The only way to be certain each and every powder charge in a batch of cartridges weighs the same is to carefully weigh them on a scale. Even so, many handloaders have discovered that a small variation in powder charge weight usually has little to no effect on accuracy out to the maximum distances they shoot, and for that reason they save time by measuring charges rather than weighing them.
In conventional benchrest competition, where targets are placed at 100 and 200 yards, top-ranked shooters customarily average less than 1/4 m.o.a. accuracy. They use the same dozen or so carefully prepared cases for each rifle and reload them between relays during a match.
Instead of weighing powder charges, they use precision-built measures to throw them. Ask one of those guys how much Vihtavuori N130 he is shooting in his 6mm PPC, and rather than giving an answer in grains, he will likely tell you what setting he uses on his Harrell or Culver powder measure.
Back when I bumped off a lot more prairie dogs each summer than I do today, I decided to compare the accuracy of ammunition prepared with weighed powder charges and those thrown by the measure of a progressive press. Several different powders and bullets were loaded in the .223 Remington. I shot the ammo in two custom rifles in .223 Remington, both capable of shooting inside half an inch at 100 yards. I also tried the loads in my rail gun.
Groups fired out to 500 yards revealed no difference in accuracy between weighed and measured charges. Some of the ammo loaded on the progressive press was actually a bit more accurate in the rail gun, but I chalked that up to fluke rather than uniformity differences in how powder charges were dispensed.
Benchrest shooters and varminters shoot small cartridges that burn relatively small powder charges, but measuring powder for cartridges with bigger appetites can work equally well.
Before a rifle built by Kenny Jarrett is shipped to a customer, it must consistently shoot three bullets inside half an inch at 100 yards. I used to visit his shop quite often and, just for the fun of it, occasionally accuracy-tested rifles for him. The powder charges of all test ammunition used in Jarrett's shop are thrown by RCBS and Redding measures. A scale is used, but only when adjusting the measures to throw the desired charge weight. Same goes for his line of custom ammunition.
For most of the reloading most of us do, powder measures available from RCBS, Lyman, Lee, Hornady and Redding are precise enough. Benchrest shooters usually go for precision-machined units like the Micro-Measure from Neal Jones as well as the Harrell and RFD/R from Sinclair International.
Benchrest competitors sometimes tweak the loads they are shooting between relays, and the custom measures they use have more precise repeatability of adjustment than some mass-produced measures. I started using a Jones measure during my benchrest shooting days and continue to use it today. With many powders it is not a whole lot better than high-quality measures available from other sources, but it does operate more smoothly and throws charges of large-grained powders a bit more consistently.
How a measure is used is extremely important. To throw charges with minimum variation, it has to be operated exactly the same for each charge of powder. If you bounce the handle hard against its stops on one charge and then operate it softly on the next, the two charges will likely vary more in weight than if the handle is operated exactly the same both times.
I consider a charge weight variation of 0.3 grain acceptable in the larger cartridges such as .30-06 and up, but a good measure, operated properly and filled with a smooth-flowing powder, should hold that to no more than 0.1 grain. When measuring extremely heavy charges, consistency will improve with some measures when two half-charges are thrown into the case rather than one whole charge. In other words, the handle is operated twice on a 40-grain setting rather than once on an 80-grain setting.
Finely granulated powders meter through any measure more accurately than coarser powders. This is why benchrest shooters use either ball powders or fine-grained stick powders such as Vihtavuori N130 and IMR-8208 XBR. Same goes for varmint cartridges.
Any time I don't use a ball powder such as W748 or A-2015 in the .223 Remington, I'll use a small-grain stick powder with Benchmark and V-N135. If H4831 and IMR-7828 are your favorites for the .270 Winchester, finer-granulated versions of those two powders designated H4831SC (short cut) and IMR-7828SSC (super short cut) flow through measures more uniformly than the originals and their burn rates are the same.
Digital dispensing systems capable of automatically measuring and weighing powder charges are available from Lyman, RCBS, Hornady and PACT. After the machine is programmed for the desired charge weight, a push on its start button or placing the pan on the digital scale causes it to trickle powder from a reservoir into the pan. In addition to being more precise with large-granule powders than a standard measure, it totally eliminates operator inconsistency in throwing charges.
To reach peak efficiency with one you may need to modify your loading sequence a bit. When using a standard measure, I charge all cases with powder before moving on to bullet-seating. Depending on the size of the charge being dispensed, a digital machine can take anywhere from five to 30 seconds to get its job done, so seating a bullet on a charged case while the machine is dispensing powder for the next one speeds up the operation a bit.
Distance to the target often has a bearing on whether a handloader chooses to weigh or measure powder charges. Out to 500 yards I don't believe it matters enough to go to the trouble, but when punching paper at greater distances those tiny variations can become big ones and for that I believe carefully weighing each charge is the way to go.
Most of us load so few rounds of big game ammunition each year, weighing charges is no big hassle. High-volume loading is where it can save a lot of time.