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7.62 vs .308 Dimensions?

This is a discussion on 7.62 vs .308 Dimensions? within the The M14 forums, part of the M14 M1A Forum category; What is the spec range for case diameter's .308 vs 7.62. Or what is the size difference in thousand's of a inch. I need to ...


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Old May 31st, 2006, 06:33 PM   #1
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Exclamation 7.62 vs .308 Dimensions?

What is the spec range for case diameter's .308 vs 7.62. Or what is the size difference in thousand's of a inch. I need to know which is bigger around and by how much. This info may solve a problem with a M14 that I am having. I know that they are very close, but there is a slight difference.
Thanks

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Old May 31st, 2006, 06:54 PM   #2
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The exterior dimensions of the two cartridges are the same.
308 Win is simply the commercial designation of the 7.62x51 military cartridge.

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Old May 31st, 2006, 07:38 PM   #3
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IIRC the headspacing is different, though.

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Old May 31st, 2006, 07:54 PM   #4
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Quote:
Originally Posted by g5
IIRC the headspacing is different, though.
i never understood this so please educate me. i always thought headspacing had to do with the rifle itself and not the ammo. if the round feeds it feeds. if the bullet properly exits the barrel than what is the problem here? i

please educate me!

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Old May 31st, 2006, 08:06 PM   #5
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Simply put, the chamber on the M14 & M14 type rifle are a bit more generous in case dirt, humidity(due), mud or several rounds have to be fired before you have time to field strip and clean. The rifle must go bang every time in a battle field situation. That being said, match rifles, as well as new commercial rifles are headspaced to 308 specs. The surplus barrels are more generous than a commercial barrel. We are getting low on surplus barrels though. HTH

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Old May 31st, 2006, 08:16 PM   #6
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It was explained to me by a gunsmith who barrels a lot of GI type rifles that the 308 headspace gauges are for commercial type bolt rifles and match rifles as Jack Orr has said. And that for a 7.62 chamber the next size up gauge is used. For example, the 308 'no-go' gauge is considered a 'go' gauge for the 7.62. And a 308 'field' gauge is a 'no-go' for a 7.62. At least that's the way I was told this works. HTH.


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Old May 31st, 2006, 08:46 PM   #7
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Check out Surplus Rifles.com or Fulton armory (faq page) and you will see the difference. Also, Google the topic of 308 vs 7.62 and you will find several interesting articles. One thing is for sure, they have different case pressures and they are different in deminsions. Here is a cut from the article on surplusrifles.com

Winchester, recognizing that any round adopted in mass by a group of countries would almost automatically become very popular, adopted the T65 / 7.62 NATO round and called their version the .308 Winchester. However, they are not identical twins.



There are headspace differences between the two rounds. The .308 go-gauge is 1.6300, the .308 no go-gauge is 1.6340. The 7.62 go-gauge is 1.6350, the 7.62 no go-gauge is 1.6405. Since NATO military ammunition can come from any NATO country, and the goal is the ability to interchange ammunition, the military chamber is larger.

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Old May 31st, 2006, 09:24 PM   #8
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Someone point me to the 7.62X51mm reloading dies then...

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Old May 31st, 2006, 09:45 PM   #9
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The reloading dies are the same, since the exterior dimensions of the cartridge are the same. However, the 7.62 NATO brass is thicker, this allows it to stretch to fill and seal the larger chamber without splitting. If you have a generous NATO chamber, you should shoot only NATO spec ammo in it. If you have a 308 Win spec chamber, you are pretty much good to go to shoot all reasonable ammos in your gun.
Finding NATO headspace gauges can be a challange. Since a NATO chamber can swallow a 308 Win headspace guage set and still be within NATO tolerences.

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Old June 1st, 2006, 05:31 AM   #10
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Old June 1st, 2006, 05:42 AM   #11
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The two essential gauges are GO and Field Reject

7.62X51 GO is 1.6355"

7.62X51 Field Reject is 1.6455"

A 1.636" .308 gauge out of the Brownells set of MATCH .308 gauges will serve effectively as a GO gauge

You can get either Clymer or Forster to cut (custom make) a 1.6455" Field Reject gauge for about $50 and a three to four week wait.

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Old June 1st, 2006, 10:22 AM   #12
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The Myth continues...

Quote:
Originally Posted by sooner44
Check out Surplus Rifles.com or Fulton armory (faq page) and you will see the difference. Also, Google the topic of 308 vs 7.62 and you will find several interesting articles. One thing is for sure, they have different case pressures and they are different in deminsions. Here is a cut from the article on surplusrifles.com

Winchester, recognizing that any round adopted in mass by a group of countries would almost automatically become very popular, adopted the T65 / 7.62 NATO round and called their version the .308 Winchester. However, they are not identical twins.



