Custom drag tables (.drg)

Goal of this article: use a projectile’s own measured drag curve, and know what the file carries, what it does not, and how it travels.

A .drg file is a measured drag curve: the drag coefficient of one specific projectile against Mach, usually recovered from Doppler radar. It replaces the “ratio to a reference shape” bookkeeping described in choosing a drag model with the thing that bookkeeping was approximating.

Where they come from

• Shipped with the application. data/drg holds a large set of radar-derived tables for Lapua projectiles, published by the manufacturer, organised by calibre.
• From a manufacturer. A handful of makers publish measured curves for their bullets.
• Built here. Tools → Approximate Drag Table derives one from a multi-BC curve or from measured downrange velocities — see approximating a drag table.

Using one

On the Ammunition tab, press Browse… beside Drag table and pick the file. Five things happen, and they are worth knowing because together they are the convention:

1. The BC becomes 1 and the table becomes GC — “custom curve”. There is no coefficient to quote: the curve is the drag model.
2. BC is Form Factor is ticked. A form factor of 1 says “as draggy as this curve says”, which is what makes the file’s own numbers the answer.
3. Weight, diameter and length are filled in from the file’s header where it carries them, converted into the window’s units.
4. Name and source are filled in if you left them empty.
5. Muzzle velocity is not touched.

That last one is the standing trap and it is not a defect: a .drg describes a projectile, not a load. The file cannot know what your barrel does with it, so the muzzle velocity is always yours to enter.

What the file carries

A .drg holds the curve plus a small header: name, source, weight, diameter and bullet length.

Two consequences:

• Weight and diameter are not decoration. The curve is stored scaled by the projectile’s sectional density, so those two numbers are part of the drag model. If a file’s header lacks a diameter, Browse… leaves the field empty and the shot cannot be computed — a form factor needs a diameter. Type it in.
• Bullet length is a recent addition. Files written before it existed store the length and source slots as 0, and the application treats non-positive as absent rather than overwriting a good value with nothing. Without a length there is no spin drift and no aerodynamic jump, so fill it in if you know it.

And what it does not carry: muzzle velocity, barrel length, powder charge, your zero — nothing about a load or a rifle. A .drg is one bullet’s aerodynamics and nothing else.

How the reference travels

The ammunition record stores the .drg file name, not a copy of the curve. That has three effects worth knowing:

• Editing the file changes your next answer. The table is read at calculation time, cached by path and last-write time, so a corrected .drg takes effect without reloading anything by hand.
• The file is found again by name. If the stored path no longer exists, the same file name is looked for in data/drg and its subfolders. So a .ammox or .trajectory you share works on another machine as long as the recipient has that .drg — one of the shipped ones, or dropped into data/drg.
• If it cannot be found at all, the shot fails. A GC coefficient has no curve to fall back on, and there is no standard table hiding behind it. That is the price of the convention.

Clearing one

Clear removes the reference. Because a GC coefficient without a table is uncomputable, clearing also resets the BC to 0.5 G1 and unticks the form-factor switch — a deliberately implausible placeholder rather than a number you might mistake for real data.

Is it worth it?

Yes, and more than any other single input past mid-range — but with two caveats worth stating plainly.

The curve must be your bullet. A radar table for the 8.8 g Scenar-L is not a table for the 9.0 g Scenar, and a table for a bullet from a different lot with a different meplat is only approximately yours. Matching the exact designation matters.

It does not fix the load. A measured curve with a guessed muzzle velocity is a precise answer to the wrong question. If you are going to the trouble of a measured drag curve, chronograph the load too.

Next

Approximating a drag table — building a usable curve when nobody has measured your bullet.

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