So i have reloading data for the .308, but I know there is a slight difference in the 7.62x51... Should I load a little lighter for the 7.62?

If, by 7.62x51 you're asking if you should load differently for something like an M1A, then no.  The NATO 7.62 round was developed in the early 50's when modern high performance powders were already developed so the NATO round is the same as the standard Winchester .308 hunting round; 150grn class bullet commercial ammo at 2800fps.  That contrasts to the 30-06 which was developed some 50 years before with lower performance powders and a 150grn class bullet commercial ammo att 2800fps vs modern 150grn class bullet commercial ammo at 3,000fps.

Yes, the lower case capacity of military cases mean you should start about 1gr lower than you would if it were a commercial case.

My LC 7.62x51 brass starting out was 54.5gr capacity .308 may be as much as 56 gr.

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Some 7.62x51 brass has thicker walls to prevent case head separation in certain machine guns, so internal capacity is reduced.  When working up a load, reduce it by 5% to compensate for reduced capacity.

fixed it for you.

Maximum loads in Lake City military brass with 168 grain Sierra Match King bullets as per NRA pressure testing:

40.5 grains of IMR-4895
40.0 grains of H4895
41.5 grains IMR-4064
39.0 grains IMR-3031
43.0 grains WW-748

and my testing can add 40.5 grains of AA-2460.

All of these maximum powder charges are listed when used with Lake City military surplus brass are a full 2.0 grains lower than maximums loaded in Winchester commercial cases. Don't mix cases, pay attention to headstamps. Well over 600 rounds of hand loaded ammo was run through a pressure gun by the former government employee responsible for establishing the data package for M14 service rifle ammo to obtain these loads. Ignore his expertise and labor at your own peril.

If you can find a copy of the now out-of-print NRA publication "Semi-Auto Rifles, Data and Comments" it has two complete articles dedicated solely to reloading Lake City brass for use in semi-auto rifles. This is the single best source available for loading surplus brass. Online used book dealers usually have them in stock.

Huh I run 45gr W748 with 175smk in my LC brass, it averages around 56-58k psi, a little bit higher than the 168gr Hornady match I have tested. Too many variables.

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2500 fps is maximum or damn near maximum velocity when using Lake City brass and 168 Sierra Match Kings when fired from a M1-A. All of the loads I posted run between 2470 fps to 2500 fps when fired from my M1-A. You'll gain another 50 to 60 fps when fired from a 24" bolt action. The velocity listed on Lake City Match ammunition boxes were fired from 24" bolt action barrels, it bears little resemblance to what happens when fired from a M1-A. 168's traveling at measured velocities of 2470 to 2500 fps are full power loads and totally normal when using Lake City brass. You can use 41.5 grains of IMR-4895 with 168's in commercial cases and get virtually identical results because the larger case capacity offsets the increased powder charge.

43.0 grains of WW-748 is a published maximum with 168 grain Sierra Match Kings when loading for M1-A's, not because of chamber pressure, but because of gas port pressure. This load only showed 48,500+/- c.u.p. chamber pressure but near maximum port pressure. A bolt action can handle more and most reloading manuals reflect that.

Ah I see, didn't see any mention of M1-A by OP.

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ok sorry took so long to respond, but I'm btw shooting the .308 through a r700 and the powder im using is IMR 4320. The powder in my speer reload book says a minimum of 44.0grs and a max of 48.0grs so should i just reload to like 43grs or 44grs? Im shooting a 150gr fmjbt  

There are three major differences to consider here.

First, the 7.62x51 NATO and .308 Win have different pressure standards.  The .308 WIn has a SAAMI maximum pressure of 62,000 psi.  The 7.62x51 NATO round is designed to operate at 50,000 CUP.  Converting between the two is not strictly linear, but 50,000 CUP equates to about 60,000 to 61,000 psi - slightly lower than the .308 Win.

Second, 7.62x51mm NATO brass has a significantly thicker web to help prevent head separations in full auto weapons that may have excessive headspace and/or with rather violent extraction.    As a result the case has less internal volume and requires a reduced powder charge.

Third, while the external case dimensions are the same for the 7.62x51MM NATO and the .308 Win, the chamber dimensions are different, with the 7.62x51mm NATO chamber being .013" longer to the datum line on the shoulder than the .308 Win.

Number three can be very important if you're shooting an M1A or a Garand, as depending on who made it, ordered it or re-barreled it, it may have a .308 Win chamber or a 7.62x51mm NATO chamber.  The .308 Win chamber is a bit snugger and may produce better accuracy, but it will start showing pressure signs sooner as the powder charge is increased.

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In my experience, you'll want to back off about 2.0 to 2.5 grains when loading military brass compared to civilian brass.  1 grain is not nearly enough reduction.  

Simply stating "150 grain fmjbt" is too generic and you primer brand plus case being used all conspire to make it impossible for anyone to give you load data. The NRA ran pressure tests on 15 different brands of 150 grain .30 caliber bullets fired through a .30-06 test barrel with identical powder charges. The pressures varied over 10,000 c.u.p. depending on who's bullet was being tested.

Speer data scares the crap out of me, especially their older manuals. If you can post your case  brand, primer and bullet manufacturer someone may have more useful information to help you. If you're loading in Lake City brass automatically subtract 2.0 grains from the starting load data and begin there. In this case 42.0 grains would be the start load and that is closer to maximum than you think if you're using Lake City brass.

There is no way I'm loading 48.0 grains of IMR-4320 in any brand case with 150 grain bullets. 46.0 grains of Varget  is a working maximum for long range Palma rifles shooting 155 grain SMK's. Varget is considerably slower than IMR-4320. Again different brand bullets, despite weighing the same, can make a huge difference in the pressure produced. Start low and work up. Chronographs are the best tool available for working up loads.

Bullet: 150grain fmjbt hornady
Case: Lake city
primers: winchester large rifle
powder: IMR-4320