.308 & 7.62 NATO handloading headspace: compass check

[dmike]

Context:
-handloading noob
-just loading for two AR10 platforms for now (bolt guns down the road)
-not BR/comp. just field prone, 350-500m paper
-Goals: 1)not blowing up self or rifle 2)getting 1/2 to 3/4 MOA 3)learning the finer points of handloading


The whole 308/762 headspace thing makes my brain hurt. I get the pressure units (CUP vs PSI) etc. POF says the P308 is chambered in both, GAP says the GAP-10 is .308. From what I've found SAAMI says 1.6300-1.6400 and NATO says 1.628-1.634...

Question 1:
If a cartridge headspace is .006 shorter than SAAMI .308 spec, is it dangerous for either rifle?

Question 2:
If a cartridge headspace measures .004 under 7.62 NATO spec, is it dangerous for either rifle?

Question 3:
If a cartridge headspace measures .002 over NATO spec, is it dangerous for either rifle?


Details:
Using a rcbs precision mic I get the following headspace values:

M80 knockoffs from Ventura...1.624-1.628
Federal GMM175...1.6252-1.6274
Fired cases from the GAP-10...1.6316-1.6353 AVG 1.6330
Cases after full-length sizing with shellholder contact on die...1.6262-1.6297 AVG 1.6279
New Lapua brass...1.6262-1.6299 AVG 1.6281

I suppose the gauge could be off but if my numbers are right that would mean:
-Neck sizing spent brass without resizing could yield a round that was .0013 longer than NATO spec (but within SAAMI 308)
-Some of these rounds/cases could be .004 under NATO and .006 under SAAMI spec

I know, I know...some of you are saying "it's not that serious" but I just want to make sure I don't blow myself up, hurt my gun, overwork brass etc so I want to try to understand this. Thanks for any help or advice you can share.

 

[FLIGHT762]

Why are you worrying about this 308 VS 7.62 NATO? Are either of your AR-10 platforms actually chambered in 7.62 NATO? I know that my GAP 10 and my SR-25 are both chambered to 308 Winchester specs (SAAMI) not 7.62 NATO. Both of my guns have snug SAAMI 308 Winchester chambers. My GAP-10 has a chamber that is about the same as my Factory Remington 40XBKS bolt gun. I can actually use the same F/L die shoulder bump setting for my GAP-10 and my 40XBKS. My SR-25 has a slightly tighter chamber and is very close to the same chamber as a custom Hart barreled 700 Remington I have, but it's only about a .002" shorter difference than the GAP-10 and 40 XBKS shoulder dimensions.

Just measure the fired cases from each rifle and adjust your F/L dies to bump the shoulders .003"-.005" back from the fired cases. If your POF fired cases do have a longer shoulder measurement than the GAP-10, you need to segregate cases for each rifle and adjust your F/L die accordingly for each rifle. I can adjust my shoulder bump lengths easily by using a set of Redding Competition shell holders or a set of Skips die shims. They both work, but the Redding competition shell holders are easier to use. If POF says your rifle is chambered in 7.62 NATO, I would bet it will be on the snug side of a NATO chamber.

If you shoulder bump about -.003"-.004" from your fired cases, you won't be over working your brass. As I said before, if the POF brass measures longer than the GAP-10 fired brass, you should segregate the brass for the different rifles. When you do your testing, be sure to use the same Mfg. of brass in each rifle to get accurate shoulder measurements from your RCBS Mic. I have a feeling your two rifles are going to be very close in the fired case measurements, but I may be wrong. Again, be sure to use the same brass when re-testing. Most factory ammo / brass shoulder measurements will be at or a little shorter than SAAMI 308 specs.

As far as worrying about over working your brass, the gas guns beat the shit out of the brass and I'm lucky to get much more than 6 firings from my SR-25 or GAP-10. I would never consider using Lapua brass for either of my gas autoloaders, they beat the brass too badly. You will also find the gas guns cannot handle hotter loads that a bolt gun can. My gas gun loads are 1-2 grains below my bolt gun loads.

Shoulder bumping measurements will vary a little due to the brass being harder or softer, depending on who made the brass and also before and after annealing and how many times it has been reloaded without annealing.

