Consistency from bullet to bullet, or using only 1 bullet, or from powder to powder or from case to case or only using one case?
Way to many variables to pick one and say, "that's it"!!
That said, when I look at cartridges for long range, I find that the larger cartridges tend to be more accurate than the smaller ones. I wondered about that for quite a while, until I sat down one day and tried to graph out why that might be, here is what I came up with:
Pick a cartridge--I'll choose the 7mm WSM, its becoming pretty well known as a good cartridge for long range. (The British F Open team just used it to win the Worlds.) It shoots bullets between 168 and 200 grains at speeds around 2900-3100 FPS. To achieve that, most guys are using powder loads of 60-70 grains of their choice of powder. I'll also choose the 180 Berger VLD, since it seems to have a pretty big following.
Lets say the rifles favorite load, determined after much workup, is 64 grains of powder. Lets say my scale has a +/- of .1 grain, meaning my loads can be anywhere from 63.9 to 64.1. (Most scales are no more accurate than that, guys, even the best ones. If you paid $500 + for a pharmaceutical grade scale AND you know how to keep it properly calibrated, AND you have a perfect clean power source, you might get it to .05 grains.)
That means that right off the bat, you have introduced a variable of .2 grains or .3%. At 3000 FPS, that's 9.375 FPS, between the top and bottom load, assuming the difference is linear, (it isn't, so this is a best case scenario.)
Now I'm going to use the assumption that your brass is 100% consistent. We all know it isn't, but we won't factor it in because no one has good enough equipment, (or the time!) to measure every case and then adjust powder loads so that pressures are identical.
Then we're going to throw in a slight variation in the bullet weight. I weighed a handful today of Berger 180's today, and they varied by .4 grains top to bottom. That's a .222 % difference. (.4/180) Using that same linear scale, that adds in another 6.6 FPS. That's a total variation of 16 FPS, and we all know that velocity is not linear to weight and powder charge. In reality, the difference is probably substantially more.
Now, go to a case that requires around 96 grains of powder to shoot a 300 grain bullet that same speed. The scale variation is the same, +/- .1 grain. That's a total variation of .2 grains, but because the powder charge is 50% larger, its only a .2% difference or 6.25 fps.
The bullet variation will be essentially the same, or .4 grains, which is now a .133% (.4/300) difference. That works out to only 4 FPS.
Total difference is then only 10 FPS top to bottom.
The larger cartridge simply makes the variances smaller as a percentage of case capacity and weight, thus making them potentially more accurate, all other things being equal.
However, if you add in the advantage of higher BC available in larger bullets, the potential difference becomes even larger.
So, to answer your question, shoot the largest caliber, with the largest case, with the highest BC bullet that you can for the purpose you need.
Bill
