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  #21  
Old 10-31-2019, 01:57 PM
dartem dartem is offline
 
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Thank you. Think I will order some of those.
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  #22  
Old 11-03-2019, 10:18 PM
mac1911 mac1911 is offline
 
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Quote:
Originally Posted by drywash View Post
Got a new RCBS 30-06 Precision micrometer. Only problem is it takes "three Philidelphia Lawers to dechipher and understand it's us. If someone can explain it's use in (simple) terms I am all ears.... Thanks. I am third on Midways backorder for Wilson. Thanks

I think the best thing to do is first measure some factory fodder and you can get a idea of what your gauge is doing.


The ideal world I think 0 on the gauge is about max of case headspace ? the minimum is -.XXX depending on the cartridge.


Once you get a base line you can now adjust your dies to get brass back to thes "0" or adjust your dies to push the shoulder back .003-005" or so.
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  #23  
Old 11-04-2019, 07:08 AM
ceresco ceresco is online now
 
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The RCBS Precision headspace gauges should be checked against headspace gauges. I have several 30-06 gauges and they do not agree. Quick fix is a piece of tape with an appropriate mark. It may be possible to heat the gauge and adjust.....haven't tried that. Without calibration, the RCBS gauge is still useful for comparison, but will not provide actual HS dimensions. The gauges are useful for a number of purposes. Additional calibration marks on the gauge and it will serve for other cartridges of similar length such as 8mm Mauser, etc. Good Shooting. ...
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  #24  
Old 11-04-2019, 10:39 AM
BRMPCF50 BRMPCF50 is offline
 
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My gauge was “off” when checked with a go headspace gauge. Contacted RCBS, sent it back and they replaced it with one that would zero with my headspace gauge.

I’ve found that factory ammo usually gauges a few 000’s “short.”
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  #25  
Old 11-05-2019, 04:31 PM
nf1e nf1e is offline
 
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Quote:
Originally Posted by BRMPCF50 View Post
My gauge was “off” when checked with a go headspace gauge. Contacted RCBS, sent it back and they replaced it with one that would zero with my headspace gauge.

I’ve found that factory ammo usually gauges a few 000’s “short.”
You really want it .002 to .006 short for an auto-loader so you are in the ballpark.
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  #26  
Old 11-09-2019, 01:52 PM
Fogtripper Fogtripper is offline
 
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Quote:
Originally Posted by ceresco View Post
The RCBS Precision headspace gauges should be checked against headspace gauges. I have several 30-06 gauges and they do not agree. Quick fix is a piece of tape with an appropriate mark. It may be possible to heat the gauge and adjust.....haven't tried that. Without calibration, the RCBS gauge is still useful for comparison, but will not provide actual HS dimensions. The gauges are useful for a number of purposes. Additional calibration marks on the gauge and it will serve for other cartridges of similar length such as 8mm Mauser, etc. Good Shooting. ...
Agreed. My precision mic seating depth part cannot zero out on its own, to get zero on the scale the knob goes all the way past to -35. They said I can ship it to them and they will replace, but I may just use it and just count tic marks.
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  #27  
Old 11-09-2019, 02:54 PM
Unclenick Unclenick is offline
 
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Funny that they got that far off. I have, I think, three of these tools and all are off, but not by more than ±0.002". So I just drop a GO gauge in and remember what it says and offset my readings by the opposited sign amount.

Incidentally, you want to calibrate with a good quality GO gauge. I've got one "armorer's set" of .308/7.62 gauges for which the PM and the case comparator adapter on my calipers both tell me the gauge labeled 1.638" is a thousandth shorter than the one labeled 1.637". A couple of others are the same length but are labeled 0.001" different. I have had good luck with Dave Manson's headspace gauges. I would expect Pacific Tool to be good and I'm sure there are others. My point is just to be aware of the inexpensive ones, as they may not be dead on.

