Gunsmithing 6.5 Creedmoor headspace question

[dondlhmn]

When I mic the headspace gauges I have (the go and no-go gauges) I find that there is a .004" difference between them when measured in the middle of the 30 degree tapered "shoulder", yet when I look at the SAAMI specs for headspace it looks to me like the tolerance is 1.541 minimum and 1.551 maximum. Doing a little math, that comes out to .010" for the headspace instead of the .004" I see in the difference between the headspace gauges and also the .004" is what I have always thought the tolerance was. So...what is the tolerance....010" or .004"? My reference for the SAMMI specs for this cartridge is http://www.saami.org/pubresources/cc_drawings/Rifle/6_5 Creedmoor.pdf and I am wondering if I am not reading that right or what? Any help would be appreciated!!

Would you guys take a look at that spec sheet and see what you come up with, please?

 

[Killswitch Engage]

.010 headspace would be bad ju ju. Your headspace gauges are correct. +.004 for no go. I set my chamber at go +.002.

 

[Turd in the Pool]

Tried looking at some of the other specs to no avail. Go with what you know and stick with .004". I like to be snug on a Go, but that's just me.

 

[dondlhmn]

.010 headspace would be bad ju ju. Your headspace gauges are correct. +.004 for no go. I set my chamber at go +.002.

Yeah...I agree with THAT! But when you read that spec sheet, what do you see? It sure looks to me like it says .010"!! I, also, do like my headspace set on the tighter side of the tolerances and that .010, which is what I think I am seeing/reading on that spec sheet sure looks all wrong....I'd call that BLIND/bad ju-ju!!

Just for grins...read the spec sheet...that is the one I pulled up from the SAAMI web site.....what do you see on the spec sheet in reference to headspace??

 

[Killswitch Engage]

No sure. They show a .010 window on 308 it looks to me. http://www.saami.org/pubresources/cc_drawings/Rifle/308 Winchester.pdf

 

[dondlhmn]

No sure. They show a .010 window on 308 it looks to me. http://www.saami.org/pubresources/cc_drawings/Rifle/308 Winchester.pdf

Yeah...I just looked at the spec sheet that link takes me to and it looks that way to me, too. There JUST HAS TO BE something here I am not getting or not reading right...as .010" seems WAAAYY too much to me!!

 

[Garyc]

This is from Forster's website:

Forster offers three lengths of headspace gages per rifle caliber. In order from the shortest to longest, they are: GO, NO-GO and FIELD:
1.GO: Corresponds to the minimum chamber dimensions. If a rifle closes on a GO gage, the chamber will accept ammunition that is made to SAAMI’s maximum specifications. The GO gage is essential for checking a newly-reamed chamber in order to ensure a tight, accurate and safe chamber that will accept SAAMI maximum ammo. Although the GO gage is necessary for a gunsmith or armorer, it usually has fewer applications for the collector or surplus firearms purchaser.
2.NO-GO: Corresponds to the maximum headspace Forster recommends for gunsmiths chambering new, bolt action rifles. This is NOT a SAAMI-maximum measurement. If a rifle closes on a NO-GO gage, it may still be within SAAMI specifications or it may have excessive headspace. To determine if there is excessive headspace, the chamber should then be checked with a FIELD gage. The NO-GO gage is a valuable tool for checking a newly-reamed chamber in order to ensure a tight and accurate chamber.
3.FIELD: Corresponds to the longest safe headspace. If a rifle closes on a FIELD gage, its chamber is dangerously close to, or longer than, SAAMI’s specified maximum chamber size. If chamber headspace is excessive, the gun should be taken out of service until it has been inspected and repaired by a competent gunsmith. FIELD gages are slightly shorter than the SAAMI maximum in order to give a small safety margin.

As I understand it:

The GO gauge corresponds to SAAMI's minimum headspace dimension, the NO-GO gauge isn't an actual SAAMI dimension but rather an intermediate length that the gauge maker determined that a properly chambered rifle should conform to. The FIELD gauge corresponds to the SAAMI max headspace dimension and if the gun will chamber the FIELD gauge then the chamber is bigger than SAAMI's max and considered unsafe. GO plus .004" sounds about right for NO-GO. GO plus .010" should correspond to FIELD. I agree that .010" is too much, but it's still safe and the case shouldn't rupture if fired in a chamber that's oversized by that amount. It's not real good, but it's not going to come apart on you.

 

[chipsfan]

As I understand it:

The GO gauge corresponds to SAAMI's minimum headspace dimension, the NO-GO gauge isn't an actual SAAMI dimension but rather an intermediate length that the gauge maker determined that a properly chambered rifle should conform to. The FIELD gauge corresponds to the SAAMI max headspace dimension and if the gun will chamber the FIELD gauge then the chamber is bigger than SAAMI's max and considered unsafe. GO minus .004" sounds about right for NO-GO. GO minus .010" should correspond to FIELD. I agree that .010" is too much, but it's still safe and the case shouldn't rupture if fired in a chamber that's oversized by that amount. It's not real good, but it's not going to come apart on you.

