Airgun pellet ballistic coefficient, explained.

BC is a ratio. It compares your projectile's deceleration to that of an idealised reference shape, so a higher BC means “sheds speed more slowly, drops less, drifts less.” Centrefire shooters lean on it heavily, and for a sleek rifle bullet a single G1 or G7 BC is a reasonable shorthand. The trouble starts when that same shorthand is stamped on a tin of diabolo pellets, because the reference shape it is measured against looks nothing like a pellet.

Why a diabolo breaks a rifle BC

A diabolo pellet is wasp-waisted and high-drag by design — the flared, hollow skirt and pinched middle are what make it stable and forgiving at airgun speeds. That shape simply does not share the drag curve of a streamlined bullet. So when a single G1 (or G7) BC is fitted to a pellet, it can only be made to agree at one velocity. Everywhere else along the flight — faster near the muzzle, slower downrange — the model and the pellet quietly part company, and your predicted drop drifts away from the real one. The number is not wrong so much as it is the wrong kind of number for the job.

Airgun drag models: GA, GA2 and friends

The honest fix at the physics level is to stop approximating a pellet with a rifle curve and use a drag law built for the projectile. PelletArc carries airgun-specific drag laws for exactly this: GA and GA2 for domed diabolos, WC0 for wadcutters, SLG0 and SLG1 for slugs, and GS for round ball. Alongside them it keeps the rifle standards G1 and G7, RA4 for .22LR rimfire, and a custom Cd(Mach) table when you have your own curve. Each law's provenance is cited in the app — the airgun work traces to Miles Morris (Ballisticboy), with JBM Ballistics, McCoy/BRL and ChairGun reference data behind the rest. These airgun drag laws are not present in any other mobile app.

A drag law is not a product claim or a guess; it is a measured description of how a class of projectile sheds speed across the Mach range. Using GA for a domed diabolo is the same in spirit as using G7 for a boat-tail bullet — you are matching the model to the shape instead of forcing one curve to cover everything.

Why published pellet BCs disagree

Pick three sources for the same pellet's BC and you will often get three different figures. That is not carelessness — it is the reality that pellet BC numbers come from wildly different places and quality levels: careful chronograph measurement, a manufacturer's catalogue, a community spreadsheet, or no reliable basis at all. PelletArc ships 1,680 sourced pellets, and instead of papering over this it makes the uncertainty visible: every entry is tagged by data quality — measured, manufacturer, community, or BC-unknown. Crucially, it never fabricates a coefficient. When a reliable BC simply isn't known for a pellet, PelletArc says so and points you at the one method that doesn't depend on anyone else's number.

The fix: true it yourself

The cleanest BC for your setup isn't printed anywhere — it lives in your barrel. BC truing is the answer: instead of trusting the tin, you fit a velocity-dependent BC to a handful of drops you actually observe. PelletArc takes two or three of your own measured drops and calibrates the coefficient to your rifle and your pellet, so the answer reflects reality rather than a reference shape from a textbook. Log the drop, tap once, and your barrel defines the BC.

That is the whole philosophy in one line: a number off a box is a starting guess; your own drops are the truth. If you want the wider picture, the airgun ballistics calculator guide shows how the drag models and truing fit together in practice, and you can read how PelletArc compares to legacy tools in PelletArc vs ChairGun. The physics and truing sections cover the same ground from the product side, and the full guides hub gathers the rest.