Running .308 rounds in a 7.62x51 NATO chamber

[mackie]

"Many people will tell you that 308 will blow a 7.62x51 gun because they'll claim 308 is much higher pressure, it's not.

This comes from the fact that 7.62x51 and 308 were tested in two different ways with CUP (Copper Units of Pressure) being used as the unit in the 7.62x51 testing.
7.62x51 = 50,000 cup (not psi)
308 = 62,000 psi

People keep changing cup to psi without converting the numbers which is why you keep seeing 7.62x51 being quoted at being 50,000 psi."

If that's the case then it would seem there's no issue with using .308 in a 7.62 rifle.

[HALODIN]

Yes there seems to be some confusion, especially as the CUP scale doesn't directly convert to PSI on a 1:1 basis.

I've spoken with LDT and they say it's fine, so I bought 200 x Hornady 168g A-MAX, which should improve matters. I couldn't lay my hands on Hornady 155g TAP FPD, but we'll see how we go.

[rufrdr]

I've been running some tests and speaking to every man and his dog about this, but I'm curious if anyone can offer an explanation for this.

I've been advised the LDT-M14 was proofed at the Belgian proof house in Liege before delivery and is safe to shoot .308 as tested in Visier Magazine "Visier N°6...LuxDefTec M14 US Klassiker-Jetzt neu aus Luxembourg" (see pages 36-43)

There is part of the article here http://issuu.com/vsmedien/docs/visier_06_2013_leseprobe

1 (Good) to 7 (Bad)
147grs RWS Cineshot HP, .................................2
147grs Sellier & Bellot FMJ, .............................4
150grs Tulammo FMJ, ......................................7
155grs Hornady TAP FPD, ................................ 1 (5 shot group 1.9cm IIRC)
168grs RWS Target Elite HP-BT..........................6
168grs Sellier & Bellot HP-King M' King...........3
168grs Remington HP-BT M'King......................5

So it seems like it should be OK, however we downsized a .308 round from 62,000 PSI (.308) to 50,000 PSI (7.62x51 NATO) and 2 of the 3 test shots caused the primer to stand proud out of the back of the brass (see example pic below). I don't believe the brass was new and whilst I don't know very much about self loading yet, could case trimming have caused this? i.e. with new .308 commercial brass would this problem disappear?

The case on the left appears to have excessive headspace. I was shown a quick headspace test once: in a resized empty case load a fresh primer about half seated. Slowly and carefully chamber the case and lock the bolt. Extract the case. If the primer is fully seated then the headspace is good, if the primer protrudes, then there is excessive headspace. 7.62 NATO chambers are a tad longer than .308 Winchester chambers.

[HALODIN]

Yes that was the conclusion we drew, so I was surprised when LDT stated .308 is fine, as it doesn't explain why this happened on 2 of the 3 50,000 PSI .308 rounds we tried.

Good test, I'll remember that thanks!

The case on the left appears to have excessive headspace. I was shown a quick headspace test once: in a resized empty case load a fresh primer about half seated. Slowly and carefully chamber the case and lock the bolt. Extract the case. If the primer is fully seated then the headspace is good, if the primer protrudes, then there is excessive headspace. 7.62 NATO chambers are a tad longer than .308 Winchester chambers.

[HALODIN]

I've finally shot my first batch of home loads and the accuracy on the M14 has improved a lot. I've excluded 1 flier from the 5 shot group as the barrel was cold and it's grouped around half MOA (0.839" -.308 = 0.531") with the 43gr & 155gr Hornady A-MAX. If I include the flier it still groups <1 MOA (1.209" -.308 = 0.901"). 43.5gr shows even more promise, but I had to exclude 2 fliers of a 5 shot group (0.631" -.308 = 0.323"). Still work in progress, so I'll see if I can repeat these results and then experiment with seating depth. I was only hoping for 1 MOA, but it looks like it will do a lot better, which is impressive considering I bedded this rifle in!

