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Discussion Starter · #1 ·
I just picked up an RFB on Gunbroker at a price I couldn't refuse.
I've been doing some reading while I wait to see one of these rifles in real life.

Anyway, I came across an interesting discussion on thefirearmblog.com.
A guy named Derrick, who says he works at KelTec, explains that .308 and 7.62 are different.
Basically, the .308 has more energy which implies a bit more recoil.

Anybody here have any comments regarding .308 vs. 7.62 mm ?
Any favorite brands?

By the way, Derrick posted this interesting link on the subject:
http://www.thegunzone.com/30cal.html
 

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That link contains a myth that has surrounded 7.62x51 in that it states that 7.62x51 is 50,000 psi, It's not, it's 50,000 CUP (copper units of pressure) People keep changing CUP to PSI without converting. 7.62x51 and 308 are the same pressure in PSI, 62,000. When they were tested two different methods were used, 7.62x51 was tested with the crush copper method with which CUP is used as the standard unit. 308 was tested by SAAMI using the piezoelectric method with results using PSI as the unit. Even if there was a big difference in pressure per NATO docs 7.62x51 (7.62 NATO) chambers are proofed to 75,000 PSI.

This link expains it better than I can http://home.comcast.net/~ehorton/The Truth About 308 Win and 762 NATO.pdf

There is however a slight head space difference between the two which is only an issue if your head space is borderline, plus 7.62x51 brass is a little thicker to better survive hard handling.  
 

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My first experience firing 7.62x51 NATO was in my issue M14. I didn't start shooting this round as a civilian untill 1964 in a "G" series FNFAL imported by Browning Arms Company and a commercial semi-auto G3 (as the HK91 designation was not yet in use). The general understanding at the time was that the .308 was the sporting version of the NATO round
and was almost identical except for a thinner cartridge case which had a slightly greater powder capacity. Early shooters scrounged brass where they could. I remember someone I knew cutting down .30-06 brass to make NATO. A couple of grains less powder in the thicker military case was  advised for reloading. This was pretty much what we went by untill the internet told us how little we really knew about the thousands of rounds we had each fired in the previous 30 to 40 years..........
Finally this is being straightened out as the newer posters are discovering that the highly touted pressure differences are more a matter of differing measuring systems than anything else.........  Now those who shoot these rounds can look for something else to worry about..........;D
 

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Discussion Starter · #4 ·
Thanks for the clarification guys!

It sounds like I can use either .308 or 7.62 mm in my RFB.

There are are some good deals on surplus 7.62 mm.
I plan to steer clear of the absolute cheapest ammo.
And, I've heard warnings about steel case. So I'll avoid that as well.
But, any other recommendations regarding ammo would be appreciated!

3dogs
 

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The RFB seems to be happier with the heavier bullets. Most of what I have read and my various handload recipies bear this out so far. There was one fairly recent report of good performance with 150GR silvertips, but there has been no followup to that. My dabblings in the 147-150GR area have not been successfull, but I am not finished yet, as you can get 147GR bulk surplus bullets a bit cheaper than 168's, so I am not giving up on them just yet:) Most agree that the 168's shoot well, and my experience also agrees with this.
 

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Here's a way to look at it simply:
If you have a 168 grain bullet leaving at 2700 FPS in a .308
and a 168 grain bullet leaving at 2700 FPS in 7.62,
The math HAS to equal. It takes the same pressure to push the same bullet
the same speed. The difference would be only if the bullet diameters or the
bore diameters varied.
If one had more pressure, given the barrel and bullet diameters being equal,  it would leave at a higher velocity!
 

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I've even told people on other forums who were arguing that 308 will blown up a 7.62x51 rifle to please send me their 308 ammo and I will personally give each and every round a test firing just to be sure. After testing I will sent the used rounds back to them proving they didn't blowup. So far no one has taken me up on the offer.   :-[
 

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WLJ said:
I've even told people on other forums who were arguing that 308 will blown up a 7.62x51 rifle to please send me their 308 ammo and I will personally give each and every round a test firing just to be sure. After testing I will sent the used rounds back to them proving they didn't blowup. So far no one has taken me up on the offer.   :-[

I'm going to try that on a couple forums I frequent....I wonder if anyone will take me up on it! ;D
 

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Just like not all 5.56x45 is created equally with regard to charge; some surplus might have been down pressured fairly significantly to make it more controllable with rapid fire. I know the Japanese did this significantly when the US demanded all allies switch to the 7.62x51 (not the largest people on average) I don't know who else might have.
 

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Discussion Starter · #11 ·
I received my new RFB and I now have a definitive answer to my question.

KelTec must have ESP, because there was an insert in the owner's manual!
The title is "Cartridge Difference Between the .308 Winchester and 7.62 NATO Cartridges"

To summarize, it elaborates on the difference in the specs for these two cartridges.
However, the last paragraph says it all:

"As you can see from the evidence above, the .308 Winchester is not the same
as the 7.62 NATO even if they are largely interchangeable. In testing we have found that
commercial .308 Winchester is perfectly safe for use in the 7.62 NATO RFB despite the
different headspace criteria. There is no perceptible difference in accuracy for most shooters,
and the RFB is often more accurate than nearly all other semi-automatic 7.62 NATO caliber rifles tested."

Thanks for clearing that up KelTec!
3dogs
 

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For those who also have AR-10s
From Armalite
ArmaLite’s® larger AR-10 series rifles are all chambered with 7.62 NATO
chambers. .308 Winchester (SAAMI standard) ammunition functions perfectly in the
7.62 NATO chambers.
 

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I received my new RFB and I now have a definitive answer to my question.

KelTec must have ESP, because there was an insert in the owner's manual!
The title is "Cartridge Difference Between the .308 Winchester and 7.62 NATO Cartridges"

To summarize, it elaborates on the difference in the specs for these two cartridges.
However, the last paragraph says it all:

"As you can see from the evidence above, the .308 Winchester is not the same
as the 7.62 NATO even if they are largely interchangeable. In testing we have found that
commercial .308 Winchester is perfectly safe for use in the 7.62 NATO RFB despite the
different headspace criteria. There is no perceptible difference in accuracy for most shooters,
and the RFB is often more accurate than nearly all other semi-automatic 7.62 NATO caliber rifles tested."

Thanks for clearing that up KelTec!
3dogs
Do you still have that insert? If so would you please post an image of it?
 
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