7.62 vs. .308 cases

[MMH]

How much less velocity will I get when using 7.62 cases compared to commercial .308 cases. Obviously I would redevelop loads. Is there any other downside in using 7.62 cases?

 

[Howland]

Military brass is typically thicker than commercial brass. If exterior dimensions are equal, that means case volume is less for military brass. The same powder charge in each is going to generate greater pressure in the military brass.

Even different brands of commercial brass - or different lots of the same brand - can have different average case volume. Most folks weigh the amount of water an empty case holds to closely estimate case volume.

 

[Zero333]

7.62 nato brass is heavier/thicker so it has less internal volume and takes less powder to generate same pressure as commercial 308win cases.

Velocity wise, there is no real difference unless you're using a slower powder and can't fit as much in the case. But realistically Same Velocity can be attained with both 7.62 Nato and 308win.

5.56 Nato is NOT any heavier/thicker than commercial 223rem brass, unlike the 7.62 Nato and 308win.
5.56 nato brass will have crimp(s) on the primer pockets. And some would of been fired in machine guns and beat to hell. Some 5.56 Nato brass can be of very good quality.

 

[MMH]

7.62 nato brass is heavier/thicker so it has less internal volume and takes less powder to generate same pressure as commercial 308win cases.

Velocity wise, there is no real difference unless you're using a slower powder and can't fit as much in the case. But realistically Same Velocity can be attained with both 7.62 Nato and 308win.

Would you expect similar velocities for varget & 4064. I currently load 43.0 grains of both under a 178 gr. bullet. w/ my 20" barrel I get approximately 2525 fps. I am guessing that my 7.62 brass loads will require about 1 grain less powder, but, I will start lower than that & work up a load.

 

[Laurie]

7.62 nato brass is heavier/thicker so it has less internal volume and takes less powder to generate same pressure as commercial 308win cases.

Velocity wise, there is no real difference unless you're using a slower powder and can't fit as much in the case. But realistically Same Velocity can be attained with both 7.62 Nato and 308win.

Not so. Ultimately, MV depends upon the amount of energy provided by the charge x the cartridge's thermal efficiency factor and affected by maximum pressures used. Assuming pressure is the same, a larger capacity case allows a larger charge and that means a greater energy input. Basic physics!

Although the differences aren't vast, until the small primer Lapua 'Palma' case appeared with its various benefits including an ability to safely survive higher pressures and still give a life of many firings, most US FTR competitors used the very high capacity Winchester brass despite a high reject rate and a lot of batching / prepping needed. It would allow a modest but still valuable MV increase over heavier, lower capacity commercial products, and that would translate into a yet greater increase over most 7.62mm examples. For the same reason, I used a quantity of mid 1980s Norma 308 Win brass that deliberately or otherwise had been manufactured very thin resulting in higher capacity. In brass fireformed in a 'minimum SAAMI' chamber, water capacities were:

Lapua ................. 56.1gn
Winchester .......... 57.0gn
Thin Norma ......... 57.4gn

Run these through QuicklOAD with Hodgdon VarGet and the 185gn Juggernaut at an optimum COAL and you get maximum calculated charge weights of:

Lapua ............. 43.9gn / 61,802 PMax / 2,736 fps MV (30-inch barrel)
Norma ............ 44.7gn / 61.639 Pmax / 2,752 fps MV

with Winchester in between.

Some ordnance 7.62 brass will have 1 to 2gn less water capacity than the Lapua figure quoted and will see a corresponding MV drop at the same pressure.

Whilst a smaller capacity case produces the allowable peak chamber pressure from a smaller charge, it isn't the peak pressure alone that determines MV, it is the total area of pressure 'under the curve' as shown in a pressure v time / bullet movement graph.

For the shooter who is running modest pressures for short range plinking or whatever loads, there aren't any worthwhile difference to speak of, although I'd still prefer using a good quality commercial case bought new or with a known history - and not one that has involved slack chambers and case-wrecking automatic weapon actions.

