.308 Reloading - Different volumes of different brass, need opinion

Mike,

The standard measure is case water capacity in grains of water weight. You want the capacity measured in cases as fireformed to your chamber, and not the resized case water capacity. The former is what helps determine peak chamber pressure in a gun at rifle pressures, not the latter. I have an Excel file with instruction that you can download free from my file repository, here. It also works in Calc, the spreadsheet program in the free Open Office suite, which may be downloaded from this location if you don't have Excel.

Powders pack with considerable variance in bulk density. Volume determined that way is not very reliable, so follow the water capacity instructions. Just use your fired cases and still take an average. At the least, use fired cases whose weight is average for the brand among the fired samples you have. Leave the spent primers in place as the flash hole plugs.

As you've discovered there is considerable difference in the capacity of .308 cases. In the late 80's Winchester got the contract to design and produce special high capacity cases with semi-balloon heads for the 1992 Palma match. Those cases were popular and used less brass, so Winchester later started making all their .308 brass to that design. Old and new Winchester .308 are therefore not the same. The new stuff weighs an average of about 156 grains. You'll find Remington weighs around 168 grains. Lapua around 171 grains, Lake City around 180 grains, and IMI around 186 grains. The last two are military spec. Since cartridge brass has a density of 8.53 (from Matweb.com) that 30 grain spread adds up to just over 3.5 grains difference in total water capacity. IMR rifle powders have bulk densities of around .88 if metered gently (you can pack them higher with a drop tube), so this is about 3.1 grains difference in powder capacity, regardless of the size they are blown out to in a chamber. It works out to almost 7% capacity difference, depending on the case and powder. Pressure has an exponential relationship to powder quantity, so it doesn't mean the powder charge can be increased by that percentage. Usually it's closer to half that going from the IMI to new Winchester brass, but it varies with the powder characteristics and the bullet weight. You still end up working up loads for each one.

Federal brass is famous for being soft and not lasting, so it is not worth puting a lot of work into it. Some is known to get loose primer pockets on the first firing of the commercial load Federal puts into them. Dan Newberry says flat out he does not consider it suitable for reloading. I have also run into folks who reload it all the time and don't have an issue. I think, as long as you are not loading near maximum pressure, if you have a lot of it and it hasn't been pre-stretched, then you can use it at least once. Keep a sharp eye on it. I wouldn't want to sell it because I wouldn't want to stick someone else with the problem. I don't much like the reliability of case head expansion as an absolute pressure indicator, but if you apply an OD thimble micrometer with ten thousandths resolution to the heads and monitor them and they don't expand, then you probably don't have a load in a hazardous range for those particular cases.

I once pulled a bunch of Federal Gold Medal match with the 168 grain SMK. It used 43.5 grains of IMR 4064. This was before ATK bought Federal Cartridge, Co. ATK also owns Alliant, and that round is now loaded with Reloader 15, same as military M118 LR. I don't know that those were canister grades, so you may find a different charge is best in your gun? It's not often an asmmunition manufacturer uses canister grade powders, but a few exceptions have arisen over the years. It makes sense to me that where higher than normal lot-to-lot consistency is desired, it would be done. Usually the powder's name designation is changed when it is a non-canister grade, so I expect these are among the exceptions. Just don't take that as gospel, and work your own loads up carefully, watching for pressure signs.

The point of measuring it expanded is that it is the expanded volume that the powder builds to final pressure in, so that is what figures into the expansion ratio and all the other load pressure related behavior. It also gives you the percent difference for your chamber accurately. If you consider the sizing die a chamber, the absolute difference in grains may not be much different measured either way, especially not if it was all fired in the same gun before.

Nonetheless, I think you are right to standardize on one headstamp if you can. It makes load development simpler. A lot of match shooters buy new brass so it has never been fired in another gun, then take good care of it. I like to fire a squib load through it on its first trip to the range. This won't stretch the head back because the pressure isn't great enough to stick the brass to the chamber. That way I wind up with a fireformed case with no pressure ring stretch. If I then neck-size only or resize the minimum 0.002" shoulder setback for magazine feed, I am stretching and aging the brass only very slowly. Not much chance of a head separation, then.