.308 vs 7.62 question

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azhobo

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Was reading today that one of the differences between these two cases is brass thickness, the military being the thicker, for use in BMG's. The author's statement was, based on his research, that CUP could be considerably higher in the 7.62 cases, because of effectively smaller powder area.
My question therefore is.......
how much reduction in powder load would be recommended in military brass, compared to the reload charts for .308? I'm thinking ~ .5 gr. I found nothing in any of my reloading books.

Included in my reading was an opinion that 7.62 ammo could safely be fired in all civilian .308 rifles, but NOT the converse. Had to do with head spacing issues. Your thoughts?

Thanks,
Bob
 
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Well in my FN FAL, I ran 7.62 surplus, 7.62 PMC, I reloaded 308 Win brass in the thousands.. dumped lot's of empty brass on the ground.

Then there's the Sig 716, factory 7.62 was fine, reloaded 308 Win or 7.62 LC brass would fail to go into battery and lock up the bolt, even after chamber checking each round.

So it all depends the gun. At least it has for me.
 
Respectfully-- you are looking for a short cut answer if you want a percentage or straight number (0.5 grains?) when taking a short cut isn't the best or safe way to approach this situation.

If you have a known and loved load in commercial brass and you want to switch to military brass, an arbitrary number isn't the answer. Load development is the answer. Start low, proper work-ups, looking for pressure and asking what your goal is and aiming for that goal.
 
".....308 or 7.62.... for use in BMGs...."
you mean GPMGs? Or, did the author say BMGs?

BMG = Browning Machine Gun = 50BMG (often the M2, M82 and others)

Like others, I have loaded gobs of 308 and 7.62 sans worry - mil-spec Lake-City to Lapua. Now, bounce anything off of the "rev-limiter" and YMMV.

Size right, deal with the mil-spec primer and you should be good.
Once fired out of any MG (full auto belt-fed) may give you sizing issues - may not.
 
Back in the old days the standard answer was to reduce .308 loads 2 grains when switching from commercial to military brass. Presumably the expected powders were 3031, 4895, or 4064. I’ve never heard of anyone who followed that recommendation having trouble, but I certainly agree with Sevens that catch-alls like that, though likely based on lots of practical experience, nevertheless should be used carefully (as in “working up”). No one wants problems when there is 50,000 PSI a couple inches from your eyes.
 
OK, .308/7.62 x 51 mm headspace is the same, the dies are the same. The chambers are slightly different. I've fired a lot of 7.62 in .308 chambers. Musty memory suggests that the MAP of .308 is higher, but since the chambers are different..........? The major issue with some military weapons systems is that the thinner brass of the .308 may cause extraction issues/excessive load on the extractor because the thinner brass increases extraction loads.

The chronograph is your friend with loading in mil spec brass. With 175 gr SMK bullets the difference between WW commercial and LC Match brass is 2.2 gr with IMR 4064 to produce similar velocities. OTOH, various commercial manufacturers use different case designs resulting in different case capacities/pressures. FWIW, the most uniform .308 brass I've found is Black Hills.
 
Sevens said:
Respectfully-- you are looking for a short cut answer if you want a percentage or straight number (0.5 grains?) when taking a short cut isn't the best or safe way to approach this situation.

If you have a known and loved load in commercial brass and you want to switch to military brass, an arbitrary number isn't the answer. Load development is the answer. Start low, proper work-ups, looking for pressure and asking what your goal is and aiming for that goal.
Click to expand...

I've only recently acquired a .308 (Savage 99) so I don't have a loved load. Maybe I didn't communicate well.... what I'm wanting to know is a starting point to work up from. I get the need to not jump up to max loads. My problem is not having a starting point so low as to create squibs, but still low enough to preclude damage to either me or the rifle. I'm not a stranger to reloading, but have had no need to address this particular issue in the past.
Thanks,
Bob
 
AmStaff64 said:
".....308 or 7.62.... for use in BMGs...."
you mean GPMGs? Or, did the author say BMGs?

BMG = Browning Machine Gun = 50BMG (often the M2, M82 and others)

Like others, I have loaded gobs of 308 and 7.62 sans worry - mil-spec Lake-City to Lapua. Now, bounce anything off of the "rev-limiter" and YMMV.

Size right, deal with the mil-spec primer and you should be good.
Once fired out of any MG (full auto belt-fed) may give you sizing issues - may not.
Click to expand...

The author of the article used the written out words "belted machine gun." I used the abbreviated form BMG. My bad, I should have NOT capitalized the abbreviation. Sorry for the confusion.
Thanks,
Bob
 
Try comparing the Weight of the cases. It's been a 8 years or so but my memory is that Lake City and Federal Cases were basically identical in weight, Remington a bit {1/2 grain) lighter and Winchester cases were all over the place. The heaviest cases that I came across was Lake City Match and it was only 0.5 to 0.7 grain heavier than standard Lake City.

Note, I suggest that you try doing this for yourself. Because to be totally honest my memory aint what it used to be and I could be incorrect in regards to the specifics. BTW my memory about Winchester being all over the place is rather distinct and the reason why I don't reload Winchester cases.
 
I have found that some military brass weighs more than some commercial brass and also vice versa. I have seen no meaningful differences in MV between GI and commercial cases for identical loads. FYI, the official Lake City weight specification for the GI M80 and M118 cases is 170 to 190 grains. For best grouping performance from GI cases it is best to segregate cases by weight.
 
