The Ammunition tab

Goal of this article: describe a projectile completely enough for the terms you care about — by hand, from a saved load, or from a measured drag curve — and know which fields change the answer and which are only there to help you recognise the load later.

This is the first tab of the Shot Parameters dialog, and the only one that must be filled in: nothing can be computed without a bullet. It is reached from Trajectory → New or, for a window already open, View → Edit Parameters (Ctrl+E).

A .223 69 gr Sierra on a G1 coefficient of 0.365, imperial window.

The fields

The bold labels are the ones the application cannot do without. Everything else is either optional or descriptive.

Field	Imperial	Metric	What it is
Name	—	—	What this load is called. Also becomes the trajectory window’s title and the default file name when you save it
Weight	grain	gram	Bullet weight
BC	—	—	The ballistic coefficient, plus the drag table it is quoted against — G1, G2, G5, G6, G7, G8, GI, GS, RA4, and GC for a custom curve. G1 is the default
BC is Form Factor	—	—	Read the number above as a form factor rather than a coefficient. See when diameter is needed — this switch makes the diameter mandatory
Drag table	—	—	The .drg file holding a measured curve, with Browse… and Clear. Empty reads (standard table)
Muzzle Vel.	ft/s	m/s	Muzzle velocity, from your chronograph or the box
Diameter	inch	mm	Bullet diameter. Optional — until it is not; see below
Length	inch	mm	Bullet length, nose to base. Optional in the same way
Caliber	—	—	Descriptive only — see the last block
Bullet Type	—	—	Descriptive only (FMJ, HP, …)
Barrel Len.	inch	mm	Descriptive only — the barrel the muzzle velocity was measured in
Source	—	—	Descriptive only — where the data came from

Which units you see depends on the window’s measurement system, chosen at Trajectory → New; see Your first trajectory.

Name the load. It is not enforced — the dialog will accept a blank name and compute the shot — but the window title comes from it, so an unnamed load opens a window with an empty title bar, and several of them are impossible to tell apart in the Windows menu.

Entering a load by hand, and keeping it

The minimum is three numbers, and for most factory ammunition all three are printed or published: weight, BC with the table it is quoted against, and muzzle velocity.

1. Type a Name — anything you will recognise: .223 69gr Sierra (16in).
2. Enter Weight.
3. Enter the BC and pick its drag table from the dropdown beside it. Take the table from wherever you got the number: a G7 coefficient entered as G1 is simply a different bullet. If your data sheet gives a form factor instead of a coefficient, tick BC is Form Factor.
4. Enter Muzzle Vel. — measured in your barrel if you have a chronograph. A box figure is another rifle’s answer.
5. Add Diameter and Length if you want spin drift and aerodynamic jump — see below.
6. Optionally fill the descriptive block at the bottom, then press Save.

Save writes an .ammox file — by default into the data/ammo folder beside the application, under the load’s name — and stores the whole record: the three required numbers, the drag table, diameter and length if you set them, a reference to the .drg file if you attached one, and all four descriptive fields. Saving does not close the dialog and does not affect the shot; it is purely so you never type this load again.

Saving is silent, including when it fails. If nothing appears in the folder you chose, the usual cause is a folder the application cannot write to — see Installation.

Loading a load you already have

Load, the button beside the name, opens a file dialog on data/ammo and accepts:

• .ammox — what this application saves.
• .ammo — the format of the original WinForms Ballistic Calculator .NET. Its sample library ships with this application, so there is a usable set of cartridges to try before you have saved anything of your own.

Loading replaces the whole tab: the three numbers, diameter and length, the descriptive block, and any custom drag table the file refers to. It does not touch the other five tabs — the load changes, the conditions and the rifle stay as you had them. A file that fails to load leaves the tab untouched and says nothing, so if pressing Load seems to do nothing at all, suspect the file rather than the button.

A loaded value keeps the units it was saved in. This is deliberate — a load saved as 0.308 in is not rounded to a metric control’s precision on the way in — but it means the tab can show a metric weight in a window that is otherwise imperial. The shipped library has both: 7N6.ammo is stored in grams and metres per second, 45acp 230gr FMJ (4in).ammo in grains and feet per second. The numbers are correct either way and the calculation does not care; only the labels look inconsistent.

Driving the shot from a measured drag curve

A .drg file is a projectile’s own measured drag curve — the Cd against Mach that radar produced for that bullet — and using one is the single biggest accuracy gain available in this application. A large library of radar-derived Lapua tables ships in data/drg.