Did you actually "zero" your RCBS precision Mic using a 1.630" headspace Go gauge? Sometimes, the RCBS mic can be a little off from true 1.630". You can go with the numbers you are using since they are a reference off of the gauges "0" point.

I don't know why you are mentioning neck sizing for these rifles, you should never neck size for autoloaders, always F/L size.

I do have a couple of NATO chambered rifles, a M1-A with a 1960's GI barrel and a Brazilian FN FAL. When I reload for them, I F/L size and usually bump back to near minimum SAAMI specs.

If you measure the fired brass from each of your rifles and adjust the F/L die if necessary, you will be fine without hurting the brass or yourself staying within the -.005"
from your fired cases.

 

[dmike]

Flight762, thanks for all the info.

Why are you worrying about this 308 VS 7.62 NATO? Are either of your AR-10 platforms actually chambered in 7.62 NATO? If POF says your rifle is chambered in 7.62 NATO, I would bet it will be on the snug side of a NATO chamber.

So the POF site says it's chambered in "7.62x51mm NATO/.308 WIN". You are exactly right, the cases from the POF are about .001 longer at 1.634 (the upper end of the NATO spec)...impressive that you expected that.

Did you actually "zero" your RCBS precision Mic using a 1.630" headspace Go gauge? Sometimes, the RCBS mic can be a little off from true 1.630". You can go with the numbers you are using since they are a reference off of the gauges "0" point.

Didn't know about zeroing with a go gauge. I figured it could be off but wasn't sure how to get a conclusive calibration so I'll pick one of those up.


Thanks again for all the info, it's a big help.

 

[FLIGHT762]

You don't really need to zero your precision mic with a 1.630" Go gauge. Save your money. If you are curious, just ask your local gunsmith if he could put his 1.630" Go gauge in your RCBS mic and see what it reads on the dial, then make a note. That's what I did with my Hornady gauge. Using the RCBS gauge as is, is fine. The numbers are relative to what you are doing.

There was a thread here not too long ago where a member zeroed his RCBS mic with a 1.630 Go gauge and he found the RCBS mic read -.002", not at the 0 mark he expected it to. Only if you needed to know the exact 1.630" measurement in your chamber. My Hornady Headspace / shoulder .400" bump insert reads 1.611" on a 1.630" Go gauge. This is because my gauge has a .019" chamfer on it's rim, that's why it reads .019" short of 1.630". I made notes of this and any shoulder bumping @ 1.611" is actually 1.630". I just read the gauge where it reads. If I have a case that reads 1.614", it is .003" over the 1.611" (1.630" minimum).

Another note, on my Remington Custom Shop 40 XBKS, the stainless barrel is actually marked "7.62 NATO" for the caliber, not 308 Winchester, but the chamber is in the low (tighter) end of the 308 Winchester SAAMI specs. It can get confusing.

As far as the chamber head space specs between 308 and 7.62 NATO, I get a little different numbers than you did.

7.62 NATO GO: 1.6355" / NO GO: 1.6405"/ Field Reject: 1.6455"

308 Winchester Go: 1.6300" / NO GO: 1.6340" / Field reject: 1.6380"

7.62 NATO has a bit more headspace allowed.

Just measure the cases fired in your rifles using your RCBS gauge and shoulder bump from there.

Don't worry about the pressure differences between the two. You don't have pressure measuring equipment anyway. You load ammo that is safe to shoot in your rifles without showing excessive pressure signs.

 

[bigedp51]

Traditionally "military" chambers are larger in diameter and longer in headspace than SAAMI civilian specifications. You can forget the headaches your reloading dies are made to SAAMI specifacations and all you need worry about is the correct shoulder bump with "YOUR" fired cases.

Second, we are Americans and all pressures are in pounds per square inch, the older copper crusher method is in PSI "BUT" to differentiate between the copper crusher method and the newer electronic transducer method one was called CUP (copper units pressure) and the other was called psi.

To put it simply 50,000 cup equals 60,000 psi and this is where the confusion starts, the military lists all pressures as psi regardless if the copper crusher or transducer method is used. This makes people think there is 10,000 psi difference between the .308 and 7.62. On top of this NATO pressure standards use the European CIP method of chamber pressure measured at the case mouth and not at mid case as we "Americans" measure the pressure.

