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Old February 19, 2005, 19:40   #1
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7.62x51 vs 308 Win. Ammo & HS differences

The 7.62x51 vs .308 Winchester ammo size/pressure debate has waged for years.
Others have already covered the pressure differences, I'll attempt with the size difference.
While there are differences in the chamber specs between them, ammo for both calibers are ALL manufactured to basically the same size and are interchangable. I'll try to prove it to you, and more important, I'll show you how to test and prove it to yourself

My weapon of choice will be the RCBS precision micrometer in 308 Winchester. This gauge, that's under $40. will perform quite a number of functions for us, but I'll get to the other uses later. Foremost here, we'll measure and compair some 7.62x51 with some commercial 308 Win. ammo in the same exact method that we use to head space rifles, by SAAMI spec as measured from the base to the datum line on the case shoulder.

The RCBS gauge consists of a base, two micrometer heads and an adjustable dummy round. For our test, we'll only be using the head space micrometer head and base. Loaded rounds are placed in the base and the micrometer for head space is screwed down until light contact is felt. This micrometer head is calibrated to measure ( 0 ) at the SAAMI minimum dimension for 308 head space, that is 1.6300" from the base to the shoulder datum line.
I double check my RCBS precision mic. using a Forster "go" gauge, sized at the same 1.6300" and it double checks at precisely zero. A check of the nogo and field gauges also resulted is precise readings in relation to the sizes printed on the gauges themselves.

The pic's to follow show a sample of both commercial 308 Winchester and 7.62x51 surplus and NATO rounds being checked in the gauge. Remember, Zero on the gauge is the minimum measurement for a commercial 308 chamber and would supposably be quite a bit under size for 7.62x51. As you'll see, there's basically no size difference between the two types of ammo. In fact, after almost 15 years of randomly checking most all of the 308 & 7.62x51 that I've shot, I've found that not only does the 7.62x51 usually measure SMALLER that commercial 308 but that theres usually quite a bit more variance between the rounds. While most commercial 308 will be uniform between a few thousands, I've seen differences of up to .008" in a single box of surplus NATO ammo.

So, not only are the 7.62x51 chamber specs LARGER than commercial 308, but the NATO/Surplus/etc. ammo is on average the same or SMALLER than commercial 308. This translates to head space being GREATER while firing 7.62x51 in a 7.62x51 chamber then it would be firing commercial 308 in the same chamber.

Other uses for the precision mic.include measuring fired cases, this will accurately tell you how much you'r brass grows when fired and the actual size of you'r particular chamber. As such, it can also be used to check and track any increase in the head space, for as the fired brass grows so does head space. The second micrometer head measures where bore diameter is met in relation to the case base, so it can be used with the dummy round to check the chamber throat to track errosion or for precision bullet seating in relation to throat standoff to bullet ogie. All in all, the RCBS precision mic. is a very worthwhile investment of $40. or less for any serious shooter or reloader and will hopefully help correct the common misconception of size and head space differences between 7.62x51 and 308 Winchester.

<http://www.rcbs.com/default.asp?menu=1&s1=4&s2=3&s3=28>
<http://www.midsouthshooterssupply.c...?sku=0004488329>

Pic's to follow, Cheers, YOOO VINNY

Last edited by yovinny; February 19, 2005 at 19:57.
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Old February 19, 2005, 19:42   #2
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FNM, Port, this lot showed differences of up to .004", from the .003" undersize in the pic, to .001" over.
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Old February 19, 2005, 19:44   #3
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Our beloved Hirt, showed differences of .002", from .001" under to .001" over
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Last edited by yovinny; February 20, 2005 at 00:11.
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Old February 19, 2005, 19:49   #4
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80 LC ball, .002" differences, from .002" under to zero.
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Old February 19, 2005, 19:50   #5
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Federal Gold Medal 168gr Match, .001" differences, from .002" to .001" over.
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Old February 19, 2005, 19:52   #6
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Win USA ball, .002" differences, from .002" under to zero.
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Old February 19, 2005, 19:54   #7
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Good old, real, RG ball, .005" differences, .003" under to .002" over.
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Old February 19, 2005, 20:06   #8
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These examples have shown that ALL the surplus or NATO ammo averages at or below undersize for the minimum 308 commercial chamber and that the largest cases on average were actually the Federal Match brass.
So the next time someone tell's you you'll blow up a 7.62x51 chambered rifle by shooting commercial 308, because the head space is overly large, you can pull out your case gauge and show them, that it ant' so

Last edited by yovinny; February 19, 2005 at 20:12.
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Old February 19, 2005, 21:19   #9
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Im waiting for the Wit-Nits to respond. Kind of hard since you provided Pictures.