Almost have it. Go+.004" is NOGO (not minus)- depending on who makes the gauges, I saw one with a .006" interval once. Go +.008" is field for most manufacturers. Go+.010" (depending on the cartridge, but using 308 as an example this holds true) would be max SAMMI. Some cartridges allow more (check out 9mm Parabellum, some less, check out a belted magnum)

There is no SAAMI spec on go, nogo and field. It's up to the tool and die maker.

FYI-in case you care- go for an ackley is typically parent cartridge go minus .004".

Check the Brownells catalog, they have a warning about mixing go and nogo gauges from different manufacturers. Dave Manson did a headspace lecture at the gunsmith conference last year and spoke about this at length.

You should also take a look at the SAAMI cartridge drawings. They are smaller then the chambers. Ammo runs short, chambers run long and the rounds should feed and chamber.

I run my chambers within go plus .001". Forster makes gauge sets in 308 and 223 in .001" intervals: MATCH RIFLE HEADSPACE GAUGES | Brownells

 

[dondlhmn]

This is from Forster's website:

Forster offers three lengths of headspace gages per rifle caliber. In order from the shortest to longest, they are: GO, NO-GO and FIELD:
1.GO: Corresponds to the minimum chamber dimensions. If a rifle closes on a GO gage, the chamber will accept ammunition that is made to SAAMI’s maximum specifications. The GO gage is essential for checking a newly-reamed chamber in order to ensure a tight, accurate and safe chamber that will accept SAAMI maximum ammo. Although the GO gage is necessary for a gunsmith or armorer, it usually has fewer applications for the collector or surplus firearms purchaser.
2.NO-GO: Corresponds to the maximum headspace Forster recommends for gunsmiths chambering new, bolt action rifles. This is NOT a SAAMI-maximum measurement. If a rifle closes on a NO-GO gage, it may still be within SAAMI specifications or it may have excessive headspace. To determine if there is excessive headspace, the chamber should then be checked with a FIELD gage. The NO-GO gage is a valuable tool for checking a newly-reamed chamber in order to ensure a tight and accurate chamber.
3.FIELD: Corresponds to the longest safe headspace. If a rifle closes on a FIELD gage, its chamber is dangerously close to, or longer than, SAAMI’s specified maximum chamber size. If chamber headspace is excessive, the gun should be taken out of service until it has been inspected and repaired by a competent gunsmith. FIELD gages are slightly shorter than the SAAMI maximum in order to give a small safety margin.

As I understand it:

The GO gauge corresponds to SAAMI's minimum headspace dimension, the NO-GO gauge isn't an actual SAAMI dimension but rather an intermediate length that the gauge maker determined that a properly chambered rifle should conform to. The FIELD gauge corresponds to the SAAMI max headspace dimension and if the gun will chamber the FIELD gauge then the chamber is bigger than SAAMI's max and considered unsafe. GO minus .004" sounds about right for NO-GO. GO minus .010" should correspond to FIELD. I agree that .010" is too much, but it's still safe and the case shouldn't rupture if fired in a chamber that's over sized by that amount. It's not real good, but it's not going to come apart on you.

Hmm....Interesting info. I guess that is another reason I have never relied upon "Field Gauges"....sure sounds like a lot of headspace and way more than good, repeatable accuracy would call for. Maybe safe, but still sounds scary much to me...and also sounds like it would REALLY shorten case life, especially if they were full length re-sized every time!! Good info, though..! Thanks. And it does sound like I was reading that spec sheet/diagram right in light of the max allowable with a field gauge.

 

[chipsfan]

I find that there is a .004" difference between them when measured in the middle of the 30 degree tapered "shoulder"

It should be at the .400" diameter part, thats the datum point for the headspace on this cartridge. Look at the drawing you linked too, you'll see .400" there...

 

[dondlhmn]

It should be at the .400" diameter part, thats the datum point for the headspace on this cartridge. Look at the drawing you linked too, you'll see .400" there...

Yeah..looked. Read. Saw. Did that before posting. Still find .004" diff pretty much as long as you measure it in the same place (however you want to define that spot) on BOTH gauges. BTW, the more or less middle of the tapered area between the "body" and the beginning of the case neck (If a gauge had a neck) IS where .400" happens to be if you check it. That all checked out fairly closely with the fired cases I have lying around (Plus/minus "spring back" and etc...) and the gauges, too and made sense to me. That is another reason I wondered about the .010".