I do not have any military 7.62 pressure data but I do have the pressure data for the 5.56. "BUT" you "MUST" understand the following.

All three pressures listed below are the SAME exact chamber pressure and are just measured using three different methods.

52,000 cup (copper crusher, American SAAMI)
55,000 psi (transducer, American SAAMI)
62,000 psi (transducer, European CIP)

Below are the military (milspec) pressure requirements for M193 ammunition for commercial contract military 5.56 ammunition.

MIL-C-9963F
15 October 1976
SUPERSEDING
MIL-C-9963E
12 May 1970

MILITARY SPECIFICATION
CARTRIDGE, 5.56MM, BALL, M193

3.7 Chamber pressure.

3.7.1 Measurement by copper-crush cylinder.-The average chamber pressure of the sample cartridges, conditioned at 70° ± 2°F, shall not exceed 52,000 pounds per square inch (PSI). The average chamber pressure plus three standard deviations of chamber pressure shall not exceed 58,000 PSI.

3.7.2 Measurement by piezoelectric transducer.-The average chamber
pressure of the sample cartridges, conditioned at 70° ± 2°F, shall not
exceed 55,000 PSI. The average chamber pressure plus three standard
deviations of chamber pressure shall not exceed 61,000 PSI.

Bottom line, the European CIP the equivalent of our American SAAMI considers the .223/5.56 and the .308/7.62 to be the same cartridge and loaded to the same approximate chamber pressures.

Long ago I made the photo below and the following humor about .308 and 7.62 chamber pressures, meaning American pressures versus European pressures.

"As you can see below it is perfectly safe to put 32 psi in your tires, BUT if you put 220 kPa in your tires they will blowup and kill everyone in a 300 yard radius."

Now click on the link below and order the "2014 Hodgdon Annual Manual", it has an article in it entitled "Reloading the .308 Winchester for ARs" and stop getting headaches.

New Products

 

[dmike]

As far as the chamber head space specs between 308 and 7.62 NATO, I get a little different numbers than you did.

7.62 NATO GO: 1.6355" / NO GO: 1.6405"/ Field Reject: 1.6455"

308 Winchester Go: 1.6300" / NO GO: 1.6340" / Field reject: 1.6380"

7.62 NATO has a bit more headspace allowed.

Just measure the cases fired in your rifles using your RCBS gauge and shoulder bump from there.

Will do. These were my sources for the specs. Looks like the 1.6300-1.6400 SAAMI values provided in the RCBS precision mic pamphlet must correspond to the chamber spec, not the case range as suggested?

 

[dmike]

You don't really need to zero your precision mic with a 1.630" Go gauge. Save your money. If you are curious, just ask your local gunsmith if he could put his 1.630" Go gauge in your RCBS mic and see what it reads on the dial, then make a note. .

Flight762: thanks for the continued education. I went ahead and got a forster FIELD, primarily to zero the mic but figured if I was going to buy one HS gauge that would be the most useful one. My mic reads about .0015 under the gauge. Not sure to what precision forster can produce stuff, I figure +/- .001 would be impressive so that would make it tough to say for sure which gauge was right, seems like the important thing is that they're close.

Anyway I have been getting a lot out of your posts so thanks for taking the time to share some knowledge.

 

[Lancer308]

RCBS Precision MIC as compared to the Reddding Instant Indicator / Case Comparator

This thread is of particular interest to me, as it touches on just exactly what I was searching for on the google. What really sparked my interest is the comment about someone putting their "Go Gauge" in the RCBS precision mic, and it was off. I have been reloading .308 for about a year now, and as I learned more about it, I ordered both the RCBS Precision MIC, and the Redding instant Indicator / Case Comparator. So, I put the brass "Set up Gage" for the Redding II/CC into the RCBS P. MIC, and I was surprised to find that it read .004" under the SAAMI minimum standard. The instruction booklet for the Pre. MIC say it is set to "zero" at the minimum of 1.630". I was looking for a "zero" reading. I measured the brass set up gauge with a Mitutoyo digimatic from the bottom of the case, to datum line, and it seems to be right on the money at 1.63. I get the feeling that the brass insert for the Redding II/CC is right on spec, and that the RCBS Pre. MIC is off by .004" (relative to the Redding). But after reading this thread, I think not to agonize about the difference, and concentrate more on the comparative readings that I get, as I use them to set up my dies. I like the RCBS Pre. MIC, it's handy and easy to use. I think that Redding II/CC to be an incredibly accurate measuring device. I have read this thread several times now, and thanks to all for the intricate, but easy to read / understand common sense approach to the subject. Awesome guys, outstanding information!

