M231 Firing Port Weapon
- Category
- Personal Defense Weapon
- Barrel
- 396 mm
- Weight, empty
- 3.33 kg
- Introduced
- 1980
About
What it is, and why it matters
The M231 Firing Port Weapon (FPW) is a highly modified, automatic-only derivative of the Colt M16A1 developed by Rock Island Arsenal and produced by Colt for use from the firing ports of the M2/M3 Bradley Fighting Vehicle. Introduced in 1980, it uses a unique open-bolt fire-control group and a very high cyclic rate of around 1,225 rounds per minute. The base weapon has no iron sights and a telescoping steel-wire stock, and a threaded forend lets the barrel pass through a Bradley firing port while remaining removable for dismounted use.
Firing roughly 1,225 rounds per minute and lacking any sights, the M231 was never meant to be aimed; Bradley crews used it to hose the ground around their vehicle, and surplus examples are prized today as one of the rare open-bolt members of the M16 family.
Open-bolt operation and the absence of conventional sights make the M231 a suppressive firing-port weapon rather than an aimed individual arm. When dismounted, crews could fit the wire stock and fire it from the shoulder. It saw service in the Gulf War and Iraq War and remains in inventory.
Specification
Everything measured
IdentityWhat it is, who built it and when.
- Manufacturer
- Colt's Manufacturing Company LLC
- Category
- Personal Defense Weapon
- Country
- United States
- Status
- In service
- Produced
- 1980
DimensionsAs issued, without a magazine or an optic.
- Barrel
- 396 mm
- Overall length
- 717 mm
- Weight, empty
- 3.33 kg
- Weight, loaded
- 3.78 kg
ActionHow it cycles, what trips it, and what feeds it.
- Action
- Gas operated long stroke
- Firing mechanism
- Open bolt
- Trigger
- Single stage
- Capacity
- 30
- Magazine
- Detachable box
MaterialsEach swatch is the material itself, not a colour chosen to stand for it.
- Frame
- Aluminum
- Barrel
- Steel
- Stock
- Steel
- Finish
- Phosphate
BallisticsFrom the service cartridge, not a handload.
- Muzzle velocity
- 930 m/s
- Muzzle energy
- 1730 J
- Effective range
- 300 m
- Rate of fire
- 1225 rpm
Trajectory
What the round does downrange
Solved from the muzzle figures and the bullet. The dashed rule is the effective range on the record, and the crosses are the steps the solver returned.
Features
What it has
What the record lists as fitted, drawn from the catalogue.
Open bolt full auto only
No iron sights
Threaded forend for firing port
Telescoping wire stock- High cyclic rate
- Based on m16a1
Standing
How it compares
Each figure against the firearms of the same type and cartridge that record it, or of the same type where that cartridge has too few. Positions are read to the nearest tenth.
| Measurement | This firearm | Median | Position |
|---|---|---|---|
| Empty weightAmong 54 personal defense weapons | 3.33 kg | 2.23 kg | Heavier than about 90% |
| Overall lengthAmong 53 personal defense weapons | 717 mm | 582 mm | Longer than about 80% |
| Barrel lengthAmong 55 personal defense weapons | 396 mm | 168 mm | Among the longest tenth |
| Magazine capacityAmong 57 personal defense weapons | 30 | 30 | Holds more rounds than about 70% |
Answers
Questions about the M231 Firing Port Weapon
How much does the M231 Firing Port Weapon weigh?
The M231 Firing Port Weapon weighs 3.33 kg (7.34 lb) empty and 3.78 kg (8.34 lb) loaded.
What caliber is the M231 Firing Port Weapon?
The M231 Firing Port Weapon is chambered in 5.56x45mm NATO.
How many rounds does the M231 Firing Port Weapon hold?
The M231 Firing Port Weapon holds 30 rounds, fed from a detachable box magazine.
How long is the M231 Firing Port Weapon?
The M231 Firing Port Weapon is 717 mm (28.2 in) long overall, with a 396 mm (15.6 in) barrel.
Who makes the M231 Firing Port Weapon?
The M231 Firing Port Weapon is made by Colt's Manufacturing Company LLC (United States). It was designed by Rock Island Arsenal.
When was the M231 Firing Port Weapon introduced?
The M231 Firing Port Weapon was introduced in 1980.
How does the M231 Firing Port Weapon operate?
The M231 Firing Port Weapon uses a gas operated long stroke action, and its firing mechanism is open bolt.
Compatibility
Which parts fit this firearm
Studio
Every part that mounts on this, resolved
Not a list the maker typed out: the answer comes from the mounts this firearm exposes and the interface each part needs, so it stays right when either one changes.
- Every part that mounts on any firearm in the catalogue
- Interface standards, platforms and fit overrides
- The same answers through the API and the MCP server
Requires a signed-in GunSpec account on the Studio plan. The answer is computed per part, so it is not part of the public record.
Provenance
What the record holds
Every figure above was taken from one of these pages. The confidence reflects how much of the record came from a page a claim can be checked against.
In service
- United States Army
- 1980
Provenance
Confidence in this record
70%
Stockists
Where to buy
Offers link straight to the shop. GunSpec never takes the order.
No shop has listed this firearm yet
Stock and prices come from the shops themselves. If you sell this firearm, you can list it and it appears here.
List a firearmMCP
Ask an assistant about it
The same record, reached the way a person asks for it rather than the way a program does.
Ask GunSpec
Works with
Point an MCP client at GunSpec and it answers from the catalogue rather than from memory. A free key covers the read tools.
API
The call that returns this record
The same fields, in the same units, from the same endpoint the page reads.
curl --request GET \ --url 'https://api.gunspec.io/v1/firearms/m231-firing-port-weapon' \ --header 'X-API-Key: your_key'{ "id": "m231-firing-port-weapon", "name": "M231 Firing Port Weapon", "manufacturer": { "id": "colt", "name": "Colt's Manufacturing Company LLC", "countryCode": "US", "foundedYear": 1855, "website": "https://www.colt.com", "logoUrl": null, "description": "American firearms manufacturer headquartered in Hartford, Connecticut. Founded in 1855 and now a subsidiary of Colt CZ Group, continuing production of iconic firearms including the 1911 pistol.", "status": "active", "defunctYear": null, "stateOwned": null, "makesFirearms": null, "parentId": null, "predecessorId": null, "createdAt": "2026-02-11 23:44:38", "updatedAt": "2026-02-18 03:43:38", "version": "7143c7ff3f7e156063a240f2df7b4746ee1c0394d7cd5f592d05bac43233d637" }, "category": { "id": "personal-defense-weapon", "name": "Personal Defense Weapon", "description": "Compact firearms designed for non-frontline military personnel" }, "country_of_origin": "US", "year_introduced": 1980, "barrel_length_mm": 396, "weight_empty_g": 3329, "action_type": "gas_operated_long_stroke"}