Special "Tactical .308 Headspace Gages" for AR Type Rifles???

[BigBore77]

I will not name builder or barrel manufacturer, but I checked recent .308 WIN upper purchase for headspace and found the combination of barrel and provided bolt would turn on my Forster NOGO gage.  Also tried a buddy's NOGO (same brand) and two other bolts with same result.  Sent upper back to builder and he checked with his .308 "tactical" NOGO for AR type rifles and says the build is fine.  Has anyone ever heard of "tactical" headspace gages for AR type rifles?  I cannot find any reference on web search.  Is this guy pulling my leg or have I missed a chapter in the book? ???

 

Any help with this issue would be great.  THANKS!!

[FaRKle!]

What the hell is a "tactical" head space gauge? Does he mean a field gauge?

 

It also might be different if he's using a 7.62x51 NATO gauge.

Edited February 12, 2014 by FaRKle!

[98Z5V]

What the hell is a "tactical" head space gauge? Does he mean a field gauge?

 

 

 

That's what I was thinking when I read the OP's stuff - even though the barrel maker might not know what the fuk he's talking about...  <dontknow>

[BigBore77]

After the builder told me I was wrong and basically screwed up, he stated he used "special tactical headspace gages for SR type rifles", whatever that means.  I have searched a hundred or more headspace references on the web and only found a reference to differences in gages for pure bolt guns and autoloaders…however it was a passing reference and i could find no more info.  Autoloaders do stretch brass a bit more than bolt guns, but that only means you have to work the case a little harder in the resizing die.  It seems to me a .308 WIN chamber is a just that…a 308 WIN chamber????

 

Also found a caution to always use the same brand of gages…don't mix Forstner and Clymer or others.  Explanation is that Forstner gages tend toward the minimum end of SAMMI tolerances and Clymer tend towards the maximum end of SAMMI tolerances.  I guess I should buy a set of Clymer gages and check with them.  Maybe that's the difference.

 

Anyone have other info??

[98Z5V]

After the builder told me I was wrong and basically screwed up, he stated he used "special tactical headspace gages for SR type rifles", whatever that means. 

 

Anyone have other info??

 

Only have a little bit more, brother - that builder just might be full of $hit! 

 

Edit - he might be talking about the gauges that have the cut-out in the base, so you don't have to remove the ejector...  <dontknow>

 

 

 

 

Edited February 12, 2014 by 98Z5V

[survivalshop]

Head space on a 7.62 & .308 should be the same , so gage stamp should not make a difference . Special head space gage ? I would try your gage's in another 308 rifle, just to make sure its not the gages, they can get damaged easier than you think .

 

98 

Thats the first HS gage I have seen with the ejector cut out , cool ! I haven't bought one since the 300BLK & its a Forster, the only ones that were made at the beginning of the 300BLK .

[Ksgunguy]

so were can I go buy those gauges with the cut-out on the bases?

daddy like alooot

[FaRKle!]

Head space on a 7.62 & .308 should be the same , so gage stamp should not make a difference. 

 

They're close, but 7.62NATO has a larger headspace.

 

7.62x51 NATO:

-Go: 1.6355"

-No-Go: 1.6405"

-Field: 1.6455"

 

.308 Win:

-Go: 1.630"

-No-Go: 1.634"

-Field: 1.638"

 

Therefore you need to make sure your smith's using the right/same gauges you are.

[shibiwan]

They're close, but 7.62NATO has a larger headspace.

 

[snip]

 

Therefore you need to make sure your smith's using the right/same gauges you are.

 

Yup... methinks this be your problem. Most 308-AR barrels are chambered for 7.62x51 these days.

Edited February 12, 2014 by shibiwan

[survivalshop]

I ment to say about the same , but I still believe it shouldn't seat on a " No Go" gage, ether one . Unless there is some thing wrong with the chamber  The different manufacturers have a little different spec's on what there gages read .

 

I wonder if the below applies to this case, this is off the Clymer web sight, Ya never know what gages these are , his or there's  !

 

 

Summarizing to this point, headspace gauges for “improved” versions of rimless, shouldered cartridges

have the following characteristics:

•

The

gauge

shoulder angle is the same as the shoulder angle of the standard chamber. See Fig. 5.

