7.62 NATO vs .308 Win brass

[Larry Gibson]

I plan on loading to roughly M80 specs with either BL-C(2) or H4895. I have no reason to want to push this round hard, I have other guns that will do that far better. I think what I'll do is do my load development in the 7.62 cases and see how those loads perform in the 308 cases afterwards. If they're not to my liking, I'm keep the cases separated and work up a load for them that is close to the 7.62 load.........Thanks fellas.

I've loaded thousand of M80 (U.S. spec) equivalent loads with both those powders and a couple others (H335 & AA2230). As popper says if the range to be used at is short then the differences in cases won't matter that much.

However, I do suggest sorting all the Winchester cases out if you are loading for a gas gun. The Winchester cases are considerably thinner than M80 cases in the web area and incipient case head separation can be a real problem. I also suggest the use of a standard 308W RCBS X-die to size the cases if used in a gas gun as case life will be considerably longer. Run a "search" for the test i did on them. I got 20+ loadings per case with LC brass using a M118SP load.

If you've a gas gun with a 22" barrel then you'll want 2750 fps with 145 - 150 gr bullets to duplicate U.S. M80 velocities. Also, with either of those powders with loads at that velocity the pressure will be fine.

[MostlyLeverGuns]

Weigh the brass. For 308/7.62x51 I've got Lapua at 178 gr, IMI - 180 gr, Peterson - 180 gr, Winchester - 159 gr, Frontier - 162 gr, IVI -180 gr, FX - 180, there are differences, various military usually run about 180 grain for me/my scale, all are averages of 10-15 cases, most vary plus minus a grain or two. Winchester is much lighter but they do vary. Weighing will tell you how it compares. Electronic scale works well for checking or sorting brass. I sort by headstamp, don't usually weigh every case, but do check 10-15 of each headstamp to avoid problems.

[Pirate69]

I thought I would take a look at the 308 Win/7.62 mm NATO case issue using QuickLoad. I have always heard the need to reduce the powder charge when using military cases and have taken the advice at face value. Guess, at this point, I need a better understanding as to why a charge reduction may be needed.

I selected at 308 Win load that is close to the SAAMI max and ran it through QuickLoad. The same charge was then run through QuickLoad as a 7.62 mm NATO load. The chamber pressure difference was very surprising.

So, with all things being equal, what is driving the significant difference is predicted chamber pressure? 59,752 psi compared to 72,398 psi seems to be significant. Note ion the table below, the NATO brass has 0.26 cc less case capacity than the commercial brass. It is assumed that this is a result of the “thicker” NATO brass when compared to commercial brass.

For this load, an 8.1% decrease in usable case capacity results in a 21.2% increase in chamber pressure. The decrease is only 0.26 cc. This is about 2 and ½ drops of water from a pipet; not a lot.

How much difference in weight of commercial and NATO brass is necessary to yield a 0.26 cc difference? Given that the SG of brass is about 8.5 grams/cc, it is calculated that a difference is the weight of the commercial and NATO brass is about 34 grains.

Is that a good number or is QuickLoad employing a safety factor? I just complete the resizing and trimming of both commercial and NATO brass. I pulled four commercial FC casts and four different NATO cases; one being a LC case. The FC cases weighed 175.4, 174.5, 177.4 and 177.3 grains. The NATO cases weighed 179.2, 172.0, 178.8 and 178.5 grains. Not a lot of difference in this small sample.

If you look at the extreme difference in weights between commercial and NATO cases, it is only 4.7 grains. This equates to only a 0.0358 cc difference in usable case capacity.

If the usable case capacity, for the 308 Win load, is decreased by 0.0358 cc, the pressure rises from 59, 752 psi to 61,210 psi.

I realize I am beating the data to get an answer and this is a limited sample of NATO and commercial cases. There may be cases with much different weight and that is baked into the QuickLoad software.

I think it is safe to say, that if using commercial cases only, the 308 Win data is appropriate. if using NATO cases only, the NATO data is appropriate. If using a mixture, a default to the NATO data is the safe bet.

Edited to add photo of chart. https://imgur.com/HTCLSkJ

Load: 168 grain Nosler CC. 46.0 grains IMR 4895



308 Win 7.62 mm NATO
COL 2.800 COL 2.800
CL 2.014 CL 2.014
Case Capacity 56 grains water 3.636 cc Case Capacity 52 grains water 3.373 cc
Usable Case Capacity 49.304 3.201 cc Usable Case Capacity 45.3 2.941 cc
Chamber Pressure 59,752 psi Chamber Pressure 72,398 psi

[Pirate69]

Win brass was some of the lightest with more case capacity than others. It was preferred for long range shooting as more powder could be used safely. Win 308 brass weighed around 155 grains iirc.

Military brass meant for use in semi and full auto rifles had a thicker/stronger case head that made the capacity less than most commercial cases. Some LC late 60's match brass weighed 177-180 grains.

Recommendations I have read were to drop the powder charge .5 grain for every 5 grain increase in case weight. I believe the info was provided by the NRA when they had a technical staff.

The 0.5 grain powder decrease per 5 grain increase in case weight caught my attention. I posted on a comparison of an identical load of 308 Win commercial brass pressure vs 7.62 mm NATO brass; as calculated by QuickLoad. QuickLoad does not indicate brass weights; only case volume. However, the difference in brass weights can be calculated (estimated) from the difference in case volumes. For the comparison that I did, the difference in case weights was 34 grains. Lets say 35 grains. From the rule of thumb that you posted, a reduction of 3.5 grains would be recommended to ensure a safe load.