 

[DAVETOOLEY]

I just ran into this same topic this past week. My reamer maker makes a go gage +.006" for the no go. Perfectly acceptable. Customer had gages made by a different company. Go gage + .004" for his no go. Using his no go gage the bolt touched with some resistance on closing. He was apprehensive about the headspace since the bolt would close on his no go gage. Both of us were right and it was perfectly safe either way. So he would feel comfortable I set the barrel back a few thousandth so it would not close on his no go gage. It took longer to sort out the dimensional differences in the gages than to shave a few thou off the barrel shoulder.

Here's something you can do if have a rifle where the bolt closes on a no go gage. Put a piece of masking tape on the head of the no go gage. Try it again. If it still closes with no resistance then you have a problem. If it closes with resistance you can measure the thickness of the flattened tape and then decide. Remember if you reload you adjust your sizing die to fit the chamber which means after firing the brass one time headspace is not an issue. It becomes an issue if you push the shoulder back to far. I have about 125 different chamber reamers and most are wildcats that require fire forming. It is not an issue unless you fire a case in a larger diameter chamber and the case wall is not supported. I've seen a lot of stupid shit over years.

 

[dondlhmn]

Here's something you can do if have a rifle where the bolt closes on a no go gage. Put a piece of masking tape on the head of the no go gage. Try it again. If it still closes with no resistance then you have a problem. If it closes with resistance you can measure the thickness of the flattened tape and then decide. Remember if you reload you adjust your sizing die to fit the chamber which means after firing the brass one time headspace is not an issue. It becomes an issue if you push the shoulder back to far. I have about 125 different chamber reamers and most are wildcats that require fire forming. It is not an issue unless you fire a case in a larger diameter chamber and the case wall is not supported. I've seen a lot of stupid shit over years.

Yeah...I have used the tape trick before and have seen people that actually use only a "GO" gauge and tape to figure out/set headspace when changing barrels (mostly bench rest guys, IIRC)

Not having been a lifelong gunsmith, I haven't seen a LOT of stupid shit in that field (A lot of shooters doing/thinking some pretty goofy stuff, though!), but then in the field where I spent my working (well...earning money...) career, I DO have a fair bit of expertise and BELIEVE ME...I did see a lot of stupid shit there...so I can just imagine what YOU have seen with firearms based on my somewhat limited exposure there....!!

BTW, the gauge set I am using are PT & G, whom I pretty much trust. These are rented, as I don't really need them every day (yeah..I SHOULD just break down and buy some...) as I figure it would be pretty hard to screw up gauges where a guy wouldn't want to use them....now REAMERS, on the other hand...Way to easy to screw up, so would NEVER use/trust rented reamers...I was talking to a guy the other day that rented some from a place that claimed to ALWAYS check reamers and never let any out the door that weren't good. HA ha....I don't know how that place defined "good", but the ones this guy got were apparently not very "good"!!!!

 

[chipsfan]

I just ran into this same topic this past week. My reamer maker makes a go gage +.006" for the no go.

PTG?

 

[DAVETOOLEY]

I don't name names particularly when everyone is right. I like numbers on the side of my gages not just go or no go. That really doesn't matter but it tells me someone was paying attention all the way through to marking the gage. Also we're talking about .002" difference in a gage which I may or may not be able to determine here in the shop without some extra work. It was a very brief trouble shooting period with this barrel with numbers on the side of the gages.

 

[Killswitch Engage]

I use a go gauge and a piece of scotch tape on the back for my headspace. I cut my chamber so the bolt will fall about half closed on go +.002

haven't had any problems that way so far.

 

[Garyc]

Almost have it. Go+.004" is NOGO (not minus)- depending on who makes the gauges....

Thanks, I fixed it in my original post. I was trying to picture it in my head and got my plusses and minuses mixed up!

 

[vitalemj]

PTG?

Pacific Tool and Gauge

http://pacifictoolandgauge.com/metr...ml?search_query=Headspace+Gauges+&results=338

 

[scudzuki]

I use a go gauge and a piece of scotch tape on the back for my headspace. I cut my chamber so the bolt will fall about half closed on go +.002

haven't had any problems that way so far.

I don't even have no-go gages.
A go gage and scotch tape is my no-go.

 

[scudzuki]

It should be at the .400" diameter part, thats the datum point for the headspace on this cartridge. Look at the drawing you linked too, you'll see .400" there...

As long as the shoulder angle is the same on both gages, it doesn't matter where on the shoulder 2 gages are compared.
If one is .004" longer at a .400" diameter gage line, it will be .004" longer at a .300" gage line, or any other for that matter.

 

[Red_SC]

I only used go gauges and tape until I started doing barrels for other people; I use both gauges now.