•

The breechface-to-neck/shoulder junction dimension is .004”

shorter

for the “GO” in an “improved”

chamber. See Fig. 6.

•

The “GO” gauge for the standard chamber becomes a “NO-GO” gauge for the improved chamber.

This means that an “improved NO-GO” will drop in a standard chamber since it is the same length

as a “GO” gauge for the standard chamber.

In practice, cutting an “improved” chamber in a barrel blank is not greatly different from cutting a

standard chamber. Whether a roughing reamer is used or not, the finisher is run in until the action will

just close on the appropriate “GO” gauge - in this case, an “improved GO” gauge. 

 

 

[EasyEJL]

I ment to say about the same , but I still believe it shouldn't seat on a " No Go" gage, ether one . Unless there is some thing wrong with the chamber  The different manufacturers have a little different spec's on what there gages read .

look again

7.62x51 NATO:

-Go: 1.6355"

-No-Go: 1.6405"

-Field: 1.6455"

.308 Win:

-Go: 1.630"

-No-Go: 1.634"

-Field: 1.638"

the 7.62 go gauge is larger than the 308 no go, and only 3 thousandths short of the field gauge. Its definitely possible for your chamber to be inside of specs for a 7.62 - say 1.639" - which would still close on a 308 field gauge.

So more important than barrel builder, we need to know what was the barrel actually chambered for? What is it marked as? and which gauges did he use.

[survivalshop]

I've Gaged a lot of 7.62x51 chambers & never had a .308  "No Go" gage fit , not saying its not possible , just in my experience , it didn't seat , but that all ,like you said , depends on how it was reamed & what it is & who's doing the Head spacing .

 

Just remember that his builder said he has a special gage , that's why I posted that about the "Improved "

Edited February 13, 2014 by survivalshop

[EasyEJL]

Yeah, i'm guessing his builder was shall we say mentally challenged :D

[BigBore77]

Guys, sorry for the delay in replying to thread, but I was babysitting for grandkids and got caught in the NC blizzard last week.  Thank you all for your help, comments, and info.  The following is long, but gives what I know about the issue to date.  

 

I have my upper back from the builder and the barrel is clearly marked .308 WIN.  However, it could be chambered for 7.62x51 for all I know.  If it is, it is tightly chambered on the 1.6355 GO dimension for 7.62x51, because it barely turns on the 1.634 308 WIN NOGO gage, with the builder supplied bolt (first  one listed in the dimensions below). Note, the rifle builder and barrel company are not the same.  I have ordered a new set of Clymer gages in 308 WIN from Brownells and will check with them when they arrive.  

 

When I first talked with the barrel company, I did not speak with the owner, but he called me back and we discussed the issue.  He says he trys to get a 1.632 chamber (obviously a 308 WIN value), but he cannot tell me anything about the bolts he uses in the process.  He also says if it just turns on the NOGO, it is safe to shoot and I should just shoot it.  So what the hell is a NOGO gage for???  

 

Now, if the bolt used by the barrel company was perfect, the chamber is 1.6325 or so, to turn on his 1.632 gage.  Since there is no Mil Spec for a 308 bolt, and I still cannot find a dimensioned drawing, I don't know what the correct dimensions are, but I think the bolt face recess is nominal 0.125 and the lugs are are nominal 0.312 (my estimate and tolerances are anyone's guess). 

 

I have measured three bolts in my possession. The three actual recesses are 0.1255, 0.1265, and 0.1280 with lug lengths of 0.3110, 0.3115, and 0.312 respectively.  I have used two different measuring tools (a Federal Spec depth micrometer and a Swiss made, Brown & Sharpe electronic caliper that has papers traceable to international standards), so my measurements are good to 0.0005 or better.  There is 0.0025 difference in recess depth and 0.001 in lug length.  This would mean an increased headspace of 0.0015 to 0.0030 for those bolts.  So if the chamber made with a perfect bolt is 1.6325 or so, then adding 0.0015 to 0.0030 will make the headspace between 1.6340 and 1.6355 or more.  I'm pretty sure this stack up is why the bolt turns on my headspace gage.

 

Now I see why some of the best barrel companies supply the bolt actually used in chambering the barrel or ask you to send a bolt for the job.  I have a sad feeling that many barrels not made to a specific bolt are not right with the bolt being used.  

