Been picking at this one for a while over the past couple of months and finally got to the point where I can say it's complete. I give you the GMX-399-12 Gundam Rhongomiant, Missile Configuration! I'll eventually knock out what I had originally intended to be its base form, which is a long range sniper variant.

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Model Number: GMX-399-12/M
Code Name: Rhongomiant Missile
Unit Type: Gundam machine experimental long range engagement unit
Manufacturer: Commonwealth Space Forces
Operator: Commonwealth Space Forces
Accommodations: Single-seat integrated support artificial intelligence user interface cockpit in mid-torso
Armor material: Truesteel plating on gundamite frame
Dimensions: Head height 20.2 meters
Weight: empty weight 74.3 metric tons
Powerplant: 2 x Ultra-compact g-reaction cores, located on back and in chest
Propulsion: Photon thrust emitter x many, located across body
Equipment and design features: Modular body frame; EM shielding; ether generators x 2; external weapons latches; "Orna" long-range photonic sensor and deflector
Fixed armaments: 2 x 100mm double-barrel ether anti-air CIWS, mounted to torso; 2 x ether cutter, located in hands; 2 x 4-tube medium missile launcher, located in knees; 4 x 9-tube micro missile launcher, located in legs; 2 x 4-tube medium missile launcher, located in side skirts; 2 x 4-tube medium missile launcher, located on arms; 2 x 28-tube micro missile launcher; 2 x 25-tube micro missile launcher; 2 x 3-tube medium missile launcher; 2 x 3-tube large missile launcher, each missile containing 15 micro missiles
Optional armaments: 2 x XML-Y399-MML.1r12 3-partition micro-missile launcher; 2 x MBS-M399-BST.4r9 type-4 ether saber, stored on skirt armor; 1 x XSUY399-ABST.1r9 ether pile shield; 1 x XML-Y399-MR.1r1 penetrating photon missile rifle

Technical and Historical Notes:

The Rhongomiant is the twelfth and final Gundam-class weapon manufactured by the Commonwealth Space Forces, depleting their limited supply of usable gundamite. The Rhongomiant is built on the same type of frame used by the GMX-397-05 Durandal, however the frame has seen several improvement in regards to its weight ratio, allowing the Rhongomiant to be equipped with more dense Truesteel plating without impacting the unit's baseline performance too much. The Rhongomiant is considerably heavy for a unit of its size, which makes its overall mobility and top speed lower than that of the Durandal; however it was built with long range engagement in mind rather than engaging at close range with enemy units, and its optional configurations tend to reflect long range combat. Several design elements of the Rhongomiant would be borrowed and refined in the development of the Gladius-M, the mass production mobile suit built off of the Durandal frame concept.

As a prototype unit, the Rhongomiant was designed to test long range weapons technology. It equips the photonic sensor and deflectors used by the destroyed GMX-397-06 Gundam Orna, allowing it to locate targets using hard light, or ether, infrastructure and target them from outside of their effective range. To this end it is the only mobile suit to employ an effectively downsized beam weapon, derived from technology tested in the XHB-Y397-11-BZT.3r3 beam bazooka, in order to shoot down enemies defending themselves with powerful photonic shielding from afar. It is also a unit that has successfully tested and refined the photonic missile; a missile with a high-yield explosive payload that utilizes photons for propulsion and maneuvering, allowing it to strike targets from multiple angles with extreme precision when paired with the Orna's photonic sensor.

This particular configuration is the most used, and referred to as the Rhongomiant Missile. It is a ballistic support and suppression configuration, loaded to the brim with medium and micro-sized missiles. It can fire medium-sized missiles at long range to hammer enemy ships, while a payload of over one hundred micro missiles allows it to suppress enemy units that may get too close. For those that manage to bypass its missile curtain, it has a pair of ether cutters built into its hands, as well as beam sabers mounted to its skirt armor. While costly due to the sheer numbers of the missiles it can fire, it has proven highly effective in supporting squadrons of Gladius-M mobile suits by taking out enemy carriers and smaller interceptor units that have difficulty avoiding the smaller missiles. It is currently one of two Gundam units still in active duty with the Commonwealth, the second being the Durandal, which only recently became active.