---
title: "Calculate ballistics for a firearm"
method: GET
path: "/v1/firearms/{id}/calculate"
tags: ["Ammunition"]
---

# Calculate ballistics for a firearm

`GET /v1/firearms/{id}/calculate`

## Request

- Base URL: `https://api.gunspec.io`
- URL: `https://api.gunspec.io/v1/firearms/{id}/calculate`
- Auth: one of:
  - API key in header `X-API-Key`
  - HTTP bearer

Computes muzzle performance for a firearm using its own barrel length and a chosen ammunition load, and compares the result against the figures the catalog records for the firearm.

Returns muzzle figures only. For the sampled trajectory out to 1000 m use `/v1/firearms/{id}/load`.

When the firearm has no recorded barrel length the response is still `200` and carries an `error` string in place of the computed blocks.

**Requires Builder tier or higher.**

## Path parameters

- `id` string, required

## Query parameters

- `ammo_id` string, required

## Response `200`

Calculate ballistics for a firearm

- object
  - `success` boolean, required: Always true on a successful response. Every error carries `false` and an `error` object instead.
  - `data` object, required: Muzzle solution adjusted for this firearm barrel length, against its published figures.
    - `firearm` BallisticsFirearm: The firearm the solution was computed for. `barrelLengthMm` is absent when the firearm has no recorded barrel.
      - `id` string, required: URL-safe slug identifying the firearm, e.g. `glock-g17-gen5`.
      - `name` string, required: Display name, as the maker writes it.
      - `barrelLengthMm` number: Barrel length in millimetres, measured from the breech face.
    - `ammunition` object: The load the calculation used.
      - `id` string: Slug of the ammunition load that was fired.
      - `name` string: Display name, as the maker writes it.
    - `calculated` object: The figures derived for the barrel length asked about, rather than the reference one.
      - `muzzleVelocityMps` number: Muzzle velocity in metres per second, for the default load.
      - `muzzleEnergyJ` number: Muzzle energy in joules, for the default load.
      - `terminalBallistics` TerminalBallistics: Terminal performance indices computed at the muzzle.
        - `tkoFactor` number, required: Taylor Knock-Out factor.
        - `momentumKgMs` number, required: Momentum in kilogram-metres per second.
        - `sectionalDensity` number, required: Bullet mass divided by the square of its diameter. Higher penetrates better, all else equal.
        - `hatcherRSP` number, required: Hatcher Relative Stopping Power.
        - `energyDensityJCm2` number, required: Energy per unit frontal area, in joules per square centimetre.
    - `sourceReported` object: The figures published for this firearm, where the catalog records them.
      - `muzzleVelocityMps` number, nullable: Muzzle velocity in metres per second, for the default load.
      - `muzzleEnergyJ` number, nullable: Muzzle energy in joules, for the default load.
    - `delta` object: Computed minus source-reported. Null where no source figure exists to compare against.
      - `velocityMps` number, nullable: Remaining velocity at this distance, in metres per second.
      - `energyJ` number, nullable: Remaining energy at this distance, in joules.
    - `error` string, nullable: Why the calculation could not be completed, or null when it succeeded.

## Other responses

- `400`: Validation error
- `401`: Missing or invalid API key
- `403`: The credential is valid and the caller is not permitted. Three causes, none of which reissuing a key changes: the plan is below the endpoint's tier, the account is suspended, or the key is not attached to the thing being acted on, meaning a user account for `/v1/me` or a shop for `/v1/vendor`. Distinct from 401, which means the credential is missing, malformed or expired and a new one can work.
- `404`: Resource not found
- `429`: A limit was reached. `RATE_LIMITED` is the per-minute limit: wait `Retry-After` seconds (a minute) and retry. `DAILY_CAP_EXCEEDED` is the plan's daily limit, per key: nothing will be served until the counters return to zero at midnight UTC. `MONTHLY_CAP_EXCEEDED` is the plan's monthly allowance, for the whole account: nothing will be served until midnight UTC on the first of next month, or until the plan changes. For both, `Retry-After` is the seconds until then and `error.resetsAt` says when as an instant. Calls made after a refusal are refused too and do not spend the month, and a free key that keeps calling is paused, so stop at the first one and resume at the reset.
- `500`: Internal server error

---

[API](https://skmtc.dev/gunspec/apis/gunspec-io-api.md) · [All operations](https://skmtc.dev/gunspec/apis/gunspec-io-api/llms.txt) · [OpenAPI document](https://skmtc.dev/gunspec/apis/gunspec-io-api/revisions/f2e974d8b05c?raw)