[svandamme]

as Mackie said, they are both about the same 61-62000 PSI
7.62 NATO is not 50000 PSI!!! it's 50000 cups.. which is about 62000 psi


M14's will eat .308 all day even with a loose chamber

The only important part is to not use Heavy bullets with to slow powders because it will hammer the oprod on an M14
But i presume in the UK you don't shoot semi's.. in which case it shouldn't matter much.



Regular 150 grain .308 should be pretty much the same as 7.62 Nato in terms of pressure and headspace.
slightly thinner brass on .308 perhaps.. depends on the manufacturer but that shouldn't be a problem.

[HALODIN]

Yes the pressure difference seems to be a bit of an urban myth. We're only allowed .22 semi's in the UK, so no worries there.

7.62x51 GGG which is 148gr produces awful accuracy in the M14, about 1.5"-2" MOA. Now my .308 brass is fire formed, I'm looking forward to the next batch of tests.

[svandamme]

that's one advantage to the straight pull conversion.
you can fire form.

i have to full size em each time.
and brass life is limited because the m14 's cycle is a bit rough on the brass.

M14forums says 3-4 times. i've managed 5 times and and no case separation yet.. though they are soft loads..

[Laurie]

As pointed out, the 7.62's allowed MAP (Maximum Average Pressure) of 50.000 psi was actually quoted in CUP (Copper Units of Pressure) an older measuring system where the pressure barrel is drilled with a small chamber holding a small copper slug above the aperture. The copper cylinders use closely specified dimensions and metal purity values and are measured pre and post the test firing to see how much the gas pressure has changed the dimensions, ie crushed the cylinder (or gauge). The change is referred to a set of standard tables which shows the pressure needed to effect the change. This is a higher pressure development of the system adopted for black powder cartridges which used softer lead gauges and where pressure units were shown as LUP.

The more modern and now standard system used by SAAMI and CIP is the Piezo crystal system, a strain gauge based method that is more accurate and which links directly into a computer giving far more data than just the peak maximum pressure produced in the chamber.

With 7.62 and .308 Win introduced and adopted by SAAMI in 1954 / 1952, it covers both eras and older loads are quoted in CUP, more modern ones in psi (Piezo). They are NOT comparable - if they were, 50,000 psi on the modern system would see a substantial performance shortfall and would reduce the 7.62's efficiency to less than that of many early 20th century cartridges, the 7.92 S-Patrone of 1903 for example rated at way above 50,000 psi.

When the modern system is used, current 7.62 milspec ammo is rated identically by the CIP (European standard body) to .308 Winchester, at 4,150 bar / 60,091 psi. US .308 Winchester is allowed to be loaded to a slightly higher level by the US standards body SAAMI at 62,000 psi MAP not the previously quoted 63,000 psi, although the top values in a range that produce an average of 62,000 psi will in all likelihood see a few sample cartridges in a batch achieve or slightly exceed 63,000 psi.

So, even the hottest US factory .308 Win commercial ammo will be loaded at most to a little above what 7.62mm's maximum rating is, 62,000 psi v 60,191 psi.

The other issues mentioned exist and can be significant, the 7.62s slightly longer maximum chamber / headspace measurement and bullet diameters. 7.62mm NATO is specified at 0.3073-0.3077" bullet dia., whereas .308 should be as the name suggests 0.3080", although many commercial match bullets run at a fraction more 0.3082-0.3083". The combination of an extra two or three thousand psi, slightly excessive headspace and a tighter bullet fit in the barrel can see .308 Win commercial ammo produce excessive pressures in a 7.62mm rifle, although it probably wouldn't cause any problems in a strong turnbolt design, but could well cause extraction issues in a straight-pull. In reverse, 7.62mm ammo will almost invariably fit in a .308 chamber, but will produce sub-standard pressures and often but not always poor groups due to (a) bullet quality / consistency, and (b) slightly undersize bullets for the barrel's groove diameter.