Most .308 and 7.62 new cartridges I've measured ranged from the SAAMI minimum headspace -0.003 to right on at 0.000 (some foreign surplus), with the majority being -0.002". I think that's where the idea that sizing to set shoulder location back by -0.002" for adequate feeding and maximum case life came from.
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Last edited by Unclenick; 11-10-2019 at 09:56 AM.
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  #28  
Old 11-10-2019, 09:29 AM
mac1911 mac1911 is offline
 
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Wink Confusing about the RCBS gauge "headspace"

I think the big problem with the RCBS precision mic is the "headspace" wording.



The Gauge itself is not to measure "headspace" as you think of "chamber headspace". You can use it to compare a factory case to a fired case to see what your "fireformed" case now compares with the unfired cases. I think this is why Hornady calls their set up a comparator..



RCBS 30-06 in their instructions says the min Cartridge Headspace or dimension A is 0 on the gauge or 2.0487

Now where did they come up with that? I know thats min Chamber" spec but why zero on that?







SAAMI

https://saami.org/wp-content/uploads...sting-Copy.pdf
pg 94


Says that the "CARTRIDGE" specs dont even call the measurement off the datum "headspace"


its just referenced at .375" the MAX is 2.0526" and gives you -.0070" allowed BELOW the 2.0526" or "min" of -.0070" = 2.0456"


Now this is all good if your gauge is made to very tight specs. We see from some posts they are not. With my simple tools my RCBS 30-06 case gauge short side that touches the shoulder is .373"





So basically if you have a dead nuts perfect min Chamber spec 2.0487" and a max Factory Cartridge "shoulder/headspace" spec of 2.0526" your .0039" over your min go gauge----so your cartridge will not fit right? no it will all but the weakest action will smash that .003" back and chamber.....wont it?


Its all very confusing until you just start using the gauge to compare "factory" brass with fired brass.


For example :
I measured all the HXP years I have and with the RCBS precision mike all samples 64/68/70/77 came in at 0 on the mic +- .001"


so I have always loaded my bulk 30-06 fodder to zero on the mic.
Then when I was looking to load more accurate ammo for my new barreled rifle I was just adjusting my dies until I got about .003" bump. Worked out just fine.


Later I bought the Hornady comparator. Measure the same HXP brass and it reads 2.044" mind you now you have another tool with its inconsistencies to deal with BUT again your using these tools to compare readings.


so with HXP coming in at 2.044" on my gauge that puts me .0016" below "SAMMI" min for the Cartridge shoulder/headspace min....


oh the horror...


As far as the bullet seating depth gauge. Its not that hard to use. It gets you close. I think some bulet designs it might not work well with ?
I ended up buying the horday gauge and the "modified" cases its easier and quicker PLUS you get to use the bullet you intend to.. I also still use the poor mans gauge with my cast bullet rifles. simple a slip fit case mouth and black sharpie on the bullet type to "see" contact.

Last edited by mac1911; 11-10-2019 at 09:33 AM.
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  #29  
Old 11-10-2019, 11:05 AM
Fogtripper Fogtripper is offline
 
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I have a couple bags of Winchester brass for my 7mm Rem Mag. I took a piece of my once fired (federal I think) and measured it, and then measured the new winchesters. The shoulder on the Winchester is comically far short of my fire formed. I had actually hoped be able to resize it while loading so it would be closer to my chambers actual spec, with a little wiggle of course, but until I fire them once the full size die is not reaching those shoulders. For what I want to do, once fired would be more valuable to me.

What does all of that have to do with the mic? As mentioned above about the hornady version, I basically use it as a comparator, checking between my fire formed and potential reload brass.

Quote:
Originally Posted by mac1911 View Post


As far as the bullet seating depth gauge. Its not that hard to use. It gets you close. I think some bulet designs it might not work well with ?
I ended up buying the horday gauge and the "modified" cases its easier and quicker PLUS you get to use the bullet you intend to.. I also still use the poor mans gauge with my cast bullet rifles. simple a slip fit case mouth and black sharpie on the bullet type to "see" contact.
I use the hornady seat gauge as well, but while it is useful to measure the ogive distance when touching the lands, it still leaves one hunting and pecking to adjust the micrometer seating die, so one can adjust bullet jump. I had hoped the rcbs bullet “dummy” would bridge that gap. For reference, I am using berger vlds.

