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G1 vs G7 Ballistic Coefficient: Where One G1 Number Stops Working

For a long boat-tail bullet the G1 BC that matches the real path holds steady above Mach 1.7 and then slides fast. Where a single G1 number starts to miss for a 6.5 Creedmoor and a .308, and by how much.

7 min readSolved with the Ballista engineUpdated October 2026

Fit a G1 ballistic coefficient to a .308 Winchester 175 gr match bullet at 300 yards and it reproduces the full drag solution within 0.02 mil all the way to 800 yards. Then it falls apart. At 1,000 yards the same number puts you 5.9 in low, and at 1,200 yards 24 in low. The G1 number didn’t drift slowly with velocity, as the usual explanation goes. It held, and then it broke where the bullet slowed toward the speed of sound.

What a BC is, and what G1 and G7 mean

A ballistic coefficient compares your bullet’s drag to a reference projectile’s drag. The reference is a drag curve: drag coefficient against Mach number, measured long ago for one standard shape. G1 is a short, flat-based projectile. G7 is a long boat-tail with a long, pointed nose, much closer to a modern match bullet. The BC is a scale factor. A bullet with twice the BC loses speed half as fast as the reference at the same Mach number.

That only works if your bullet’s drag curve has the same shape as the reference curve, so that one scale factor fits at every speed. If the shapes differ, the scale factor that fits changes with speed. That’s the whole G1 versus G7 argument in one sentence.

The G1 BC of a G7 bullet, speed by speed

Take a bullet whose drag follows the G7 curve exactly. Divide the G1 drag coefficient by the G7 one at each Mach number, multiply by the G7 BC, and you get the G1 BC that bullet would show at that speed. The two curves come from the same standard tables the engine uses.

The 6.5 Creedmoor leaves the muzzle at Mach 2.43 in standard air. Over most of a typical flight its equivalent G1 BC barely moves. Hornady’s published numbers for this exact bullet, the 140 gr ELD Match, show the same thing: G7 0.326 and G1 0.646, and 0.326 × 1.992 (the G1/G7 drag ratio near Mach 2.4) is 0.649. The trouble lives at the slow end of the chart.

Fitting G1 at one range, then checking the others

A shooter who trues a G1 BC adjusts it until the solver matches an observed drop. The engine can do the same: for each range, it finds the G1 BC whose come-up matches the G7 solution there.

For the 6.5 Creedmoor the matching G1 number is 0.648 at 300 yd and still 0.647 at 1,000. The bullet is above Mach 1.33 the whole way, where the two curves are nearly proportional. For the .308 the fitted BC holds at 0.483 to about 800 yd and then drops to 0.475 at 1,000 and 0.466 at 1,200. The .308 crosses Mach 1.2 at about 790 yd in this air and drops below the speed of sound at about 970 yd.

What one G1 number costs at distance

Now dial the G1 solution and let the bullet fly its real path.

Three things follow from the table.

At normal ranges a G1 number from the box is fine for this kind of bullet. The 6.5 with the published G1 0.646 stays within 0.01 mil of the G7 solution out to 1,000 yd at sea level.

Below about Mach 1.2 a single G1 number goes wrong fast, and always the same way. Below Mach 1.4 the G1 drag coefficient falls as the bullet slows, while the G7 coefficient keeps rising until just above Mach 1. A long boat-tail behaves like the G7 curve. So the G1 solution expects less drag than the bullet really meets, predicts too little drop and calls for too little elevation. The shot lands low.

Truing G1 far away doesn’t fix it, it moves the error. Fit the .308’s G1 BC at 1,000 yd (0.475) and the 1,000 yd shot lands within a fraction of an inch. At 600 yd that number is 0.8 in high, which is negligible, but at 1,200 yd it’s still 13 in low. Once the bullet is in the slow part of its flight, any single G1 value is right at one distance only.

Bullet makers who publish G1 numbers for long boat-tails often publish several, each for a velocity band, to work around this. That’s a patch for a reference curve with the wrong shape. A G7 BC fixes the shape.

G7 is a better fit, not a perfect one

The G7 curve is a better match for long boat-tails. Litz’s book makes that case from measured drag of match bullets. Real bullets still don’t follow it exactly. Hornady publishes Mach-specific G7 BCs for the 140 gr ELD Match: 0.326 at Mach 2.25, 0.320 at Mach 2.0 and 0.310 at Mach 1.75. That’s a 5% slide, against the G1 number’s much larger slide through the transonic zone. Every number in this article treats the G7 solution as the bullet’s true path. For a real bullet the gap between the two can be smaller or larger, and only measured drops at long range settle it.

When a bullet in Ballista’s library has both, the app solves with the G7 number, or with the maker’s G7 velocity bands where they’re published, and the library line marks which one is used. A profile can still be set to G1 or G7 under Drag model when your data is G1 only.

What to use

• Long boat-tail match and hunting bullets: use the maker’s G7 BC. If only a G1 is published and you shoot past the point where the bullet drops under Mach 1.2 (about 1,340 fps in standard air), expect to true at long range.
• Flat-base and short, blunt bullets: G1 is the closer shape. It’s what the standard was built on.
• True at the distance you care about, and if you true a G1 BC, check it at a second distance farther out. A G1 number that matches at 600 yd says nothing about 1,100.
• Watch the Mach number, not the yardage. Thin air keeps the bullet faster for longer and pushes the breaking point out. The transonic article shows where each rifle slows below Mach 1.2. The velocity vs BC article shows what a plain 5% BC error costs.