A particular issue arises using modern .308 Win match ammo in a first generation TR rifle such as rebarrelled 7.62mm Enfield No.4s with Enfield Lock hammer forged match barrels. Because of the undersize bullet diameter, many first generation TR rifles had internal barrel measurements considerably reduced over SAAMI .308 Win's 0.300" bore dia. / 0.3080" groove dia. 0.296 and 0.297" bore diameters were used by some gunsmiths with groove diameters down to as low as 0.3055", although a thou' more was probably more common. The current NRA / RWS .308 win match ammo with the Sierra 155gn MK has been loaded to suit current TR rifle barrels which are usually specc'd a little 'tighter' than SAAMI, but nothing like those regularly used during the late 60s through to early 80s. Speaking from memory, I think modern TR/Palma 'tight' barrels are usually 0.2985 / 0.3075". So, the RWS ammo may produce substantially lower than spec MVs from a true .308 Win (0.300 / 0.3080") barrel as used in most custom built F/TR rifles for instance. In the reverse, direction this ammo and other full-pressure .308 win match jobs may produce considerably excessive peak pressures in elderly TR rifles. What probably protects many people in this situation is that most such rifles have heavily eroded chamber throats after thousands of rounds through them and this reduces pressures substantially. However, should you find a tight-bore 7.62mm low round-count 'sleeper', be very careful what you put through it!

So far as the case to chamber match goes and straight-pulls, the former may see a rifle only shoot really well with handloads after the (.308W) brass has been fireformed. I've got a friend who started F-Class in the early days with a secondhand AI that did amazing things, but he always said it didn't shoot worth a damn with newe Lapua brass handloads. There is another issue to this - be VERY, VERY careful on how you set your (.308 Win) sizer die up in the press is handloading for a 7.62 as it'll likely be very easy indeed to 'bump' the shoulder back too much creating excessive headspace. (This is an issue that anybody who handloads for a semi-auto or staright-pull has to be aware of anyway and it's really essential to set the die up using a case 'headspace gauge' to bump the shoulders 0.004-0.005" and no more, less on a turnbolt where one or two thou' is what's wanted in precision loads.) So far as hard extraction in straight-pulls goes, that'll be partly a 308 pressure / case expansion issue, but also likely to be down to different brass alloy specs between milspec and commercial ammo as the hardness has a considerable effect on ease of extraction (and chambering too in a really hot rifle in rapid fire). Hatcher says of the .30-06 that early brass was very hard and ideally suited to the M1903 turnbolt, but it regularly produced separation induced jams in MGs where they's been set up with a little too much headspace, so a softer / 'stretchier' alloy was adopted that wouldn't separate so easily, but produced much stickier operation for the infantryman with his bolt-action piece.

Incidentally, many modern special purpose 7.62mm cartridges use commecial HPBT match bullets (or OTM [Open Tip Match] as the authorities insist in calling them, maybe terrified of the hollow-point tag) and these bullets such as the 175gn Sierra MK loaded in the US M118LR sniper round are true thirties at a bit over 0.308 dia. Modern 7.62s including semi-auto 'marksmens' rifles don't seem to have much trouble coping with this allied to some pretty warm loadings

[Sandgroper]

Yes the pressure difference seems to be a bit of an urban myth. We're only allowed .22 semi's in the UK, so no worries there.

7.62x51 GGG which is 148gr produces awful accuracy in the M14, about 1.5"-2" MOA. Now my .308 brass is fire formed, I'm looking forward to the next batch of tests.

From what I've read it seems the main difference seems to be the chamber and the headspace of each, as well as watching the max pressure in some older rifles.

http://www.thefirearmblog.com/blog/2007 ... 2x51-nato/ and http://www.303british.com/id36.html for a flavour of the differences.

Edit: Beaten to it (and better explained) by Laurie...