.308 AR Headspace

[Benito]

I recently bought some .308 headspace gauges and started playing with them.
I have 1 Armalite AR-10 complete factory rifle. It closes on the Go, and does not close on No-Go. OK, no issue there.
I have 1 set of parts from different manufacturers for a .308 AR build. The bolt & barrel/chamber combo closes on the Go, also closes on the No-Go, but does not close on the Field. Is this setup safe to shoot??? If so, are there other downsides like perhaps reduced precision, etc?

Please advise.

 

[NukeMMC]

What GO and No-Go gauges do you have?
For .308, SAAMI says: 1.630 GO, 1.634 NO GO, 1.638 FIELD
For 7.62x51 NATO: 1.6355 GO, 1.638 NO GO, 1.6455 FIELD

As you can see, it looks like you have a barrel setup for 7.62 NATO, not 308. Depending on its use, you are fine. If you're handloading for it, pay attention to shoulder setback as you will get case head separation pretty quick if you over-size your brass.

 

[Benito]

My headspace gauges are marked as 1.630 GO, 1.634 NO GO, 1.638 FIELD.
For the combo that closes on GO and NO GO (but not FIELD) what Ammo would be safe to use? Commercial .308? Surplus 7.62x51?

 

[NukeMMC]

It should be safe to shoot either 308 or 7.62 NATO, but you should have it verified by a competent gunsmith. I would suggest that if you intend to reload, you would certainly want to tighten the headspace to where your AR10 is (1.631-1.633) with a different bolt, if possible. Ultimately you would want your rifles to be able to shoot the same reloads without over-sizing the brass to fit the shorter headspaced one and being as much as 007" too short for the longer headspace. Doesn't always happen without either buying extra parts or doing some searching and measuring for the right parts.

 

[sinister]

If the bolt closes on a NO-GO gage you are at the maximum safe headspace for that cartridge. It must not close on the FIELD gage -- the FIELD gage denotes excessive headspace and an UNSAFE condition.

308 Winchester and 7.62mm are identical cartridges -- it's the chambers that are different.

308 Win and 7.62 Nato are NOT identical cartridges.

1. 7.62 brass is thicker in the base with generally less case capacity
2. 308 Winchester is loaded to higher pressures than 7.62 NATO

Rule of thumb is 7.62 is safe in a 308win chamber but you should not shoot 308win ammo in a 7 62nato chamber

 

[TitaniumBlue]

On semi auto platforms, a lot of manufacturers actually aim for the NoGo headspace gage as nominal. This allows for a more reliable gun when everything gets dirty. A chamber at Go isn't going to cycle as well as something at NoGo. For reference, I used to be in the barrel manufacturing business making barrels for numerous AR companies.

 

[bfoosh006]

From Ted Brown on another forum...

Link...http://m14forum.com/ted-brown/104224-head-space.html

[IMG2=JSON]{"data-align":"none","data-size":"full","src":"http:\/\/m14forum.com\/images\/icons\/icon14.gif"}[/IMG2]On Head Space

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Not to break any ones bubble, but…

It’s been driving me nuts reading all the posts about head space and the difference between military and SAAMI specifications. The fact is that there is actually no difference other than the military specification tends to be on the longer side of SAAMI in order to insure the proper functioning of weapons used under combat conditions. Do I hear shock and awe? Maybe just shock…

While researching information about the use of reaming to adjust head space in chromium plated barrels, I came across some very pertinent information about head space for .308 Winchester and 7.61x51 NATO spec chambers. Are you all sitting down?

SAAMI publishes the SAAMI Voluntary Performance Standards which all manufacturers use to insure their ammunition and barrel chambers are consistent across the board. On Page 66 of this standard the drawing specifications for .308 Winchester indicate that minimum head space is to be 1.630” and the maximum permissible is 1.640”… Did you catch that? 1.640” max!

Now, I know most all head space gage sets use 1.634” as the no-go and 1.638” as reject. Better safe than sorry I guess.

The military starts at 1.634” and the no-go is 1.638”, but Field Reject is 1.6445”. That’s really stretching it! In actuality, the military won’t accept anything over 1.640”. We were told that even National Match barrels with chambers of 1.638” should not be rejected as long as they shot well. The feeling was that long chambers would probably not last as long as those with shorter head space, but I never had the opportunity to test this out.

The point is that .308 and 7.62 are pretty much interchangeable in barrels with head space between 1.630” and 1.640”. Shoulder angles and datum points are the same for both specs. My experience has been that barrels with 1.630” head space will not reliably chamber NATO spec ammo. Bolts won’t always lock completely with minimum head space. While this is especially true with imported military ammunition, I have even had problems with US military M118 and Special Ball in rifles with minimum chambers. I recommend at least 1.631” and I lean toward 1.632” as minimum for the M14.

I love this stuff and I hope you all have a great day. I’m sorry it’s been awhile since I last contributed to my column. I know some readers eagerly await anything that spews forth from my vast cesspool of knowledge. I’ve really got them buffaloed. Don’t take any of it too seriously.

 

[Benito]

Taking the build into the gunsmith tomorrow. He says for a semi-auto it's OK for it to close on the 1.634" NO-GO, as long as it doesn't close on the 1.638" FIELD.
Hope he is correct. I'll take it to the range and run a bunch of different .308 through it.

 

[BigBlastin300win]

Sorry to kick and old thread but when I measured my headspace on my once fired brass they are all about 1.625 some 1-2 thousandths longer. When I checked my prepped brass that was sized they are mostly at 1.620-1.622(except for one that is 1.617). Is this acceptable or is there something I am doing wrong? This is the first gasser i've reloaded for and I just want to make sure im sizing correctly.

Using virgin starline lrp brass. That test loads i ran seemed to do pretty well but even with my die set at 2.800(dummy round measured at that) they are seating to 2.805.2.810.

Any help would be greatly appreciated.

 

[nn8734]

Sorry to kick and old thread but when I measured my headspace on my once fired brass they are all about 1.625 some 1-2 thousandths longer. When I checked my prepped brass that was sized they are mostly at 1.620-1.622(except for one that is 1.617). Is this acceptable or is there something I am doing wrong? This is the first gasser i've reloaded for and I just want to make sure im sizing correctly.

Using virgin starline lrp brass. That test loads i ran seemed to do pretty well but even with my die set at 2.800(dummy round measured at that) they are seating to 2.805.2.810.

Any help would be greatly appreciated.

Edit: seems like Hornady comparator. Looks fine to me and is consistent with measurements from my 308s using Hornady gauge.

And to clarify you’re not actually measuring the true headspace dimension, the tool is giving you s reference measurement.

 

[BigBlastin300win]

Edit: seems like Hornady comparator. Looks fine to me and is consistent with measurements from my 308s using Hornady gauge.

And to clarify you’re not actually measuring the true headspace dimension, the tool is giving you s reference measurement.

Yes, you are exactly right. Just a reference point and yes a hornady comparator. This is the first time loading a gasser and all the stuff i read online had me worried about head space. So I wanted to double check. All the reloads went well on the first batch(27rds) and was gearing up for some 178's so I wanted to make sure I was on the right track. Thank you for the quick and informative response. Just what I was looking for.