Should have saved them for later. BTW, Did you check any Aussie? I have my first reloads ready to go from 91 brass.

Thanks for the info.
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Old February 19, 2005, 22:24   #10
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Heavier .308 bolt gun ammo can and will do harm to the average gas operated 7.62 NATO battle rifle, but I thought the reason for not firing some .308 ammo in a 7.62 chamber had to do with bullet weight and powder burn rates.
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Old February 19, 2005, 22:48   #11
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Jerrymc,
I did'nt get to the Aussie, I shot up the case I had opened, the rest is squirreled away

Sgt,
Lets stick with comparing apples to apples. That's ammo sizes between whats commonly used for 7.62x51 military, surplus or NATO loads and .308 Win loads up to 170gr. A heavier payload hunting load in a gas gun is a whole different subject and outside the scope of this test. As for powder burn rates, IMHO, I dont think you'r going to find much difference between compairable loads to make a difference. Their's little practical velocity and pressure difference between them, though their loaded all over the world with all different powders and components.
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Old February 19, 2005, 23:06   #12
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Given that the 7.62 NATO wasn't really the standard that the 5.56 turned out to be, there are some fairly significant differences in bullet weight, jacket construction, and case size. Your gauge test shows the size issue quite nicely, but I must be missing something else. Is there an issue with firing commercial .308 ammo such as Federal's Ameerican Eagle brand in a 7.62 NATO chambered rifle? I'm curious becaue eventually we are going to run out of surplus 7.62.
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Old February 20, 2005, 00:09   #13
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Quote:
Originally posted by Sgt_Gold
Given that the 7.62 NATO wasn't really the standard that the 5.56 turned out to be, there are some fairly significant differences in bullet weight, jacket construction, and case size. Your gauge test shows the size issue quite nicely, but I must be missing something else. Is there an issue with firing commercial .308 ammo such as Federal's Ameerican Eagle brand in a 7.62 NATO chambered rifle? I'm curious becaue eventually we are going to run out of surplus 7.62.
Nope, you'r not missing a thing
The only people that missed the boat are those that insist firing commercial 308 ammo, in an equivelent loading, in a 7.62x51 chambered rifle will wreck it
Cheers, YOOO VINNY
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Old February 20, 2005, 12:11   #14
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Good work Vinny, I love a good reality check every now and then.

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Old February 20, 2005, 12:46   #15
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Thank you

I should also mention something about reloading.
If you'r using full length or small base 308 dies to reload and using them to the full depth, that is shell holder to die contact, then you'r reloads are all probably undersize also.
Using the gauge to check resized brass, both surplus and commercial, out of 3 different brands of full length dies (RCBS, Lee & Lyman) showed brass that averaged .003" undersized.

Cheers, YOOO VINNY
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Old February 20, 2005, 13:12   #16
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"One test is worth a thousand expert opinions."
H.P. White labs

Nice work.
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Old February 22, 2005, 06:37   #17
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Quote:
Originally posted by Uncle Buck
"One test is worth a thousand expert opinions."
H.P. White labs

Nice work.
I like that Thanks Uncle Buck.


So nobody else has any questions, comments or flames to throw about this ??
Nobodys going to mention internal volume/brass thickness differences between the two ??
You guy's must be slacking
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Old February 22, 2005, 08:26   #18
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No flames.

That's a neat tool. Maybe I should have one of those since I reload too.

My question? Man...are snowed in over there? I usually don't perform test like that unless I'm bored or hiding from the wife. "don't bother me now honey,
I'm in the middle of a very important test!"

Thanks for taking the time and posting the results.

Nice to have the pics with the facts

As Shultz would have said " veeerrrryyy interesting
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Old February 23, 2005, 09:44   #19
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Quote:
Originally posted by yovinny


I like that Thanks Uncle Buck.


