.001 gr accuracy.
Not needed, ever, too many variables. Barrel heating after first shot changes ever so slightly in length and diameter and after shooting 5 rounds in a ten round comp the barrel really changes dia and length. This would negate the need for .001 gr accuracy as the barrel variable alone cancels out any three place decimal reading device.
My friend who shoots at Williamsport weighed his bullets like this and after many, many hours of weighing the resultant group sizes of bullets was so small he couldn't tell if it was the weight or just simply some other variable as the gun shot equally well with the three place weighed bullets versus the 2 place weighed bullets.
As far as powder goes the quality of each individual grain would have to be exactly the same size and then you may see some insignificant amount of change probably even un-measurable. Your grains of powder would have to fall exactly the same and lay in the case exactly the same and probably even then the burn rate would be at best so far apart that calling it close would be the same as horseshoes and handgenades, not a factor once again as you cannot control this aspect.
I thought about getting the .001 scale but with all the other variables associated with loading and shooting this one was truly a "doesn't matter you'll never see it" issue.
You would need an optical comparator to measure each and every bullet, each and every grain of powder, each and every case, each and every primer and then weigh each and every bullet each and every case, each and every primer, some optical comparators will allow you to measure in millionths .xxxxx decimal places, this is absurd as like I had said before barrel heat alone would cancel out this time and effort.
Ignition time and energy of the primer could be studied, you'll find this too doesn't matter, maybe for the most anal of benchrest shooters would this matter but it's only in your mind where it matters as the numbers are so small the effort exerted does not outweigh the gains as they are almost un-measurable.
For the benchrest crowd this may be what they do to some extent, but for us and extreme long range shooting 2 place decimal reading devices will suffice quite well for our needs. Don't let me stop you though from getting the scale it is a fine piece of equipment and a great price the Ohaus I was looking at, the AR1530 is $770.00 and measures 3 places, the repeatability is .001, the VIC-123 is .001 and repeats +/- 2d which means it will read into the 3 decimal places but only repeat in the 2 decimal places so the 3 in this case is just gee whiz info.
These scales are also very sensitive to magnetism, wind, and leveling for them to perform as suggested by their specifications.
I have the RCBS where the scale talks to the powder dispenser, it reads in 2 decimal places so I don't know how close it is to the next second decimal reading which may contribute to accuracy but you would have to pick the high side or the low side of the reading and stick with that for averages if you had a three place reading device.
Then again I have seen factory Rem 700's with only one bolt lug contacting shoot extremely small groups with federal factory ammo suggesting that maybe all of what we do is in vain, but then again even a blind squirrel finds a acorn once and a while so what does this matter, I'm not sure, but if the gun shoots great with two place reading equipment and I cant tell the difference between groups from three place reading equipment then it doesn't really matter does it?
Dave