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Rock Island Arsenal M231 FPW (Firing Port Weapon)

Special Vehicle Carbine [ 1980 ]

The M231 Firing Port Weapon was a specialized variant of the M16A1, used by Bradley vehicle occupants through the available firing ports presented.

Authored By: Staff Writer | Last Edited: 08/15/2018 | Content ©www.MilitaryFactory.com | The following text is exclusive to this site.

The M231 FPW ("Firing Port Weapon") was a highly-modified, specialized variant of the Colt M16A1 assault rifle series. Also chambered for the 5.56x45mm NATO cartridge and operating on the same gas system, the M231 was specifically-designed for the crew and passengers of the American Bradley Infantry Fighting Vehicle (IFV), engaging enemy infantry through the available firing ports of the Bradley. Design work on the product began in 1972 by Rock Island Arsenal to coincide with the arrival of a new mechanized infantry combat vehicle - this eventually becoming the Bradley vehicle. Introduced in 1980, the weapon saw use in the 1991 Gulf War as well as the 2003 Iraq war and remains in circulation today (June 2014).

Much of the general form and function of the M16A1 was retained though an open bolt design was accepted. A threaded section along the forend was used for passing the barrel through the available firing ports of the Bradley vehicle. The weapon could be fixed in this manner or removed and operated as a stand-alone carbine weapon in-the-field. Since the gunner would be engaging through the vehicle's vision blocks and periscopes, there were no iron sights fitted though the carrying handle could be used as a rather basic aiming reference. Feeding was through the same 30-round STANAG curved detachable box magazines featured in the M16A1. The M231 FPW showcased a rate of fire of about 1,225 rounds-per-minute though short bursts were generally the call of the day. Effective range was out to 330 yards which offered the needed protection against infantry.

For its intended role, the dimensions of the M231 were purposely compact - an overall length of 28.25 inches with a barrel measuring 15.6 inches.

Today, the Bradley line has seen most of her firing ports removed save for the rear-facing positions which, in turn, has limited the tactical value of such a specialized weapon as the M231 FPW.©MilitaryFactory.com
Note: The above text is EXCLUSIVE to the site www.MilitaryFactory.com. It is the product of many hours of research and work made possible with the help of contributors, veterans, insiders, and topic specialists. If you happen upon this text anywhere else on the internet or in print, please let us know at MilitaryFactory AT gmail DOT com so that we may take appropriate action against the offender / offending site and continue to protect this original work.


Service Year

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United States


Special Vehicle Carbine

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(OPERATORS list includes past, present, and future operators when applicable)
Automatic Function
Features a mechanical function to automate the firing action.
Special Purpose
Special purpose weapon for a specially defined battlefield role.

Overall Length
717 mm
28.23 in
Barrel Length
396 mm
15.59 in
Empty Wgt
7.34 lb
3.33 kg



Gas-Operated; Open Bolt

Open Bolt
System utilizes internal mechanism where bolt and associated working components are set to the rear of the receiver with no cartridge chambered; trigger actuation moves bolt forward, chambers cartridge, and fires bullet in single action - ultimately resetting during the complete cycle by way of the generated energy.
Gas-operated system is featured, typically involving a gas cylinder and rear-driven piston directing energy to the bolt component.
(Material presented above is for historical and entertainment value and should not be construed as usable for hardware restoration, maintenance, or general operation - always consult official manufacturer sources for such information)


5.56x45mm NATO

Sample Visuals**

Graphical image of a 5.56mm intermediate rifle cartridge
Rounds / Feed

30-round detachable box magazine
Cartridge relative size chart
*May not represent an exhuastive list; calibers are model-specific dependent, always consult official manufacturer sources.
**Graphics not to actual size; not all cartridges may be represented visually; graphics intended for general reference only.
Max Eff.Range
984 ft
(300 m | 328 yd)

M231 FPW - Base Series Designation

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