So nobody else has any questions, comments or flames to throw about this ??
Nobodys going to mention internal volume/brass thickness differences between the two ??
Well, I wasn't going to bother. But, since you asked...forget it
Good work. Thanks for your effort.
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Old February 23, 2005, 23:38   #20
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Finally! Thanks for taking the time to perform these tests. I am going to save this thread in .pdf format for the archives...

- N20
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Old March 10, 2005, 15:06   #21
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Quote:
Originally posted by yovinny
These examples have shown that ALL the surplus or NATO ammo averages at or below undersize for the minimum 308 commercial chamber and that the largest cases on average were actually the Federal Match brass.
So the next time someone tell's you you'll blow up a 7.62x51 chambered rifle by shooting commercial 308, because the head space is overly large, you can pull out your case gauge and show them, that it ant' so
Well, respectfully, USGI 7.62-MM headspace is way too long for commercial .308 Winchester ammo. Elsewise, why isn't commercial .308 Winchester No-Go 1.6455"? Hmmm?

To empirically demonstrate the effect of shooting .308 Win in a USGI 7.62-MM chamber, get a Navy Garand or true M14. Shoot several boxes of Rem-UMC white box .308 Win (the flimsiest we've found) through it. You'll get about 50% case head separations. Not my idea of a good time at the range.

I can't speak for foreign NATO ammo; however, 7.62-MM USGI ammo is made with heavier brass than .308 Win to handle what is essentially excess headspace (with clean ammo and a clean chamber; dirt/fouling can take up some of the excess space).

We've had some fun getting under .308 Win No-Go with Beretta BM 59E and BM 59 barrels in 7.62-MM, in custom applications. Chambering our M14s in .308 is a piece of cake, thankfully, as we've designed our receiver and our barrels such that we can get 1.632" easily.

But, note that I'm referring to the USGI 7.62-MM chamber, not any other NATO chamber spec. For example, referring to the Ozzie L1A1 "Technical Brochure Supplement," I find that the Ozzie L1A1 NO-GO headspace spec [U}is[/U] the .308 Win commercial spec (1.638"), not the USGI 7.62-MM spec (1.6455")! What do the Ozzies know, and when did they know it?!

Your welcome research has vailidated our long-standing contention that there is no difference in cartridge dimensions between .308 Win and 7.62-MM ammunition, whatever the source. We've known that for twenty years. Yet, the USGI 7.62-MM and .308 Win chambers remain different, and the 7.62-MM USGI chamber is still too long for the (relatively) weaker commercial brass used with much (most?) .308 Win commercial ammunition.

Here at the FAL Files, I'd love to see if anyone has further tech info documented on the rifles themselves. Is the FN chamber spec the .308 Win chamber (as is the Australian L1A1), or the USGI 7.62-MM chamber? The G1? StG 58? Brit? Canadian?

Inquiring minds...

In the meantime, continue to use 1.632"; go forth and enjoy.

Very best regards,

Walt
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Old March 11, 2005, 00:06   #22
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Hi Walt,
Thanks for the input
This post was basically because I'm tired of hearing people cry about not being able to shoot commercial 308 Winchester ammo in the commercial rifles that are claimed to be chambered in 7.62x51 NATO. Which is anything but true.

If you look back to post 17, you'll see I was looking to talk about the brass differences and how it relates to the different chamber sizes, but it did'nt seem anyone was interested

The 1.6455" you quote is the USGI field reject size and quite a bit bigger than the 1.640" commonly accepted for FN & NATO reject size.
In fact the commonly accepted 1.640" NATO reject size is only .002" larger than SAAMI 308 reject.
I dont know where you'd even find a rifle chambered near that large, except maybe a worn out, service rejected M14. Certainly not on any commercial rifle, on any FAL built with knowledge gained here, or on anything that's been looked at by anyone with any sense or with any commercialy available gauges.

Any FAL built using the common knowledge found on this site for correctly headspacing a FAL to 7.62x51 NATO or any commercially available rifle in "7.62x51 NATO" should easily handle commercial 308 Winchester ammo without a problem if headspaced correctly.

And yes, I also personally like 1.632" and chamber my M1A's, FAL's and even my one 7.62x51 M1 Garand to that spec, they shoot everything quite well. Even better when chambered with an M852 throat length reamer

Cheers, YOOO VINNY
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Old March 11, 2005, 07:04   #23
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Quote:
Originally posted by yovinny
Hi Walt,
Thanks for the input
This post was basically because I'm tired of hearing people cry about not being able to shoot commercial 308 Winchester ammo in the commercial rifles that are claimed to be chambered in 7.62x51 NATO. Which is anything but true.

