Go Back   CMP Forums > Welcome Center > How To's
Register FAQ Members List Calendar Mark Forums Read

Reply
 
Thread Tools Display Modes
  #11  
Old 12-09-2010, 08:58 PM
Ericc Ericc is offline
 
Join Date: Oct 2009
Posts: 2,495
Default

Quote:
Originally Posted by en bloc View Post
Could you shed some light on the difference between these tests and the two gauge manufacturers.
I reserve the right to be wrong here, but my understanding is that there is no difference between the two gauges. The confusion comes from the military using a different datum line. A datum line is a reference line by which the length of the gauge is measured. SAAMI used one datum line, the military used a different line.

That was my understanding. To verify this I pulled the drawings for the M1 and they are in agreement with the SAAMI spec. I pulled the Clymer drawing and it's in agreement with the SAAMI spec. I pulled the forster gage drawings and they're in agreement with the SAAMI spec.

For our current headspace gages the Datum line starts where the case shoulder tapers down to 0.375" for SAAMI. If you use that datum line then a GO gauge spec's @ 2.0487" which you can see below in the SAAMI image. This is what my forster Go gauge says. It's also what the Clymer drawing says.

The military used the beginning of the taper as their datum point in which case a USGI GO gauge would say 1.940".

I've included the SAAMI spec's below.



I just pulled drawing #SA 6535464 (Mar. '40) it lists several dimensions for their "reference" headspace. I'm not sure why its shown that way. SA shows 1.940 as being the minimum dimension to the shoulder. Which would agree with the SAAMI 1.9399 above.

Springfield Drawing:



Clymer Drawing for their Go Gauge.



Notice the Clymer gage is also 2.048" from the same datum point of 0.375".

So the confusion comes from that fact that the military used a different datum point.

So lets look at TM9-1275 the maintenance manual for the M1. (Page 11)

Quote:
(c) The headspace of a rifle is measured as the distance between the shoulder of the chamber and the face of the bolt where the bolt is in a locked position. The minimum headspace measurement is 1.940 inches.
Ah ha! From our SAAMI drawing we can see that the 1.940" dimension is the exact same as the military describes in TM9-1275. (FOR THE M1). When we use the military's datum point.

So the military chamber is the same as a SAAMI chamber.

Clymer and forster gages use the exact same datum point from the SAAMI spec which agrees with the Military spec for the M1.

So both gage's are the same and they both use a datum point different then the military used.

That's my interpretation on the drawings and spec's.

Eric
Reply With Quote
  #12  
Old 12-10-2010, 08:33 AM
en bloc en bloc is offline
 
Join Date: Nov 2010
Location: Massachusetts
Posts: 252
Default Thanks Ericc

Light shed.

I will put your efforts to good use.

Thanks again,
~Matt
__________________
Eight-On-Tap!
Reply With Quote
  #13  
Old 12-12-2010, 02:32 PM
Unclenick Unclenick is offline
 
Join Date: Nov 2010
Posts: 988
Default

Matt,

I'll toss in a caveat. There is supposed to be no difference in any of the modern gauges, but it can be a case of getting what you pay for. I have a Forster's armorer's set (all sizes from GO to Field in 0.001" increments) of .308 gauges I got 20 years ago that proved to be very imprecise. When compared using the RCBS precision mic, the one labeled 1.638" is actually a thousandth shorter than the one labeled 1.637". There are two other sizes that are the same length, despite being labeled with 0.001" difference from one another. The RCBS PM I have is off by 0.002" from absolute itself, so these are relative and not absolute readings, but tell the tale just the same.

I calibrated the .308 Win RCBS PM using a Dave Manson GO gauge. His work is very high quality and is often a little less expensive than others. He used to run Clymer's shop, but eventually set out on his own. I took that GO gauge and used a granite surface plate and a height gauge and a precision reamed hole to verify that his gauge was accurate within the limits of resolution of the height gauge (half a thousandth).

I keep meaning to send the Forster gauges back to Forster and see if they will swap correct gauges into the set. Other Forster products I have, like my Co-ax press, are very well made. As I say, these gauges are 20 years old and maybe Forster went through a rough patch in QC back then that's over with now. I don't know. Just the same, at this point I'd buy from Dave Manson first. Clymer and Pacific and JGS are good brands, too, and I have reamers and/or gauges I've purchased from all of them at one time or another. I favor Manson for price and always being best or equal to the best on quality. Also, he's saved me money by giving me free advice in the past that prevented some pointless purchases, so I admit being biased in his favor based on that treatment. My .30-06 gauges are Clymer gauges made when he was still with Clymer.

