Ammunition per kilogram

Rounds per kilogram of full magazines and what a weight budget holds, derived from the record, and a basic load's mass estimated by the fitted cartridge-mass model with its error as a range.

What it computes

The ammunition answer takes one or more firearms and returns their ammunition as a weight: how many rounds a kilogram of full magazines carries, how many full magazines and rounds fit a weight budget, and what a basic load of rounds and magazines is estimated to weigh.

Two kinds of figure come back, labelled and never mixed. A derived figure is arithmetic on two weights the firearm's record states. An estimated figure comes from a model fitted to published cartridge masses, and is always given as a range.

Parameters, defaults and the response are in the reference: GET /v1/firearms/ammo-load

Derived from the record

A full magazine is the firearm's loaded weight less its empty weight, the same subtraction the load-carriage answer makes. Rounds per kilogram and the budget follow from it and the magazine's capacity.

m_{mag} = m_{loaded} - m_{empty}

n_{kg} = \frac{c}{m_{mag}}

N_{mag} = ⌊ \frac{B}{m_{mag}} ⌋, N_{rounds} = c N_{mag}

where

$m_{loaded}, m_{empty}$: the firearm's loaded and empty weights, as its record states them
$c$: the magazine's capacity, from the record
$B$: the weight budget the request gives

Only a detachable magazine comes out whole. For a firearm with a fixed magazine, a tube or a clip, loaded less empty weight is the rounds alone, so these figures are not derived and the answer says why.

Estimated by the cartridge-mass model

A loaded round is its bullet plus a case and the powder inside it, taken together as a mass per cubic centimetre of the cylinder the case occupies. The bullet is the named load's, or the cartridge's typical bullet.

m_{round} = m_{b} + 2.5 \times \frac{π}{4} D^{2} L

m_{load} = n m_{round} + N_{mag} m_{body}

where

$m_{b}$: the bullet's mass in grams
$D, L$: the case's base diameter and length, in centimetres, from the cartridge's record
$m_{body}$: an empty magazine's mass, which the model gives only as a band: at least 15 g, up to 450 g for a box magazine and 2500 g for a drum, so the magazines' range is wider than the rounds'

The model describes a metal case of powder behind a bullet, the kind it was fitted on. A plastic or paper hull, a caseless round or a load of shot is not estimated. The reference cartridges' cases run from 2.6 to 5.7 cm³; outside that the answer still estimates, with a warning that the error there is not measured.

The model's error

The model was fitted to four cartridges whose loaded mass a source states. Its largest miss on them is 8.5%, and every estimate carries that error either side as its low and high.

Cartridge	Stated	Model	Error
.45 ACP	21.19 g	21.45 g	+1.2%
5.56×45mm NATO	12.31 g	12.08 g	−1.9%
7.62×39mm	16.30 g	17.69 g	+8.5%
7.62×51mm NATO	25.40 g	23.66 g	−6.9%

Checking a derived magazine

The model also checks each derived magazine. If it weighs less than its rounds alone are estimated to weigh, even taking the model's whole 8.5% off them, one of the record's two weights must be wrong, and the answer says so beside the figures derived from it. A magazine outside the band the catalogue's own plausibility check allows is warned about too.

m_{mag} < c m_{round} (1 - ε)

A flagged magazine is still compared, but never used as the baseline the other firearms are measured against.

Worked example: two 30-round rifles

Two records as they stood on 30 September 2026, each with a 30-round magazine, on a 5 kg budget and a basic load of 7 full magazines. Every figure below is the answer's own functions, run as the page loads.

Record	$m_{mag}$	$n_{kg}$	In 5 kg	Rounds alone, estimated	Basic load, estimated
HK416 5.56×45mm NATO	400 g	75.0	12 × 30 = 360	331 to 392 g	2.42 to 5.90 kg
AK-74 5.45×39mm	300 g	100.0	16 × 30 = 480	309 to 366 g	2.27 to 5.71 kg

$m_{round} = 4 + 2.5 \times 3.222 = 12.06 g$ is the model's estimate for one round of the HK416's cartridge.

The AK-74 record gives its full magazine as 300 g. Its 30 rounds are estimated at 337 g, and at no less than 309 g even at the model's whole error, so the answer flags the record: one of its two weights is wrong.

Limitations

A derived figure is only as good as the two weights on the record. The check catches a magazine too light for its rounds, not every error.
An estimate is the model's, never a recorded weight, and holds to its stated error only for cartridges like those it was fitted on.
The magazine body in a basic load is a band, not a fitted figure, so that range is wide.
Rounds per kilogram counts full magazines, not the pouches, belts or carriers that hold them.