 

[pirate ammo]

my FNFAL will destroy a case neck bent to hell

 

[Bart B.]

Biggest problem with surplus brass is their case heads. They're virtually all out of square. They'll shoot bullets into 1/2 to 3/4 larger groups than when new and unfired.

This is why military teams never reloaded their fired brass. New stuff shoots most accurate in their semi autos 'cause none of them had their bolt faces squared up. Bolt guns are no different.

 

[Laurie]

Biggest problem with surplus brass is their case heads. They're virtually all out of square. They'll shoot bullets into 1/2 to 3/4 larger groups than when new and unfired.

Back in the days when GB 'Target Rifle' (ie sling shooting at 200-1,200 yards) mostly used 'issued' arsenal 7.62, even a lot of new cases apparently weren't that square at the back. I understand that was a primary driver of the four-lug bolt arrangement in the Swing / Paramount / RPA family of single-shot target actions. It was found that they made this ammo group noticeably better than a traditional 6 and 12 o'clock twin-lug arrangement, even a good stiff one like the single-shot Musgrave RSA Mauser system action that was popular in the UK before the original Swing appeared.

 

[Laurie]

Actually mentioning out of square after firing in semi-autos reminded me of an incident on a set of short distance ranges local to me in the north of England many years back.

I was shooting a .22 rimfire with some levergun shooters when a regional police armed response team set up on a neighbouring range and started blasting away with 5.56mm SIG semi-auto black rifles of some type or other. One of the guys on my range went off and spoke to them afterwards and reappeared clutching a bag of maybe 100 or so fired Winchester 223 Rem cases from the police rifles donated by the head copper who'd said they're yours if you'll save us the effort of picking them up.

Well, I looked at these cases, and 'battered' was an understatement! They all had an angled and deep knife-blade like dent hammered into the case-head presumably from hitting an ejector peg and necks and shoulders were badly knocked about too. So, I said words to the effect of their being ruined. 'Naw!' says the guy, 'my brother is a gamekeeper and he loves these ex-police 223 cases for his 223 Ruger 77 - they shoot just fine.' I've got to say I cringe whenever I think of this - I doubt if you could have stood these cases upright on a flat surface without their falling over.

 

[Bart B.]

Back in the days when GB 'Target Rifle' (ie sling shooting at 200-1,200 yards) mostly used 'issued' arsenal 7.62, even a lot of new cases apparently weren't that square at the back. I understand that was a primary driver of the four-lug bolt arrangement in the Swing / Paramount / RPA family of single-shot target actions. It was found that they made this ammo group noticeably better than a traditional 6 and 12 o'clock twin-lug arrangement, even a good stiff one like the single-shot Musgrave RSA Mauser system action that was popular in the UK before the original Swing appeared.

Not only messers SWenson and kING combined rifle building talents evolved the Swing action in the early 1970s, but it led to Barnard and Stolle actions with more than two bolt lugs.

Great Britian's Radway Green arsenal 7.62 NATO best lots of ball AMMO was good enough that 1:14 twist barrels shot them most accurate in temperatures above 75F. With .3070" diameter bullets in .3065" groove diameter barrels.

Don't know what Mauser 98 style single shot action came out first; the Musgrave or FN.

I've seen a few test results showing powder charge weights be changed 7/10ths grain for every 10 grains of case weight. 308 and 7.62 case weights I've checked were from 149 (WCC58 match) to 193 (some non USA 7.62) grains.

 

[Laurie]

Great Britian's Radway Green arsenal 7.62 NATO best lots of ball AMMO was good enough that 1:14 twist barrels shot them most accurate in temperatures above 75F. With .3070" diameter bullets in .3065" groove diameter barrels.