Press Browse… beside Drag table and pick a file. Five things happen at once:

• BC becomes 1 with the table set to GC — “custom curve”. There is no coefficient to quote, because the curve itself is the drag model.
• BC is Form Factor is ticked. The form-factor-of-1 convention is what makes the file’s own curve the answer rather than a scaled version of a standard one.
• Weight, Diameter and Length are filled in from the file’s header, where it carries them, and converted to the window’s units. Older .drg files store the length and source slots as zero; those are left alone rather than overwriting good values with nothing.
• Name and Source are filled in from the header — but only if you left them empty, so attaching a curve to a record you have already named does not rename it.
• Muzzle Vel. is not touched, and stays your job. A .drg describes the projectile, not the load: the file cannot know what your barrel does. This is the one field to check after every Browse….

A 6.5 mm Lapua Scenar-L on its radar curve. The BC unit is GC, the value 1.000, and the bullet dimensions came out of the file — 135.80475 gr, Ø0.26417 in, 1.33858 in, the metric source values carried across without being rounded to the control’s precision.

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

Two things worth knowing about the reference that gets stored:

• The file is referenced, not copied. The record keeps the file name; the table is read fresh at calculation time. Editing the .drg changes your next answer.
• It 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 .drg is one of the shipped tables, or the recipient drops it into data/drg. If it is nowhere to be found, the shot cannot be computed — a GC coefficient has no curve to fall back on.

If you do not have a curve for your bullet, Tools → Approximate Drag Table builds one from a multi-BC curve or from measured downrange velocities.

When diameter and length are needed

Both are marked optional, and for a plain coefficient on a standard table they genuinely are — the answer is identical whether you fill them in or not. They stop being optional in three cases:

You want	You must supply	Otherwise
Spin drift — the horizontal drift a spin-stabilised bullet accumulates downrange	Diameter and length, plus the barrel’s twist on the Rifle tab	The term is silently absent. No warning: the windage column simply contains only wind
Crosswind aerodynamic jump — the vertical shift a crosswind imparts at the muzzle	The same three, plus at least one wind zone	Silently absent, exactly as above
Any shot using a form factor — including every .drg/GC shot, since those are form-factor-1 by construction	Diameter (and weight, which is always required)	The shot cannot be computed at all: a form factor is turned into a coefficient through the bullet’s sectional density, and that needs the diameter

The first two are the important asymmetry to understand: leaving a field empty does not produce a warning, it produces a different trajectory. A shot with no bullet dimensions is not wrong — it is a trajectory with no spin drift and no aerodynamic jump in it, which at 300 yd nobody would notice and at 1,000 yd is inches of windage. If you have the numbers, enter them; bullet manufacturers publish both, and .drg files usually carry them.

The third is a hard stop rather than a quiet omission, and the case that catches people is the middle one in disguise: attaching a .drg whose header has no diameter. Browse… fills the field when the file carries it — check that it did, and type it in when it did not. OK will tell you if you did not: “Bullet diameter is required when the ballistic coefficient is a form factor.” The same check covers a GC coefficient whose .drg cannot be found — opening someone else’s saved shot that names a table you do not have — and names the file it went looking for.

Caliber, bullet type, barrel length and source are notes, not inputs

The block at the bottom of the tab — Caliber, Bullet Type, Barrel Len. and Source — never reaches the calculation. Not one of the four is read by the solver, and changing any of them cannot change a single number in the table.

They exist so that a saved load is recognisable a year later: which cartridge it belongs to, what kind of bullet it is, the barrel length the muzzle velocity was measured in, and where the data came from. They are stored with the load — in the .ammox file when you press Save, and in the .trajectory file when you save the shot — and are filled in for you by Browse… where a .drg header supplies the name and source.

Barrel Len. deserves the warning, because it is the one that looks like an input: it is a label recording the test barrel behind your muzzle velocity, not something the application converts a velocity with. Entering 16 in next to a 24-inch barrel’s velocity does not correct anything — it only records a note that contradicts the number beside it. If you want the velocity from your barrel, measure it.

Things that catch people out

• A G7 number entered as G1. Nothing warns you; the trajectory is simply a different bullet’s. Always take the table from the same place as the number.
• Muzzle velocity after a Browse…. The field is left empty on purpose, and an empty required field is easy to walk past.
• Diameter with the form-factor switch on. Required, including for every .drg shot — OK says so.
• A blank name. Legal, but the window title goes with it.
• Units after a Load. The file’s own, not the window’s.

Next

The Weather tab — the air the bullet has to fly through, and the one field on it that catches everybody.

---

← Contents