
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  • Boyd, I don't disagree with you. Dies are different, brass work hardens and needs to be dealt with. The chamber is the bottom line. One must not let an ejector hide the fit when closing the bolt.

    Offline fguffey

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    « Reply #11 on: 05:45 PM, 06/10/13 »
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  • Head Space Measurement Mysteries


    “How can this be?  What am I missing?”

    I am the fan of transfers, standards and verifying, the chamber does not get dark just because the bolt closes, I check the length of the chamber three different ways without a head space gage. I modify gages, I have one reference, my reference always starts with .000”.

    F. Guffey

       

    Online BoydAllen

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    « Reply #12 on: 06:15 PM, 06/10/13 »
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  • What ejector?  ;D My PPCs don't have one. and noting else that I shoot is reloaded for as critically. I use the Stoney Point gauge on a tight case, and set my bump by that. With the factory chambers, there is enough clearance in the body between sized cases and their chambers that simply gauging the bump to set the die works very well, and for my PPCs I use a Harrell's Vari-base die that has inserts that vary by .001 so I can set the shoulder bump where I want it, and use an insert to get the feel that I need. The only rub that I have seen has come when fellows have dies that are too large for their custom chambers. They still need to make sure that they have the feel that thy want, but if they only relied on that, some of them would have had too much bump to get to the feel that they wanted.

    Offline fguffey

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    « Reply #13 on: 07:42 AM, 06/11/13 »
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  • Bump? I have instructions that go back to the late fifties and into the early  sixties from RCBS. Back then bump was bump, had nothing to do with the moving the shoulder. I have at least 3 RCBS Rock Chucker presses, not one of them will bump, I can mortify a Rock Chucker to bump but, I am happy with lock up, jam up or ‘go into a bind’.  I can not bump a shoulder, even it the die is a bump die???? By the time the die, by any name, makes contact with the shoulder the die is contacting the case body, WHY!? Without case body support moving the shoulder squats the case causing the case body to increase in diameter (somewhere). So, I know it sounds cool, “I bump my shoulder .001”.

    Forming and sizing": It would make sense if the progressing was sizing first, then go to forming, the shoulder is not the first part of the case to contact the die is the neck, the case body hits hits next, then the shoulder Again, I have instructions, my instructions cover ‘bump’ the instructions that covers bump have nothing to do with the shoulder. I do have bump presses, all of my Herters presses bump. Bump, in the old days was a design feature of the press.

    F. Guffey 

    Offline butchlambert

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    « Reply #14 on: 09:09 AM, 06/11/13 »
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  • Bump? I have instructions that go back to the late fifties and into the early  sixties from RCBS. Back then bump was bump, had nothing to do with the moving the shoulder. I have at least 3 RCBS Rock Chucker presses, not one of them will bump, I can mortify a Rock Chucker to bump but, I am happy with lock up, jam up or ‘go into a bind’.  I can not bump a shoulder, even it the die is a bump die???? By the time the die, by any name, makes contact with the shoulder the die is contacting the case body, WHY!? Without case body support moving the shoulder squats the case causing the case body to increase in diameter (somewhere). So, I know it sounds cool, “I bump my shoulder .001”.

    Forming and sizing": It would make sense if the progressing was sizing first, then go to forming, the shoulder is not the first part of the case to contact the die is the neck, the case body hits hits next, then the shoulder Again, I have instructions, my instructions cover ‘bump’ the instructions that covers bump have nothing to do with the shoulder. I do have bump presses, all of my Herters presses bump. Bump, in the old days was a design feature of the press.

    F. Guffey




    So what is bump if it isn't bumping the shoulder a couple thousandths? Also is bumping the shoulder back a couple thousandths going to swell your case somewhere?  I just bumped a couple pieces of Lapua that is formed into 6mmppc. Both my tenth micrometer and using a fixture to hold my brass in a vee block showed no change in body dimension. Yes, I have custom dies.
    fguffey, I guess you could experience this if you bumped the shoulder several thousandths.


     

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