Ah .... but what a lottery RG was, until it produced a 155gn FMJBT version which at its best was good, but at its worst plain mediocre. Before that the GB TR staple of the 146gn RG 'Green Spot' so-called sniper round was notorious for its variability, and like they say about naughty children ........... When it was bad, it was REALLY, REALLY, REALLY bad. Competitors flew in from around the world for the annual Imperial Meeting and some vowed they'd never come back again, as they'd come to compete on a level playing field, not take part in an ammunition lottery.

In all seriousness, the RG situation for TR ammunition was so bad at times it was scandalous, and the GB NRA stuck with it for far too long. It must have done the discipline great harm over time.

The British Army itself woke up to how bad the combination of the L42A1 rifle (the .303 Lee Enfield Number 4(T) of WW2 rebarrelled to 7.62) plus the 146gn 7.62 'Green Spot' round was due to its very poor performance in the 1982 Falklands War which generated great dissatisfaction among the troops. In a post-war review process, the British Ministry of Defence recognised the issue and as the first steps to sorting it carried out trials of other NATO countries' sniper rifles and ammunition against the L42A1 / RG 146 combo - and was reputedly shocked as to how poor the British kit was. Both the Accuracy International designed and manufactured L96 rifle and new RG 155gn bulleted ammunition came out of this directly.

 

[riflewoman]

Ah
In all seriousness, the RG situation for TR ammunition was so bad at times it was scandalous, and the GB NRA stuck with it for far too long. It must have done the discipline great harm over time.

It wasn’t just the GB NRA, the DCRA, Aus. NRA etc, all followed the same path to near oblivion. The claim was that “it is the same for everybody” and “we don’t want to make this an equipment race”. But it wasn’t the same and it became an equipment race.

 

[Bart B.]

The claim was that “it is the same for everybody” and “we don’t want to make this an equipment race”.

Yes. The century old "level the playing field" concept. The icing on that cake was to issue random picked rifles to all competitors to further smooth that field. Same lot of ammo would shoot equally accurate across all rifles. The best marksman would always win.

 

[Zero333]

P

Not so. Ultimately, MV depends upon the amount of energy provided by the charge x the cartridge's thermal efficiency factor and affected by maximum pressures used. Assuming pressure is the same, a larger capacity case allows a larger charge and that means a greater energy input. Basic physics!

Although the differences aren't vast, until the small primer Lapua 'Palma' case appeared with its various benefits including an ability to safely survive higher pressures and still give a life of many firings, most US FTR competitors used the very high capacity Winchester brass despite a high reject rate and a lot of batching / prepping needed. It would allow a modest but still valuable MV increase over heavier, lower capacity commercial products, and that would translate into a yet greater increase over most 7.62mm examples. For the same reason, I used a quantity of mid 1980s Norma 308 Win brass that deliberately or otherwise had been manufactured very thin resulting in higher capacity. In brass fireformed in a 'minimum SAAMI' chamber, water capacities were:

Lapua ................. 56.1gn
Winchester .......... 57.0gn
Thin Norma ......... 57.4gn

Run these through QuicklOAD with Hodgdon VarGet and the 185gn Juggernaut at an optimum COAL and you get maximum calculated charge weights of:

Lapua ............. 43.9gn / 61,802 PMax / 2,736 fps MV (30-inch barrel)
Norma ............ 44.7gn / 61.639 Pmax / 2,752 fps MV

with Winchester in between.

Some ordnance 7.62 brass will have 1 to 2gn less water capacity than the Lapua figure quoted and will see a corresponding MV drop at the same pressure.

Whilst a smaller capacity case produces the allowable peak chamber pressure from a smaller charge, it isn't the peak pressure alone that determines MV, it is the total area of pressure 'under the curve' as shown in a pressure v time / bullet movement graph.

For the shooter who is running modest pressures for short range plinking or whatever loads, there aren't any worthwhile difference to speak of, although I'd still prefer using a good quality commercial case bought new or with a known history - and not one that has involved slack chambers and case-wrecking automatic weapon actions.

I should of been more specific to my reply. I was thinking within standard SAAMI pressures surplus 7.62 and 308w commercial brass can achieve same velocity.