When using LC match casings, many of us shooting NRA Service Rifle replicated the M852 round with 42 grains of IMR-4895 and a 168 Sierra Match King bullet seated to magazine length.

Point of aim and impact were the same and they ejected to the same spot as well.....could not have been very far of this replication.

In almost any M1A, if your rifle will not shoot this round....it isn't going to shoot.

Randy

Should work well in your Savage 99 as well. This is not a real stout load by any means but has proven itself accuracy wise for many years. Simply substitute a 165 grain hunting bullet and stack them up in the middle of your target!! Velocity is around 2650 fps.
 
You're not gonna get a squib if you start at the suggested starting load in your manual. Sooty cases from failure to fully seal the breech maybe. If all you have is max loads, reduce 10% and work up watching your velocities.

It's not so much the weight of the case as it is the capacity. The classic way is to weigh cases empty and full of water. I've filled cases to the brim with a given ball powder and weighed that to get a better/less messy idea of capacity. I've had a couple real shocks.
 
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M29since14 said:
Back in the old days the standard answer was to reduce .308 loads 2 grains when switching from commercial to military brass. Presumably the expected powders were 3031, 4895, or 4064. I’ve never heard of anyone who followed that recommendation having trouble, but I certainly agree with Sevens that catch-alls like that, though likely based on lots of practical experience, nevertheless should be used carefully (as in “working up”). No one wants problems when there is 50,000 PSI a couple inches from your eyes.
Click to expand...

Now we're gettin' down to what I was looking for, a place to start. With your "old days" guidance, coupled to what WR Moore provided, I think that starting at, say 2.5 grains reduced from a low level recommendation from IMR, would be a safe place. I'll be using 3031 with a Speer 150 gr jacketed boat tail bullet, so it looks like 37.5 grs will be the place to start.
Thanks to all,
Bob
 
azhobo said:
I think that starting at, say 2.5 grains reduced from a low level recommendation from IMR, would be a safe place. I'll be using 3031 with a Speer 150 gr jacketed boat tail bullet, so it looks like 37.5 grs will be the place to start.
Thanks to all,
Bob
Click to expand...

Apparently you don't recall the warning NOT TO REDUCE STARTING LOADS!
Don't reduce the starting loads below what's listed in the manuals. At those pressure levels you won't see anything approaching pressures where you need to worry.
 
On page 29 below it lists .308/7.62 load data using Lake City brass with 5 different powders. And IMR-3031 is one of the loads listed, with three loads listed starting at 44,000 cup to 51,000 cup max.

NRA 7.62x51mm Lake City load data for M1-A's.
NRA 7.62x51mm Lake City load data for M1-A's. - AR15.COM

Below newer dated Lake City 7.62 cases are made of harder brass and are not as thick as the older cases where you reduced the load by 2 grains.

"BUT" just start at the manuals suggested and work up and you will be safe. You will not have a squib load with Lake City cases, and below you can see the case volume of the newer Lake City cases.

I buy bulk once fired Lake City cases and just start at the suggested start load in the manuals and work up. And with all the newer LC cases the primers are protruding with the start loads. Meaning the chamber pressure with the start loads was not great enough to make the case stretch to meet the bolt face. And as a example all my 30-30 Win loads have the primer protruding at 38,000 cup or 43,000 psi or the max load range.

I think I got the chart below from a M14/M1A reloading forum from a poster who weighed and checked the case capacity of the cases and made the chart below. And this closely corresponds to the Lake City 7.62 cases I have purchased in the last year for my Savage bolt action .308.

Qvc8vVV.png


My "guess" after looking at the chart above and checking the case capacity of my LC cases is the cases are now made of a harder grade brass. And are not as thick as the older 7.62 cases since ATK took over production at Lake City. "BUT" I'm not sure of the year the change took place, so starting low and working up is always the best way to go.
 
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bigedp51 said:
My "guess" after looking at the chart above and checking the case capacity of my LC cases is the cases are now made of a harder grade brass. And are not as thick as the older 7.62 cases since ATK took over production at Lake City. "BUT" I'm not sure of the year the change took place, so starting low and working up is always the best way to go.
Click to expand...

Basically ATK is Federal! The operation of Lake City Arsenal rotates, between Federal, Remington and Winchester. Think in roughly one year increments each; however there have been times when the operating company has a longer or shorter period of operation.

Regardless of the operator, the specs for all components remain the same, the performance specs remain the same, and the quality remains the same. Basically the best military ammo in the world!

Ivan
 
Not to change the subject, but is this (case wall thickness) also an issue when switching from .223 to 5.56 military brass?
 
Soon this will MORPH into a 223 Rem vs 5.56 discussion:D
If you are not loading MAX SERVICE loads don't worry about it


Once you size and trim your brass there is not a whole hill of beans difference, The debate as with 223/556 has been around for many years. The brass these days, is so varied who the heck knows,


You can weigh the brass, calculate the H20 volume(which is better) of the brass etc, just don't load Max loads as with anything why?


Plug in 308 Win and 7.62 here on Hodgdons
Take Aim at Rifle Reloading Data | Hodgdon Reloading


Nosler


308 Winchester Load Data - Nosler
 
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Case capacity is far more meaningful than brass weight. Light brass usually has a greater case capacity than heavy brass, but it's not necessarily that way in every situation. While it's messy and time consuming to check water weight, it's worth the effort.

I recently checked the capacities of five brands of .300 Winchester Magnum cases . Weights varied more than a 10% from light to heavy. Water weights varied much less, about 4%, but still of some significance if you're using maximum loads.
 
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