Last edited by Fogtripper; 11-10-2019 at 11:14 AM.
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  #30  
Old 11-10-2019, 11:30 AM
Unclenick Unclenick is offline
 
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Mac1911,

The idea behind it being a "headspace" gauge is that by measuring a fireformed case you are making what is called a transfer measurement of the chamber. That is, you transfer the size of your chamber to the measuring tool using the fired case as the transfer gauge.

In practice that doesn't work perfectly because unless you have an over-pressure load that stretches the chamber, the case is going to spring back a thousandth or two and your reading will be short of the actual headspace dimension. To get an accurate indirect measurement of headspace, you have to fire the case, make a reading, neck size it only so the sides of the case don't make any die contact (which changes its length) and fire again and measure again. You keep doing this until the case is just getting snug chambering. At that point, you can figure your last measurement before that was pretty darn close to actual headspace size.

It's a lot of work, and just removing the ejector and extractor from the bolt and shimming your GO gauge until you just barely feel it touch the shoulder and subtracting a quarter to half a thousandth is at least as accurate and doesn't require all the shooting and measuring. So I don't think the RCBS tool is really a very good system for measuring headspace. What you can do with it, though, is figure that a single fireformed reading (no neck sizing-only shots afterward) is at least a thousandth short due to spring-back and that if you resize that case until its chamber-filling dimension (bottom of head to the shoulder at its datum diameter) is -0.001" shorter than you read on the gauge, it will be at least -0.002" shorter than the chamber actually is, and that is enough for reliable feeding, as demonstrated by the fact a lot of commercial ammo is about -0.002" shorter than a minimum chamber and still feeds into a minimum chamber reliably.

The reason the gauge zero is on the minimum chamber number has to do with how SAAMI uses dimensions and tolerances. SAAMI uses an ANSI Standard engineering practice for critical dimensions. "Critical", in this context means a dimension beyond which the device cannot be assembled to function. We are accustomed to seeing non-critical tolerances of + or - some average number. These are used when it makes no difference whether an error is over or under. But with critical values, one end of the tolerance range is an absolute limit while the other end is more flexible. In these instances, the dimension given is the critical limiting value, followed by a unilateral tolerance (minus only or plus only) in the non-critical direction. For SAAMI, a cartridge's critical dimensions are maximums beyond which you cannot close the gun on them, where undersize amounts are less important to make exact. So, for example, the 30-06 head diameter is 0.473 - 0.010". That means 0.473" is the critical maximum size that is still guaranteed to fit the tightest SAAMI-dimension chamber. It is the critical dimension. But you can make it smaller down to 0.463" and even if you went over and made it 0.460" it would still fit into the chamber, even if it might not fire or extract reliably on some guns. That is why the minimum size is not critical. For lengths expressed without a tolerance, the drawing tells you to assume -0.008" cartridge tolerance, but all those dimensions given without a tolerance are still critical maximums.

The chamber is just the other way around. For the chamber, the critical dimensions are minimum dimensions, below which you might not be able to chamber a round. So the chamber dimensions given are critical minimum values with a plus (+) tolerance. Even though the drawing explicitly shows the minimum and maximum headspace dimensions, in particular, that is just to prevent a gunsmith making an arithmetic error. The operating principle is still that the chamber's minimum dimension is the critical one to the gun being usable, while the tolerance is +0.010"*.

The bottom line here is that the RCBS gauge is zeroed on the chamber's critical headspace dimension, which is its minimum. That is the reason for choosing that end of the range for the zero point on the scale.

By the way, if you chamber your rifle with a pull-through headspacing reamer, you can be pretty sure of hitting minimum headspace closely. Your first fireformed cases fired in that new chamber will give you the amount of spring-back your loads have below chamber minimum and you can record that information with your rifle file to monitor changes in its headspace in the future.

* Note that the industry standard NO-GO gauge is usually just +0.004" or +0.005" for new chambers because they figure the size will settle in, while the industry FIELD REJECT gauges are often +0.008". I don't know why that deviates from the SAAMI standard other than the gauge makers want some wiggle room for their gauge tolerances. The military will accept up to +0.015" in the field before it rejects a .30-06 chamber.
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