There are headspace differences between the two rounds. The .308 go-gauge is 1.6300, the .308 no go-gauge is 1.6340. The 7.62 go-gauge is 1.6350, the 7.62 no go-gauge is 1.6405. Since NATO military ammunition can come from any NATO country, and the goal is the ability to interchange ammunition, the military chamber is larger.
The above is a perfect example of how this myth perpetuates; The headspace of the CARTRIDGES is for all intents and purposes the same. Also they do not "operate" at different pressure levels. Most will say the .308 has the higher pressure but I've also heard it the otherway. Problem is many times CUPs are compared against PSI which is a meaningless comparison. Further the NATO specs (right out of the TM) list the 7.62 pressures as "average operating pressure" which are +/- 7,000 to 8,000 PSI. This is often compared to the SAAMI list "maximum operating pressure". This SAAMI figure includes the 7 to 8,000 + PSIs. If one does a factual comparison of US NATO spec M80 ball you will find that it is loaded right up there pressure wise with any .308 Winchester cartridge. Actual tests using Oehler M43s have demonstrated that a lot of .308 Winchester ammo acutally does have less operating pressure than M80 Ball.

This myth that some how the two cartridges are different seems to stem from the early (mid '70s) caution not to fire Winchester (W-W) ammunition or reloaded W-W cases in M1A/M14s. This caution was/is necessary simply because the W-W brass is much thinner in the web/case head area and incipient case seperation could occur as soon as the on the second firing. It was cautioned not to use factory Winchester .308 ammo "just in case" it might happen on the first firing. This caution had absolutely nothing to do with .308 ammo being "hotter". However, with the advent of the internet armchair theorists, experts and some well intentioned experienced shooters have attempted to explain this simple fact way out of proportion to what it really is. Many of these theories and ideas have been repeated and are in print so they have become "facts". They are not facts, they are myth and thus the myth was born and continues to perpetuate.

It is a simple matter of fact that the two cartridges are the same. Many, including myself, have fired lots of commercial .308 Winchester ammo through M1As and M14s without problems for years. I do refrain from using W-W ammo in M1A/M14s though because the original caution does have some validity to it. Conversly I have shot thousands of US 7.62 M80, M118 and Special Ball along with commercial .308 ammo through numerous commercial bolt guns with regular SAMMI chambers, "national match" chambers and minimum SAMMI spec match chambers without nary a sign of any pressure problems.

We also find many of these experts attempting to show that the .223 and the 5.56 are two different cartridges. They are using the same misguided theorys used for the 7.62 is different than the .308 theory. Fact is they too are the same cartridge.

That the headspace of 7.62 military chambers is slightly larger than the headspace for SAAMI commercial .308 Winchester chambers does not make the two cartridges different. Common sense and logic should tell you that. Do not believe the myth.

Larry Gibson

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Old June 1st, 2006, 04:44 PM   #13
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Quote:
Originally Posted by Larry Gibson
The above is a perfect example of how this myth perpetuates; The headspace of the CARTRIDGES is for all intents and purposes the same. ......
......
That the headspace of 7.62 military chambers is slightly larger than the headspace for SAAMI commercial .308 Winchester chambers does not make the two cartridges different. Common sense and logic should tell you that. Do not believe the myth.

Larry Gibson
Well stated...

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Old June 1st, 2006, 08:42 PM   #14
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Quote:
Originally Posted by Larry Gibson
This myth that some how the two cartridges are different seems to stem from the early (mid '70s) caution not to fire Winchester (W-W) ammunition or reloaded W-W cases in M1A/M14s. This caution was/is necessary simply because the W-W brass is much thinner in the web/case head area and incipient case seperation could occur as soon as the on the second firing. It was cautioned not to use factory Winchester .308 ammo "just in case" it might happen on the first firing. This caution had absolutely nothing to do with .308 ammo being "hotter". However, with the advent of the internet armchair theorists, experts and some well intentioned experienced shooters have attempted to explain this simple fact way out of proportion to what it really is. Many of these theories and ideas have been repeated and are in print so they have become "facts". They are not facts, they are myth and thus the myth was born and continues to perpetuate.

Larry Gibson
I am of the opinion that you can shoot either (new) .308 or 7.62x51 in the M14/M1A rifle..

The issues of insipient case head separation come into play when the brass is reloaded. Given that the chamber is "longer" on military rifles, I believe this is a VERY valid concern.

I reload .303 British and because of the unusually long chamber it is imperative to ONLY neck-size the brass. Full-length re-sizing pushes the shoulder back too far at the expense of the case wall (stretch) near the head of the case.

Unfortunately, neck-sizing is something that should only be done for bolt action rifles. Semi-autos NEED more "sizing" to feed reliably.

I have been told by knowledgeable sources that the best way to reload for a M1A is to take a bag of fired brass (from the rifle you're loading for) and have your RCBS sizing dies made to fit. Such "custom" dies will permit resizing without excessively working the brass leading to much longer case life. YMMV

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Old June 2nd, 2006, 05:15 AM   #15
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Bottom line is...
If the carton says 7.62mm, as most surplus ammo does, whether or not it's packed loose or in 20 rnd boxes, it's GI surplus and made for a rifle that has a 7.62mm chamber.

If you buy ammo that says .308, then it is commercial ammo and will be set to commercial headspace dimensions.

You can fire both out of a Military 7.62mm Chamber, but may find that the bolt doesn't lock in battery when using 7.62mm in a .308 chamber.

Most M14's and M1A's, are 7.62mm. Some new barrel makers though made the chambers to .308, and as long as they are not chromed, can be reamed to the looser tolerance of 7.62mm. If they are chromed, .308 chambers leave them alone, and shoot good high quality commercial .308.

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