M231 Firing Port Weapon

Updated 6 October 2017 to include a link to a video about the weapon 

The M231 Firing Port Weapon is a curious thing.  It is a modified M16A1 assault rifle that has been specially adapted for use as a close range defense weapon for the M2/M3 Bradley (which was GURPS-ified earlier here), as well as a weapon of last resort for its crew.

The M231 is a successor of sorts to the M3 Grease Gun, itself used as a vehicle crew weapon up into the 1990's.  During the same project to replace the M113/M114 combat vehicles as the gave us the Bradley family, the requirement for a high volume of fire, anti-infantry close support weapon was laid out for use within.  After undergoing several revisions, the M231 Firing Port Weapon was deployed alongside the new Bradley Fighting Vehicle.

The M231

Though it is based off of and visually similar to the M16 series of weapons, there are a number of mechanical differences that set this weapon apart.

First, there is no stock; the close confines of the vehicle passenger compartment don't really allow them.  Prototypes featured a retractable wire stock, but they were removed for the final version as they did not prove resilient enough to the Army's liking, and Command was concerned that they would be used by soldiers inappropriately if they were easy to fire dismounted from the vehicle.

Second, there is a heavily modified barrel arrangement.  In addition to being a 5" shorter, it also features a screw that allows it to interface with the Bradley's firing port ball mount.  This stabilizes the weapon.

Third, the weapon is open bolt and utilizes a three spring system to support a much higher rate of fire than the M16 series:  1,225 rounds per minute.

Fourth, the weapon can only fire in fully automatic, like the M16/M16A1 it was initially based from.

Fifth, it is not designed to handle M885 ball/M886 tracer rounds.  It must utilize the older M193/M196, which performs differently than the newer M88* series.  It is normal for magazines to be filled entirely with tracers.

Sixth, the weapon has no sights, though the prototype did offer flip-up versions; these were deleted for much the same reason as the stock.  "Aiming" is achieved through a combination of tracers and the vehicle's periscope.

Lastly, the weapon cannot mount a sling.  Again, this is an internal firing port weapon, and slings catch on things.  It also discourages the M231 from being used as an infantry weapon.

Because hot brass and the chemistry that makes gunpowder work are not conducive of a safe sealed environment, the weapon utilizes a brass catching bag and venting conduit with fan when in use.

If you're curious to see the insides, here is a video that goes through disassembly of the weapon.  The historical information is a touch inaccurate, but it is very cool to see it taken apart and its mechanism of action talked through.

The Gun in GURPS

The weapon is TL7, though it is used still today.  Damage is taken straight from the M16A1 entry in High Tech p.120, since they will be firing (roughly) the same rounds.  Accuracy is reduced, and both recoil and Minimum Strength increased in accordance with the "Folding Stock" entry on HT160.  There isn't a stock, so it's basically permanently folded, in a sense.

Range represents the maximum effective range.  I searched for a mention for the weapon's maximum possible range (3,200 yards for the M16A1 in High Tech), but could not find it even in the weapon's Operator's Manual from 1997.  I believe that GURPS uses roughly maximum effective range for 1/2D range for the M16 series as is.  The M4 carbine has a comparable real life effective and maximum range to the M16A4 (longer than the M16A1 given in GURPS), so I couldn't just base it off that.  Such is life.

The weapon's Rate of Fire is 1,225/60, or 20 shots per second.  It is fully automatic only, unlike the M16A2 and A4.

The weapon's weight is taken from the Operator's Manual.  Magazine weighs one pound.

Cost is taken directly from HT's pricing on the M16A1; any savings in mass is likely reversed with the weapon's specialized nature.  I also suspect no player is going to be buying this at any market rate!

As noted, the weapon has no sights, sling, or bayonet mount; combined with its lack of stock, it doesn't make a very suitable infantryman's weapon.  Without sights or a stock, I expect that an "Aim" maneuver is either impossible or doesn't net you anything more than a +1.  If GURPS Tactical Shooting's "Sighted Shooting" (p.13) section is anything to go by, it outright removes the "All Out Attack (Determined)" option.

I did leave these stats available even if they're not really part of the weapon's stats when used as intended, just in case someone feels the need to use them as an infantry weapon anyways (with or without the stock - who knows what the brass of your alternative universe approved!).  In the modern day, they often find their way into the hands of other vehicle crews, including the humvee.

Closing

I made this post to work closely with my other one on the Bradley series vehicles.  Feel free to use it as is or as an inspiration for something in your own world, though!  This is a pretty simple mod from an existing weapon, with universally available rules to justify it.  Definitely a credit to GURPS modular flexibility!

Cheers!