I took the modeled output from the 7.62 mm load and reduced the powder charge until it equaled the pressure of the commercial brass. It took a 2.5 grain reduction in the powder charge to get equal chamber pressures for both loads. Looks the rule of thumb is a good one to remember. Hope I can remember it in the future.

[GONRA]

GONRA loaded quantities of 7.62x51mm NATO in LC 62 brass decades ago. Typical powder charges were in the 42 grain range.
IF I had used .308 Winchester brass, would have have ADDED about 2.5 grains powder based on lottsa previous load development.
Sounds like Pirate69's experience.....

[popper]

46.0 grains IMR 4895 WAY TOO HOT!

[Pirate69]

46.0 grains IMR 4895 WAY TOO HOT!

I agree 46.0 grains of IMR 4895 is a hot load in commercial cases; even more so in NATO cases. This load is not being recommended by any means. It was selected as part of a paper exercise since this load predicted a chamber pressure slightly below the 308 Win SAAMI. The objective was to see if NATO cases with this load would exceed the SAAMI; which it did. Again, this in not a recommendation for a load.

[303carbine]

So, I have not been able to find a definitive answer on this. Is there really a difference in 7.62 NATO and .308 Win brass, or for all intents and purposes, are they the same? I've read claims that the 7.62 brass is thicker and will need to be downloaded, and I've read claims that the difference in thickness is negligible. The internet being what it is, people love to pass along myths and hearsay, so I was wondering what the truth is on this.

I have a pile of .308/7.62 brass and since this is not going to be for anything even resembling a precision rifle, I'm wondering if I should even bother sorting and separating the brass. I'm going to be reloading to 7.62 NATO specs, so I'm thinking that if I do my load development in 7.62 cases, the worse case scenario at that point would be the loads in the 308 cases having slightly less pressure and a slight POI change.

Simply put, Nato brass is thicker, less internal volume , higher pressure loaded the same as commercial 308 brass.
Nato ammo is about 10 thou or so less in PSI for auto loading rifles, commercial 308 Win can damage internal parts of some rifle designs.
That's why the M1 Garand has its own loading data.

[AlaskaMike]

After reading through this thread, I have a question about an assumption that appears to have been made.

Several posts have commented on concluding that the mil brass is thicker because a mil case weighs more than a commercial case. How can we make that conclusion when we really don't know the brass alloy used in either the mil or commercial case?

Most of us here on the Castboolits forum are aware of how varying lead alloys can make a significant difference in bullet weight, despite the external bullet dimensions being identical. Why wouldn't we consider that with brass?

[beltfed]

A couple things from my years of Service Rifle competition with the M14 and M1A.
Back in "DCM" days, at Perry, we had to shoot the supplied M118 and Later the M852 white box ammo
In order to be able to practice get good zero's and shoot local matches, I developed a duplicate Handload that matched the
M852over my Oehler 33 chronograph: It was LC77 Match brass, Fed 210M primer, 41 gr IMR 4320 under 168 gr Sierra or Hornady BTHP match bullets.
( I had a couple of 20# kegs of the 4320, and it worked well, metered well(short stix))
One Important thing was, I used Forster's "308 National Match" die for full length sizing the brass. It is spec'd to exactly
bring the brass back to Mil Spec dimensions for the M14s. trimming as needed, of course.
The other important thing for the M1a/M14s was to set up the FL die for my particular rifle/Krieger barrel and chamber:
Take down the rifle, strip the bolt of the ejector plunger and extractor> Replace the Bolt Only in the receiver.
Then take a fired brass, and size it with the Forster FL die backed out a turn or so. Wipe off the lube and then drop the
case into the chamber with rifle horizontal and the bolt in Open position. Now you tip down the rifle, barrel down like
to a 45 degree angle or more until the bolt slided forward of its own weight. First try, the bolt will not close and lock itself.
Now resize the brass after turning the FL die a bit deeper into the press.
Do the bolt closing exercise again.
Repeat until the bolt will "just close" AND LOCK on the sized cast.
Now you lock that FL die ring and you have a die that will size enough, but not over re size the brass for that rifle.
Lastly, Yes, the Prison Range training brass I picked up from a friend whose brother was a guard, that was fired in M60s
was not very good. A good deal of it was stretched in the web and some would last only for one reload, even tho minimally
resized as per the above for my rifle. I got to "hooking" this brass to find and scrap those many that had incipient head separation
I finally tossed it all when I bought a couple of 20MM cans of new LC77Match brass.
beltfed/arnie

[HumptyDumpty]

I have noticed that most commercial brass has a far more prominent impression of my rifle's chamber flutes (PTR-91) after firing. Additionally, over the course of four or so reloadings, I had a neck finally split slightly. Again, this was commercial brass. I haven't been able to make a precise measurement, but I suspect that commercial 308 brass is generally thinner, or else, softer.

[303Guy]

After reading through this thread, I have a question about an assumption that appears to have been made.

Several posts have commented on concluding that the mil brass is thicker because a mil case weighs more than a commercial case. How can we make that conclusion when we really don't know the brass alloy used in either the mil or commercial case?

Most of us here on the Castboolits forum are aware of how varying lead alloys can make a significant difference in bullet weight, despite the external bullet dimensions being identical. Why wouldn't we consider that with brass?

Cartridge brass consists of approximately 70 percent copper and 30 percent zinc with no other alloying constituents.

Brass is a substitution alloy which means that zinc atoms replace copper atoms so differences in alloying percentages would be proportional to the difference in atomic weight. The difference in atomic weight is small at 3.33% so a change in alloy make up from 75:25 to 65:35 would produce a 0.34% increase in density. Not a lot.