 

[dmike]

I measured the brass set up gauge with a Mitutoyo digimatic from the bottom of the case, to datum line, and it seems to be right on the money at 1.63.

Lancer, thanks for posting. I am not familiar with this setup (that particular caliper or the brass gauge) so I am curious about a few details...is there a feature on either/both part(s) that ensures precise measurement to the datum line? Also, was the digimatic reading only to the hundredth inch? Sorry if this is a dumb question but if we were rounding to the hundredth with 4 thou in dispute it seems like the rcbs could still be very close to dead-on.

 

[Lancer308]

dmike, the "brass set up" gauge comes with the Redding Instant Indicator / Case Comparator. It is a brass mock cartridge case that just extends to the shoulder, so the datum line (as per SAAMI specs) can used to zero out the dial gauge.. What I noticed is that the brass insert from Redding, measures "less than" the minimum standard of 1.630. In the RCBS Precision MIC, it measures to be 1.626, which is .004 (four one thousands) of an in short (more headspace). However, when I measure it with a digital caliper, it seems to be right on the money at 1.630 (as close as I can get it). The Mitutoyo Digimatic is just a digital caliper, and it measures to .001, or one thousandth of an inch. Your's is not a dumb question. This can be confusing. All I am trying to say is that in the RCBS Precision MIC, the Redding brass insert measures "short". With a digital caliper, it measures just right. That's why I thing the brass insert that Redding provides, is a near perfect representation of where the datum line should be with a .308 Winchester cartridge.

 

[Lancer308]

These are an array of the instruments I'm using; Redding Instant Indicator, and the RCBS Precision MIC.


You can see the large Dial Indicator, Redding supplies the brass insert so you can set your dial to zero in terms of where you compare your own, fired cases to. It does not "Fix" anything, it just allows you to compare the headspace measurement of your cases, to the SAAMI standard (as provided by the brass insert "set up gauge).

 

[judgedelta]

For what it's worth, I once asked Gene at Lewis Machine and Tool why they changed the stamping on the MWS from .308 to 7.62. (My rifle is marked .308) His response was that their overseas customers preferred it. There was no change in the guns whatsoever...

 

[dmike]

dmike, the "brass set up" gauge comes with the Redding Instant Indicator / Case Comparator. It is a brass mock cartridge case that just extends to the shoulder, so the datum line (as per SAAMI specs) can used to zero out the dial gauge.View attachment 31530. What I noticed is that the brass insert from Redding, measures "less than" the minimum standard of 1.630. In the RCBS Precision MIC, it measures to be 1.626, which is .004 (four one thousands) of an in short (more headspace). However, when I measure it with a digital caliper, it seems to be right on the money at 1.630 (as close as I can get it).

thanks for the info and the pics, I was not familiar with that product. I like the idea of a flat at the .400 datum line, FWIW I believe SAAMI spec is "1.634-.007" so it seems like that would put the minimum at 1.627. Not sure what the Redding paperwork says whether it's just "minimum" or if it specifically claims 1.630 to the flat but if it was just a SAAMI minimum value to zero the dial gauge then the mic might not be that far off. I have heard from folks on other threads that there aren't a lot of products available that give precise resolution of these distances...like the hornady comparator being about 2 hundredths off due to chamfer etc.

 

[dmike]

regardless I'm sure indexing the mic markings to .001 during manufacture is tough so I wouldn't be surprised if they were off sometimes. Limited to common tools, it's tough to know for sure which product really has the .400 datum line exactly 1.630 from the base (go gauge vs comparator vs mic vs brass gauge etc).

