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ES and SD on Target: How Much Vertical Your Velocity Spread Really Adds

Inches of vertical per fps of standard deviation for a 6.5 Creedmoor and a .308 at 300, 600 and 1,000 yards, what that does to a half-MOA rifle's groups, and how little a 5-shot string tells you about your real SD.

6 min readSolved with the Ballista engineUpdated October 2026

For a 6.5 Creedmoor, every foot per second of muzzle velocity is worth 0.018 inches of vertical at 300 yards and 0.29 inches at 1,000. So a load with a 10 fps standard deviation adds about 2.9 in of vertical spread at 1,000 yards (one SD) and stretches a half-MOA rifle’s average group from 0.50 to 0.81 MOA there. At 300 yards the same SD adds 0.18 in, which no group can show. A common question on reloading forums is whether an SD of 12 instead of 6 really matters. The answer depends almost entirely on the distance.

ES and SD in one paragraph

Extreme spread (ES) is the fastest shot minus the slowest in a string. Standard deviation (SD) describes how far a typical shot lands from the average. For velocities that scatter in a bell curve, about two shots in three fall within one SD of the average and about 19 in 20 within two. ES depends heavily on how many shots you fire, because more shots give more chances for an outlier. SD doesn’t grow with the count, which is why ballistic calculations use it.

Vertical per fps, from the engine

We solved each rifle at its nominal velocity and 10 fps either side, with the bore held at the nominal load’s zero. The difference in impact height, divided by 20 fps, is the vertical per fps at each range.

It isn’t linear in range. The 6.5’s vertical per fps grows 4.6 times from 300 to 600 yd and 3.5 times again to 1,000. A slower bullet spends longer in the air, and the gap keeps growing as both bullets slow. The .308, with its lower BC, gets 0.43 in per fps at 1,000 yd. That’s 1.5 times the 6.5’s figure, and the .308 is subsonic by then: it drops below the speed of sound at about 970 yd in this air.

For a 95% band, double the one-SD number on each side: a vertical stripe 3.92 SD tall holds 19 shots in 20. For the 6.5 with SD 10 that’s 11.5 in at 1,000 yd. A 10-inch plate is shorter than that before the rifle, the shooter or the wind add anything.

What it does to a group

Velocity spread doesn’t add straight onto group size. It adds to the vertical part, and independent errors combine as the square root of the sum of their squares. A small spread barely shows next to the rifle’s own dispersion. A large one takes over. We simulated 60,000 five-shot groups per case with a rifle and shooter that average 0.5 MOA on their own, then added each SD on top.

This is why a rifle that groups half a MOA at 100 yd can group larger in MOA at 1,000 yd, even in calm air. Up close the rifle’s own spread dominates, and velocity spread is hidden inside it. For the 6.5 with SD 10, the velocity term becomes as large as the rifle’s own vertical spread at about 700 yd for a 0.5 MOA rifle; for a 1 MOA rifle it stays smaller all the way to 1,000 yd. For the .308 the crossover comes at 580 yd.

For the question of SD 12 versus 6, the nearest rows are SD 10 and SD 5. At 1,000 yd with the 6.5, the half-MOA rifle averages 0.59 MOA at SD 5 and 0.81 MOA at SD 10. At 300 yd you couldn’t tell those loads apart on paper. The .308 at SD 10 averages 1.07 MOA at 1,000.

What a 5-shot string tells you about SD

The SD your chronograph prints is an estimate from the shots you fired, and small strings are noisy. If the true SD is 10 fps, here is the range that 90% of measured SDs fall in, and how ES relates to SD on average.

A 5-shot string with a true SD of 10 fps reads anywhere from 4.2 to 15.4 fps nine times in ten. Comparing two loads by their 5-shot SDs mostly compares luck. Twenty shots narrow it to 7.3–12.6 fps. The ratio column turns an ES into a rough SD: a 5-shot ES is on average 2.33 times the SD, a 10-shot ES 3.07 times.

When SD is worth chasing

The hit probability guide puts velocity spread next to the wind call, range error and rifle precision. On a round plate at 1,000 yd the wind call matters more. Velocity spread matters most where the target is short and wide, the distance is long, and the wind is calm or known. The MV vs BC guide uses the same per-fps numbers for a different problem, an average velocity that’s wrong.

Using your numbers

1. Measure SD with enough shots. Five shots can read anywhere from 4.2 to 15.4 fps for a true 10. Ten narrow that to 6.1–13.7, twenty further.
2. Convert to inches at your distance. Multiply your SD by the in-per-fps value for your range. Use the table above or run your load through a solver at ±10 fps.
3. Compare it to your rifle. If the velocity term is well below the rifle and shooter’s own vertical at your distance, a lower SD won’t show on target.
4. Check the vertical band against your target. 3.92 SD of velocity is the height that holds 19 shots in 20, before every other error.
5. Keep the solver’s velocity right. SD spreads the shots around the average. A wrong average velocity moves the whole group, and no SD figure shows it.