If you look back to post 17, you'll see I was looking to talk about the brass differences and how it relates to the different chamber sizes, but it did'nt seem anyone was interested

The 1.6455" you quote is the USGI field reject size and quite a bit bigger than the 1.640" commonly accepted for FN & NATO reject size.
In fact the commonly accepted 1.640" NATO reject size is only .002" larger than SAAMI 308 reject.
I dont know where you'd even find a rifle chambered near that large, except maybe a worn out, service rejected M14. Certainly not on any commercial rifle, on any FAL built with knowledge gained here, or on anything that's been looked at by anyone with any sense or with any commercialy available gauges.

Any FAL built using the common knowledge found on this site for correctly headspacing a FAL to 7.62x51 NATO or any commercially available rifle in "7.62x51 NATO" should easily handle commercial 308 Winchester ammo without a problem if headspaced correctly.

And yes, I also personally like 1.632" and chamber my M1A's, FAL's and even my one 7.62x51 M1 Garand to that spec, they shoot everything quite well. Even better when chambered with an M852 throat length reamer

Cheers, YOOO VINNY
Vinny, we're in violent agreement! Thanks for being an island of sanity in this too-often contentious yet easily resolved non-issue.

For years we've been looking for a 7.62mm NATO cartridge longer than .308 WIn dimensions. Glad you couldn't find one either!

Warmest regards, and with a big "WHEW!" for being able to put this subject to rest,

Walt
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Old March 11, 2005, 07:43   #24
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Originally posted by Sgt_Gold:

"Is there an issue with firing commercial .308 ammo such as Federal's Ameerican Eagle brand in a 7.62 NATO chambered rifle? I'm curious becaue eventually we are going to run out of surplus 7.62."

I have fired thousands of the Federal American Eagle red box .308 Winchester in several FALs, H&K-91's and a S.A. Inc. M1-A or two that were correctly headspaced for the U.S. 7.62x51 NATO, according to my former Marine armorer/gunsmith, who also is in agreement with Vinny. No problems at all. The American Eagle and equivalent W-W were all I shot until I discovered milsurp 8-10 years ago.

I have also fired a great deal of the Winchester white box 147 grain FMJ ammo, but I seem to recall that the cases were all stamped 7.62x51, not .308 Winchester. I will have to make a trip to the basement and see.

R-P brass and ammo is never my first choice, especially in .308 Win. I believe that the military rifles may start to extract the case before pressures have dropped all the way, before the brass has had a chance to retract a bit from the chamber walls. Thin, weak brass will give casehead separations under such circumstances. We don't get the same problem in bolt or pump rifles because, frankly, none of us can work the actions that fast.

Great post, Vinny.
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Old March 11, 2005, 08:33   #25
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Quote:
Originally posted by Sgt_Gold
Given that the 7.62 NATO wasn't really the standard that the 5.56 turned out to be, there are some fairly significant differences in bullet weight, jacket construction, and case size. Your gauge test shows the size issue quite nicely, but I must be missing something else. Is there an issue with firing commercial .308 ammo such as Federal's Ameerican Eagle brand in a 7.62 NATO chambered rifle? I'm curious becaue eventually we are going to run out of surplus 7.62.
SGT, I doubt you'll ever be shooting a rifle chambered in USGI 7.62-MM. As Vinny has suggested, "7.62 NATO" turns out to be .308 Win in nearly all cases, even in the "Austrailian Army spec" Austrailian L1A1.

The criterion is not the name on the chamber; the criterion is the headspace dimension of the chamber. I'm not willing to shoot anything but USGI military ammunition in a chamber that fails .308 NO-GO, but falls within USGI 7.62-MM NO-GO.

Please note that I use "7.62-MM" as a "term of art." This is the official USGI nomenclature. By using USGI nomenclature I am not referring to "7.62x51mm" or "7.62mm NATO," as at this point I have no idea what those terms mean with respect to headspace dimensions, with the exception of the afore- and often-mentioned Australian L1A1 chamber.

Very best regards,

Walt
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Old March 11, 2005, 10:57   #26
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As Walt stated, you'r odd's of ever encountering a servicable rifle with the excessive head space of USGI field reject of 1.6455" are slim to none, even in the US Military.