 

I'm going to follow through and let everyone know how it all turns out, but my advise now is to only deal with a barrel company that supplies a bolt matched to the barrel or lets you send in your bolt for the chambering operation.  I paid the builder about $900 for this barrel installed.  It is over $600 straight out.  This has been an expensive lesson in rifle building.  So far I have not fired a shot from a rifle I have spent well over $3000 on.

[survivalshop]

 

e Forster Headspace Gage is designed to ensure accurate testing of chamber headspace.

Headspace is the distance between the face of the breech and the base of the cartridge when theaction is closed. Safety and accuracy are threatened when excessive headspace exists.

For best results with bolt action rifles, remove the extractor from the bolt. The bolt must work freelyin the receiver. It is impossible to have the proper feel for a gage if the bolt is tight or sticky.

Insert the proper gage into the action. Attempt to close the bolt on the gage.  CAUTION — Neverforce the bolt closed on a gage.

A rifle with correct headspace should close easily and without feel

on the GO gage, but should not close on the FIELD gage. If it does close on the NO-GO gage, but

not on the FIELD gage, the rifle may still be serviceable. However, it is not advisable to use it for best

accuracy with factory ammunition.

CAUTION — Any rifle that will close on the field gage may have

dangerously excessive headspace and should not be fired under any circumstances.

Three lengths of Headspace Gages are available per caliber:

z

GO length— This checks the rifle to see that the chamber will accept the cartridges made to

maximum SAAMI specifications.

z

NO-GO length— This is used by most gunsmiths as a maximum headspace gage when

chambering a rifle’s headspace dimension. If a rifle closes on a NO-GO gage, it

may have excessive headspace. If a rifle closes on a NO-GO gage, it may still be within the SAAMI Rifle Chamber tolerance. Next, it should be checked with a FIELD gage.

z

FIELD length— If a rifle closes on a FIELD length gage, its chamber is dangerously close

to, or longer than, SAAMI’s specified maximum chamber size. This is the longest of the three

gage lengths.

 

 

 

Edited February 21, 2014 by survivalshop

[BigBore77]

Finally the story ends.  I received new headspace gages by Clymer and did the check using the NOGO from this set.  The bolt would not turn on this NOGO.  So my apologies to the rifle builder and the barrel maker.  They were right and I had a faulty NOGO gage.  I checked both of my NOGO gages in a RCBS Precision Mic for .308 WIN and found that the Forster was a full 0.001 shorter (maybe even a couple of tenths more) than the Clymer NOGO.  I have had the Forster for over 30 years and used it on several bolt guns and a couple of M1-As.  These were tight enough that it caused the bolts not to close, but the AR bolts headspace a little different I guess.  The Forster gage has been re-marked as 1.633 since I can't bring myself to can it.

 

I spoke with an engineer at JP and he said that the M-16/AR-15 specifications allowed the controlling dimension on the M-16 bolt to vary by a total of 0.003.  The controlling dimension is from the rear of the bolt lugs to the plane of the bolt face recess.  This dimension is not a Mil-Spec on the AR308s, but he thought most bolt makers probably used this allowance.  JP does not.  They hold their Enhanced Bolts to much tighter tolerances so the don't have this problem.  So if you don't get a barrel chambered to your bolt or get a bolt supplied with the barrel, be prepared for the potential of swapping out bolts to get your headspace correct.  

 

When you buy a bolt by itself, check the lugs to see how close to 0.3125 they are and how close the face recess is to 0.125.  Based on all I can find out, these are the theoretical dimensions that control the bolt's contribution to headspace.  Subtracting the two gives 0.1875 or 3/16" as the theoretical distance between rear of lug and plane of recess for the controlling dimension.  Since most folks cannot measure that directly, you have to measure the two dimensions and subtract.  When you do, the farther you are from 0.1875, the more likely you will have headspace issues.  Since there is only 0.004 between GO and NOGO, you can see why a bolt that uses up 0.003 might be a problem.  

 

I learned a lot on this go around.  I guess the biggest thing to note is when there is no Mil-Spec you have to be very careful in mixing parts.  

[98Z5V]

 

I learned a lot on this go around.  I guess the biggest thing to note is when there is no Mil-Spec you have to be very careful in mixing parts.  

 

That's the most important thing about all of these 308 platforms.  Soooooo many manufacturers now, just for receivers.