I have a lot (100)of Winchester 308 Winchester brass weighed at 156 gr. and a lot (again 100) of Federal 308 Winchester brass weighed at 175.5 grain, would it be accurate to figure the difference by percentage and then reduce the powder volume by the same percentage in the Federal brass to get approximately the same pressure? I would tend to believe that this might not be the right way to figure it, as different powders weigh so much diffent by volume...Any opinions here.....Thanking everyone in advance.

Shoot Safe, Shoot Straight...RiverRat

RiverRat,

I have seen a rule of thumb (1 grain of powder for 7 grains of brass???), but I usually just drop back 10% and start over with any component change.

Be safe,
Don

Start over....that is life.

Don,
the relative mass / density of brass to smokeless powder (average) is 7.5. So yes, divide the difference in brass weight by 7 (.5) and get the number of grains to reduce the load for the same pressure in the two same caliber cartridges. The ratio of the cartridge weights would be way to much of a drop in powder charge, probably dropping below the minimum listed load. You should ALWAYS drop back to starting loads and work up a load to fit any change in components. AGREED!
PaulS

PaulS

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stay safe and live long!

I would find the difference in water weight between the two. You're talking about 20 gr difference in brass weight but what you actually want to know is the internal volume difference between the two lots.

I usually weigh 5 cases of new brass dry, then fill to the top with water and weigh again, then subtract the weights. Divide the heavier into the lighter, multiply by 100 to get the percentage of difference, then just add or subtract that percentage to arrive at the approximate grains to play with.

Heavy case = 60 gr H2O
Light case = 65 gr H20 (Maybe more volume, you don't know until you do it.)

60/65 = .923 x 100 = 92.3% difference. When you get down to it doing the 10% reduction to start is quicker and safer and you don't need a calculator to figure it out.

RiverRat,

Sorry but I have been in your boat several times over the years (why I buy brass in bulk at one time). Currently am starting over with some 338 Lapua once fired brass that I got at .30 on the dollar. It weighs 326 to 327 gr and the other brass is 340-342 gr both Lapua but different lots. It will give me a chance to work with a different bullet but you need to start over even though it is a pain.

Dave

Thanks to everyone for their input. I know it is starting over again. And I do hate to do that. Maybe I should have just stayed with Winchester brass, but the urge to try something different.......Well you all know what I mean. Thanks again for all the sage advise.....

Shoot Safe, Shoot Straight....RiverRat