When pushing the envelope the the MAX, the brass with more capacity always wins.
I still prefer Winchester brass because if I don't push it to the jagged edge it lasts forever and ever. And I've gotten some spectacular velocities whenever I wanted to with out trashing the brass.

 

[Bart B.]

When pushing the envelope the the MAX, the brass with more capacity always wins.

I still prefer Winchester brass because if I don't push it to the jagged edge it lasts forever and ever. And I've gotten some spectacular velocities whenever I wanted to with out trashing the brass.

Some people have got over 50 full length sizing cycles per case with Western Cartridge Company WCC58 match brass made for the US Army International Teams' 300 meter free rifles; loaded with ball powder under 200 grain FMJBT match bullets. Case weight's about 150 grains and has very uniform wall thickness.

2 years later, WCC60 was equally as uniform weighing about 157 grains per case. It had ball powder and 197 grain HPBT match bullets. Dozens of reloads per case.

 

[riflewoman]

Back to the OP’s question, the major difference in loading using military cases is generally about a grain of powder using powders of the 4895 and 4064 burn rate. In another thread, I stated my match loads for a 308 were 41.5 IMR 4895 in Military cases (LC Match) vs 42.5 IMR 4895 in commercial cases, both with 168 bullets.

 

[fyrewall]

Is that the stuff (7.62) that needs primer pockets to be either reamed or swaged out?

My experience with military 7.62 brass has been one of scrounging up stuff to sell or trade with the exceptions of using the stuff that came in white cardboard boxes with black & red lettering, eagles, and other info about "not for combat use". This was a long time ago. In any event I cheerfully picked up the match grade stuff that did not require primer pocket reaming and was tossed out of different types of rifles including M14's, (M1A?) and F/L sized them to use in my bolt action match rifle; this did not cause worry because another shooter who was providing me with guidance & direction used the same cases to win matches at the master level. My opinion of Lake City brass is good.

As for the distortion of fired brass causing it to be "all out of square" and causing 1/2 to 3/4 larger groups I think this is a bit of an exaggeration. My idea is that military teams don't need to mess with fired brass as they are funded by taxpayer dollars and that makes for policy. Unbiased & random examination and comparison of both new and fired high quality military brass like that produced by Lake City may not show that degree of distortion. But to effect a desired outcome, "random" could be replaced by carefully selected.

Reliance on unsupported "factoids" can make for big problems like resolving claims for arbitrary bid rejections causing extreme anguish.

As this is about 7.62 X 51 ammo; I think some info should be provided to provide forum members not having a clue of what a M14/M1A looks like inside - a source of much once fired 7.62X51 brass. As seen this weapon has a plunger ejector much like most bolt actions and the extractor is similar also. There is no slot in the bolt face to allow an ejector to kick out a fired round. Brass is not treated in a delicate manner but is not mangled either (no "ejector peg").
www.chestnutridge.com/14pic.asp

 

[Bart B.]

As for the distortion of fired brass causing it to be "all out of square" and causing 1/2 to 3/4 larger groups I think this is a bit of an exaggeration.

If one cannot shoot well enough with a precision built M1 or M14 using the best ammo, they won't see a difference.

Creighton Audette proved this back in the 60's.

Ask the benchresters how important square bolt faces are.

 

[fyrewall]

If one cannot shoot well enough with a precision built M1 or M14 using the best ammo, they won't see a difference.

Creighton Audette proved this back in the 60's.

Ask the benchresters how important square bolt faces are.

What about propping the M1 or M14 up on sand bags and carefully shooting it. Any test largely dependent on skill tends to void technical aspects. Any minor ammo tech might consider using sand bags vs. obtaining creds as a match shooter. Remove the fallible human element as much as possible.

 

[Bart B.]

What about propping the M1 or M14 up on bags.....

Most of my data from military sources is based on accuracy cradle tests; no human variables. High Master class competitor's can easily tell a half MOA difference in ammo shooting in all three positions shouldering the rifle.