 

[FLIGHT762]

Here's a post where the Hornady Headspace gauge is discussed about the chamfer on the .400" insert. The poster's insert has a .016" chamfer, my gauge has a .019" chamfer. It's simple mathematics for me to add .019" to any of my shoulder bump measurements to get the numbers that would correspond to the SAAMI headspace spec numbers. In the video in post #5, he shows how you can use a feeler gauge to set up the bump gauge insert to set the insert up to read the number of the SAAMI go gauge:

http://www.snipershide.com/shooting...5-help-headspace-measurement.html#post2490582

Knowing where the insert measures to the 1.630" SAAMI go gauge or any of the 308 SAAMI gauges really isn't necessary since you set your die up to bump .001"-.002" or whatever, from your fired cases. The only time where it could be used when talking to a gunsmith about the readings you get if there is a headspace issue with the rifle, but the rifle would be checked with headspace gauges anyway.

 

[918v]

Context:
-handloading noob
-just loading for two AR10 platforms for now (bolt guns down the road)
-not BR/comp. just field prone, 350-500m paper
-Goals: 1)not blowing up self or rifle 2)getting 1/2 to 3/4 MOA 3)learning the finer points of handloading


The whole 308/762 headspace thing makes my brain hurt. I get the pressure units (CUP vs PSI) etc. POF says the P308 is chambered in both, GAP says the GAP-10 is .308. From what I've found SAAMI says 1.6300-1.6400 and NATO says 1.628-1.634...

Question 1:
If a cartridge headspace is .006 shorter than SAAMI .308 spec, is it dangerous for either rifle?

Question 2:
If a cartridge headspace measures .004 under 7.62 NATO spec, is it dangerous for either rifle?

Question 3:
If a cartridge headspace measures .002 over NATO spec, is it dangerous for either rifle?


Details:
Using a rcbs precision mic I get the following headspace values:

M80 knockoffs from Ventura...1.624-1.628
Federal GMM175...1.6252-1.6274
Fired cases from the GAP-10...1.6316-1.6353 AVG 1.6330
Cases after full-length sizing with shellholder contact on die...1.6262-1.6297 AVG 1.6279
New Lapua brass...1.6262-1.6299 AVG 1.6281

I suppose the gauge could be off but if my numbers are right that would mean:
-Neck sizing spent brass without resizing could yield a round that was .0013 longer than NATO spec (but within SAAMI 308)
-Some of these rounds/cases could be .004 under NATO and .006 under SAAMI spec

I know, I know...some of you are saying "it's not that serious" but I just want to make sure I don't blow myself up, hurt my gun, overwork brass etc so I want to try to understand this. Thanks for any help or advice you can share.

Lookit...

Your die is bumping the shoulder .005" from it's fired position which happens to be within spec. That's about perfect for a semi-auto.

Stop overthinking this. Think about reliable chambering and about all the gas and crud floating in the chamber while the rifle cycles.

 

[bigedp51]

Below a Colt Field gauge for the AR15, 1.4736

Below my Hornady cartridge case headspace gauge reading true headspace, and no longer a comparator gauge.

When I first put the Field headspace gauge in the Hornady gauge I noticed it read .011 low and then the lightbulb came on. I loosened the set screw between the red and silver body halves and placed a .011 feeler gauge between them and then tightened the set screw. I now have a calibrated gauge that reads true cartridge headspace.

Below a fired case from my AR15.

Below the same case with .003 shoulder bump.

And finally a new unfired Federal M193 cartridge

My RCBS Precision Mic is gathering dust, it is not calibrated by RCBS and reads -.002 and the Hornady gauge is cheaper and can be calibrated. On top of this you need to buy separate RCBS Precision Mics for each caliber and you only need to buy "ONE" Hornady cartridge case headspace gauge.

 

[Lancer308]

Bigedp51, do you mean the "Hornady Lock N Load Headspace Gauge Kit" with the five gauges? And you just use the one that fits whichever caliber you use? I would also need a Colt field gauge for the .308. I don't really need much of an excuse to buy reloading tools, I see that Hornady gauge kit for $40 on amazon. I like this, it seems to make easy an often confusing issue that usually ends up with a mind warp, as in "Whattt?" Thanks for your posting. By the same line of thought, I should be able to put a Colt field gauge in my Redding Instant Indicator, I wonder if it will fit in the shell holder. Or, I could compare the Redding brass set up gauge, to the Colt field gauge, then I would know for sure. Anyway, your powers of illustration are superior, and present a clear and concise picture.
"