There has been a change in how gauging is done on rimless bottleneck cases since the Garand was designed. In the original drawings and Hatcher's Notebook and other older sources, you will find .30-06 headspace measured from the breech end to the intersection of the case wall and shoulder. There are two problems with this old method. First, because the chamber is always slightly bigger than the case, that shoulder corner is a different diameter for the case and the chamber. Second, exactly locating the vertex of the obtuse angle formed by the wall and shoulder without a stem gauge slipping is tough to do by feel. The diameter tolerance makes using a plug gauge inaccurate. On a cartridge case it is even tougher because the case has a small radius at that corner. You need an optical comparator to project the location of that intersection accurately off a case.

The modern SAAMI chamber specification for rimless bottleneck cartridges gets around those two problems by assigning a single reference diameter in mid-shoulder that serves as a measuring datum for both cases and chambers. When two tapers nest, they self-center, so the datum diameter can match in both without conflict. It is the only dimension on a SAAMI drawing can be and is identical for a case and its chamber. It is called the headspace datum, and there are several common values used among a large number of cases to cover different chamberings.

For .30-06 the headspace datum is where the shoulder is 3/8" (0.3750") in diameter. For a minimum headspace .30-06 chamber, the GO gauge is 2.0487" from breech to headspace datum. That is 0.1133" longer than the headspace as Garand and Hatcher would have measured it to the start of the shoulder.

Below is an illustration I made for the headspace datum of a .308 cartridge. The .308, having less body taper than a .30-06, has its headspace datum at a wider 0.4000" diameter. Just remember that for .30-06 it will be at 0.3750". But the principle is the same, and this dimension is what the headspace gauges provide.

If you want to measure your chamber's headspace approximately, you can usually derive it from a fired case that has not yet been resized. The easiest way is to buy one of the caliper comparator adapter heads by Sinclair or Hornady (the former Stoney Point gauge) and get the 0.375 case comparator insert. If you have the caliper but not the insert, you can improvise, though you'll need a good touch for wiggling things straight. For this you need a cylinder, like a spacer with a close-to 0.375" diameter hole in it and a digital caliper. Commercial spacers aren't precisely bored, so you can use a bronze 3/8" Oilite bushing from the hardware store, instead. Put a headspace GO gauge in the caliper with the cylinder sitting on it and zero the caliper. Then put a fired but not-yet-resized case in place of the gauge and see what the difference is. Add that difference to the GO gauge length of 2.0487". A hint: it is common for the Garand extractor to bend rims back a little, so rotate the case until you get a minimum reading. The comparator adapter tool makes that easier to do. Figure a case that was full length resized will spring back about 0.001"-0.002 after firing, so the number you get will be that much shorter than actual headspace. That doesn't really matter. You mainly want to be sure you are sizing cases adequately and pushing the shoulders back at least 0.002" from what you measure as-fired.





Improvised case measuring:





Nick
__________________
--------------
Nick

Gunsite Orange Hat Family Member
CMP Certified GSM Master Instructor
NRA Certified Instructor
NRA Patron Member
ORPA Life Member
Reply With Quote
  #14  
Old 12-12-2010, 03:48 PM
jmm jmm is offline
 
Join Date: Sep 2010
Location: G'Boro, NC
Posts: 1,707
Default

Nick - nice details. Of course both .308 and .30-'06 are "delta L" cartridges as well.
http://en.wikipedia.org/wiki/Delta_L_problem
Reply With Quote
  #15  
Old 12-12-2010, 04:54 PM
Darthnip Darthnip is offline
Super Moderator
 
Join Date: Apr 2010
Location: East Central Indiana
Posts: 553
Send a message via AIM to Darthnip
Default

Quote:
Originally Posted by en bloc View Post
Nice work Ericc.
This post should be made STICKY for posterity.

~Matt
Heck yes it should, very informative. Some one should really sticky this...wait, I can do that!
Reply With Quote
  #16  
Old 12-12-2010, 06:24 PM
jmm jmm is offline
 
Join Date: Sep 2010
Location: G'Boro, NC
Posts: 1,707
Default

Flexing those new found muscles, eh?
Reply With Quote
  #17  
Old 12-14-2010, 05:24 PM
Unclenick Unclenick is offline
 
Join Date: Nov 2010
Posts: 988
Default

Jmm,

Re ΔL, if you look at SAAMI drawings, cartridges are specified by maximum dimension with a minus tolerance, while chambers are specified as a minimum dimension with a plus tolerance. (I suppose this is intended to prevent gunsmiths getting their math wrong and fouling it up, but I don't really know.) The way the SAAMI drawings show the .30-06 and the .308 Win, there is an actual specified 0.0039 and 0.0040" interference fit between a minimum chamber and a maximum case at the same-size headspace datum on the shoulders of the two rounds, respectively. For .30-06 this is 0.0024" shorter than a NO-GO gauge, while for .308 it is the same as a NO GO gauge. The difference in dimensions from breech to shoulder cone intersect for the chamber and case are 0.0056" and 0.0081", respectively. And simply extending a maximum case shoulder to the minimum chamber diameter gets 0.0050" and 0.0032", respectively, so that doesn't explain where these 0.0039" and 0.0040" interference fits come from. Neither does a difference between CIP and US drawings, as these are just SAAMI's own drawings I am speaking of.

