Here is what I do for 1000yd BR. Our group requirements are probably a lot tighter than across the course guys, so we are probably a little more anal obsessive about prep.
I weigh all my brass and sort them into groups within 2 grains plus or minus (magnum size cases)Trim to uniform length, polish the inside of the necks with a bronze bore bush. I them turn the necks to match my chamber and uniform the primer pockets and the flash hole. spin them real quick on the lathe and polish inside and outside of the necks with "0000" steel wool. Weigh them again. The ones that are the closest together are numbered and kept as the ones used for "record fire" They are then fire formed and ready to shoot. I usually like to have loaded them at least 3 times before they are used for record fire in a big match. While the bullet is being loaded I also have a tool that measures seating pressure. Any unusually high or low pressures are put to one side and not used for record fire. These record cases are always kept separate and together. After each loading they are checked for run out on a Sinclair gage. Once they have been prepped this much there is seldom a problem unless they are damaged in loading or hit the concrete during a match.
I usually get close to 10 reloadings before the neck tension gets sloppy or the primer pockets loosen...and then it starts all over again.
Not sure which or if all of this makes a lot of difference at 1000, but I do know that if I dont do it my groups are bigger with fliers. One thing for sure that does make a lot of difference is neck tension..a extra tight or loose bullet noticed during seating for sure will make a flier at 1000 as well a tight case.
Now for the bullets........
Take the old brass you were using and the military brass you want to load. Size them both in the die to make sure their dimensions are the same.
Now weigh the old case empty, and fill with water to the top and weigh again. Do the match and you'll have an internal volume in grains.
Repeat this step with the military brass and get the internal volume of that brass in grains.
Now, again, divide the military volume (smaller volume) by the commercial brass volume, and you will end up with a percent. Let's say the division gives you a .93 on your calculator,..this means the military brass has 93% of the volume of the commercial brass,...or in other words, it has a 7% LESS volume. Sooooo, take your load from the current commercial brass, and reduce it 7% (by multiplying by .93) and this will be a very close starting point for the new load in the military brass. Within a load or two in both directions (+/-) you will have the new accuracy load in the new case.
(more coming soon!)