What the two numbers measure
Extreme spread is simply the fastest shot minus the slowest shot in a string. It is easy to understand and statistically fragile: it depends entirely on two shots, ignores everything in between, and can only grow as you add rounds. A five-shot ES and a twenty-shot ES from the same load are not comparable numbers, and the twenty-shot figure will always be the larger — not because the load got worse, but because you gave the extremes more chances to appear.
Standard deviation describes how tightly the whole string clusters around its average, so it uses every shot rather than two. It is the more meaningful number, but it needs a real sample before it stabilises. An SD computed from three or five shots is barely an estimate; it moves around dramatically as you add rounds. If you want an SD you can act on, you need a string long enough to earn it — considerably more than the three or five rounds a typical ladder step provides.
Why velocity consistency matters at all
Velocity spread matters because of what it does to vertical dispersion at distance, and only that. At 100 yards, a wide velocity spread is nearly invisible — every round drops effectively the same amount over that distance, so the group size is telling you about everything except velocity. As distance grows, the flight time difference between a fast round and a slow one turns into a vertical difference on the target, and by long range it dominates.
This gives you a practical rule for when to care: if your shooting is inside a few hundred yards, velocity statistics are close to irrelevant to your group size and you can develop a load perfectly well without them. If you intend to shoot at distance, velocity consistency becomes one of the things that decides whether you hit, and it is worth measuring.
⚠️ When the numbers and the target disagree
It is common, and initially maddening, for the load with the lowest SD not to be the load with the smallest group. Shooters run into this constantly, and the resolution is that velocity consistency and mechanical accuracy are related but separate properties. Low velocity spread removes one source of vertical dispersion at distance; it does nothing about how consistently the bullet is released, how uniform the bullets are, or where the barrel is in its whip cycle. A load can be beautifully consistent in speed and still be launched at a bad point in the barrel's vibration.
⛔ When they disagree, the target wins. Paper is the outcome you actually care about; velocity statistics are a diagnostic that helps you predict how that outcome will scale with distance. A load that groups well with mediocre SD may open up at 1000 yards. A load with superb SD that groups poorly at 100 will not improve at 1000. Use SD to choose between loads that already shoot well, not to pick a load that does not.