The idea that this was OK possibly came from Hatcher's and other's observation that a bolt rifle would push a .30-06 case shoulder back several thousandths just working the bolt fast on a rifle. 0.006" comes to mind from his measurements with an Enfield, but that's from memory, so don't take it as gospel. The number is in Hatcher's Notebook in the chapter on headspace, IIRC. Because the chamber diameter minimum is greater than the case diameter maximum, there is always room for the case to swell up and fit. The Wiki article suggestion that closing a break action rifle on an interference fit might not squeeze the case enough to get similar results to the bolt action seems perfectly believable.

Both case maximum dimensions at the headspace datum have a -0.007" tolerance. I've yet to encounter a new case or cartridge in either chambering that wasn't near the short end of that tolerance range, typically being -0.005" to -0.007" from the specified number, so no actual interference fit seems to occur normally. When NATO adopted the 7.6251, they elected to avoid any potential action closing problems with interference fit, and instead specified the chamber to be 1.635" minimum (that's the NATO GO gauge spec) referenced to that same headspace datum. Thus they neatly bypassed the whole issue.
__________________
--------------
Nick

Gunsite Orange Hat Family Member
CMP Certified GSM Master Instructor
NRA Certified Instructor
NRA Patron Member
ORPA Life Member
Reply With Quote
  #18  
Old 12-14-2010, 06:10 PM
jmm jmm is offline
 
Join Date: Sep 2010
Location: G'Boro, NC
Posts: 1,707
Default

Unclenick, it is interesting that the 7.62x51 avoided the problem, my understanding was the Winchester jumped the gun on the .308 Win. (pun intended) before the final specs were released.
Of course the military is only concerned about the first time a case is used, and they can let their chambers be at max. length, or more even, on automatic weapons. As long as the round chambers and fires, they don't care if it gets reloaded. Case stretching is someone elses problem.
The list of "Problematic" cartridges is also interesting. You would think that recent cartridges like the Lazzeroni cartridges (7.82 Warbird, 7.21 Firebird) and the .357 Sig would have avoided problems.
Don't these guys check their tolerance stacking?
Reply With Quote
  #19  
Old 12-22-2010, 05:28 PM
Unclenick Unclenick is offline
 
Join Date: Nov 2010
Posts: 988
Default

It is a bit odd. Interestingly, on tolerance, Hatcher's notebook includes some information on a special reamer he made and used to make extra long headspace to see what it would do. It seems to me he got out to 0.050" long, or thereabouts, and still got no head separations. These were new cases each time, of course. The long headspace stretch never actually occurs, though. What happens is the shoulder can only be moved so far forward before the cartridge rim starts to headspace on the extractor hook, the way a lot of 1911's unintentionally do, and any additional gap forward of the shoulder is covered by the shoulder blowing forward.

I was at the range one day when a fellow sighting in a hunting rifle handed me a case with no neck at all and asked me if it was normal? He'd borrowed the .308 rifle from a friend to take hunting and picked up some .308 Remington soft point ammunition at Walmart. I had the dubious pleasure of showing him where the chambering is stamped on the barrel. It read .30-06.

There was no damage other than to his ego. His friend had told him it was a .308. Note, by the way, that even though the heads are the same size, a .308 has less body taper than a .30-06, so there is a good chance the base of the shoulder likely would have been squeezed a little bit on chambering. Again, a rifle chamber proves it can do a little resizing.
__________________
--------------
Nick

Gunsite Orange Hat Family Member
CMP Certified GSM Master Instructor
NRA Certified Instructor
NRA Patron Member
ORPA Life Member
Reply With Quote
  #20  
Old 12-22-2010, 05:39 PM
jmm jmm is offline
 
Join Date: Sep 2010
Location: G'Boro, NC
Posts: 1,707
Default

.308 in a .30-'06 is pretty common, I've seen it probably 5 times. I haven't seen the results of a .358 Winchester (.308 necked up to .35 cal) in a .30-'06 yet, but that wouldn't end well. I'm surprised Winchester created that cartridge.
I haven't tried to keep track of all the new Short/Super Short/Ultra Mag cartridges out there, but I'm sure several new "opportunities" have been created.
Reply With Quote
Reply

Tags
field, gauge, headspacing, no-go

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off



All times are GMT -5. The time now is 07:54 PM.