I've come to think of the differences between 7.62x51 NATO ammo and .308 Winchester ammo and their respective chambers a little differently than the common misconception of using the USGI field reject size limit as the top of tolerance.

That is, not as to why standard commercial ammo and chambers are undersize from it, but to why/who made the decision to make the USGI field reject gauge so overly large ??

I know it couldent possibly have anything to do with the servicability of M14's being worth more of a premium to the military than the safety of the users

Walt, BTW;
The USGI gauge sized 1.6455" is the "Field Reject" gauge, used with a test bolt at all levels to reject rifles from ANY further service.
There were/are also USGI "go" and "nogo" gauges in armory service, used when rebuilding M14 rifles.

USGI "field reject" FSN# 4933-647-3698 (sized 1.6455")
USGI "no go" FSN# 4933-916-9275 (sized 1.6375")
USGI "go" FSN#4933-916-9271 (sized 1.6355")
*referenced from USGI manual #TM 9-1005-223-34

Cheers, YOOO VINNY

Last edited by yovinny; March 11, 2005 at 11:08.
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Old March 11, 2005, 11:06   #27
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Quote:
Originally posted by yovinny
As Walt ststed, you'r odd's of ever encountering a servicable rifle with the excessive head space of USGI field reject of 1.6455" are slim to none, even in the US Military.

I've come to think of the differences between 7.62x51 NATO ammo and .308 Winchester ammo and their respective chambers a little differently than the common misconception of using the USGI field reject size limit as the top of tolerance.

That is, not as to why standard commercial ammo and chambers are undersize from it, but to why/who made the decision to make the USGI field reject gauge so overly large ??

I know it couldent possibly have anything to do with the servicability of M14's being worth more of a premium to the military than the safety of the users

Walt, BTW;
The USGI gauge sized 1.6455" is the "Field Reject" gauge, used with a test bolt at all levels to reject rifles from ANY further service.
There were/are also USGI "go" and "nogo" gauges in armory service, used when rebuilding M14 rifles.

USGI "field reject" FSN# 4933-647-3698 (sized 1.6455")
USGI "no go" FSN# 4933-916-9275 (sized 1.6375")
USGI "go" FSN#4933-916-9271 (sized 1.6355")
*referenced from USGI manual #TM 9-1005-223-34

Cheers, YOOO VINNY
Thanks, Vinny, I grabbed the wrong dimension from Kuhnhausen yesterday.

Great posts, super info.

Thanks again!

Wamest regards,

Walt
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Old December 03, 2005, 19:20   #28
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Why not just give this a new bump?
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Old December 04, 2005, 15:39   #29
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Thanks for graphically posting something I have been saying for years. The only caveat I have, is that at least one RCBS case mike that I'm aware of didn't measure properly. Whether it came that way or was damaged I can't say. I use a Wilson case gage together with a headspace gage and an indicator. If you want to use the case mike, periodically calibrate it with a headspace gage.
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Old December 04, 2005, 16:30   #30
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Quote:
Originally posted by steve marshall
Thanks for graphically posting something I have been saying for years. The only caveat I have, is that at least one RCBS case mike that I'm aware of didn't measure properly. Whether it came that way or was damaged I can't say. I use a Wilson case gage together with a headspace gage and an indicator. If you want to use the case mike, periodically calibrate it with a headspace gage.
Absolutely correct

From my first original post;
"I double check my RCBS precision mic. using a Forster "go" gauge, sized at the same 1.6300" and it double checks at precisely zero. A check of the nogo and field gauges also resulted is precise readings in relation to the sizes printed on the gauges themselves."

As with any measuring instrument, they need to be checked or calibrated by measuring "standards", or in this case, head space gauges of known size.

From what I understand, the RCBS case mic's are calibrated at the factory and chemically "glued" at the correct setting.
Use of too much force, heat or a cleaning solvent, can very possibly cause the pieces to move.
I've also heard of others that were said to be "off", though my own selection of 8 different calibers, are all within the gauges zero resolution for their respective "go" gauges.

Cheers, YOOO VINNY
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Old December 09, 2010, 10:19   #31
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Another thread on the same topic:
http://www.falfiles.com/forums/showt...postid=2160133

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Quote:
Originally posted by W.E.G.
Ammunition “headspace” comparison










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