What BC actually measures
BC is the ratio of a bullet's sectional density to its drag coefficient relative to a standard projectile. A higher BC means the bullet decelerates more slowly — it retains more velocity at distance, drops less, and drifts less in wind.
Two bullets leave the muzzle at identical velocity. By 500 yards, the bullet with BC 0.6 might be traveling 1850 fps; the same bullet with BC 0.3 is traveling 1350 fps. That 500 fps difference means dramatically more drop and drift for the lower-BC bullet.
G1 vs G7 drag models
BC is always relative to a drag model — a mathematical description of how a standard projectile behaves. Two common models are G1 (flat-base projectile, the traditional US standard) and G7 (boat-tail long-range bullet, developed specifically for modern VLD/LRAB bullets).
Most older published BCs use the G1 model. Most modern ballistic solvers and high-end bullets now use G7. The problem: a G1 BC for a boat-tail bullet is only accurate near 2600–2800 fps. Above or below that band, G1 over- or under-predicts drag. G7 is accurate across the full velocity range for modern VLD bullets.
Rule of thumb: G7 BCs for boat-tail bullets are typically about 1/2 the G1 BC for the same bullet. A 175gr Sierra MatchKing might be G1 0.505 or G7 0.264 — both represent the same bullet.
Choosing a bullet for BC
For targets beyond 400 yards, BC matters significantly. Below 400 yards, even a modest BC difference rarely separates a hit from a miss.
The highest-BC bullets are typically long, heavy-for-caliber, and require fast twist rates to stabilize. A .308 Win with a 1:10 twist can stabilize a 175gr MatchKing (G7 0.264). A 1:12 twist cannot — the bullet tumbles and BC is irrelevant.
Verify your barrel twist can stabilize the bullet before relying on its stated BC. The Miller stability formula: Sg = 30 × m / (t² × d³ × l × (1 + l²)) where m = bullet mass, t = twist rate (inches/turn), d = diameter, l = length. Sg > 1.5 is required; > 1.8 is preferred for accuracy.
Factory ammo BCs are nominal values. Lot-to-lot variation, seating depth, and muzzle velocity affect real-world BC. Verify with chronograph + DOPE data if shooting past 600 yards.
Using BC in your solver
Enter BC and its model (G1 or G7) into your ballistic solver correctly — using a G1 BC value in a G7 field will give wrong answers. Most solvers default to G1; ensure you select G7 if that's what the bullet manufacturer published.
If you have both G1 and G7 BCs available for your bullet, always prefer G7 for shots beyond 400 yards. Use G7 0.264 instead of G1 0.505 for the 175gr SMK.
Verify solver outputs against real DOPE at 500+ yards. If the solver consistently over- or under-predicts, your BC or MV input is wrong — adjust BC (not MV) to match real-world data.
Going deeper: the Advanced Ballistics track's 'Truing Your Solution' lesson formalizes this — true muzzle velocity at mid-range first, then the drag/BC at long range, and only ever on good supersonic data.