Here's the standard configuration for the GMX-399-12 Gundam Rhongomiant that should've come first. I particularly like how this rifle turned out. Big, bulky, larger than the mobile suit, but still stylish.

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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; 2 x ether generators; external weapons latches; "Orna" long-range photonic sensor and deflector; Photonic defense curtain
Fixed armaments: 2 x 100mm double-barrel ether anti-air CIWS, mounted to torso; 2 x ether cutter, located in hands
Optional armaments: 2 x MBS-M399-BST.4r9 type-4 ether saber, stored on skirt armor; 1 x XSUY399-ABST.1r9 ether pile shield; 1 x XBR-Y399-12-BRT.1r7 High-output long-range beam sniper 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. However, these missile weapons have been relegated to a secondary configuration due to their high-yield payload.

The standard configuration of the Rhongomiant is not used as often as the missile configuration, but is still devastating in terms of its power output compared to other mass production mobile suits. The XBR-Y399-12-BRT.1r7 High-output long-range beam sniper rifle, while larger than any ether-based weapon due to the technology involved, it is capable of firing a high-energy, high-output particle beam from extremely long ranges that is capable of penetrating photonic defenses. Though powerful the weapon has a limit; it must be clip-fed a specially designed generator to supply power to the rifle, and each generator can only support the firing of three shots before they are spent. It carries an additional two generators, giving the rifle nine shots before it runs out of ammunition. It also has a slower firing interval than standard ether weaponry, needing a few seconds between each shot for the rifle to cool down. The Rhongomiant can snipe safely from the rear line to support allied troops engaging up close and personal with enemy squadrons, thanks to the Orna's photonic sensor giving it unparalleled targeting capabilities.

Rhongomiant also features a similar defensive mechanism to enemy units; a photonic defense curtain. As enemy units primarily utilize ether/hardlight weaponry, such a defense has proven highly effective, though highly expensive to produce, due to the number of ether generators involved. While only capable of being produced in limited quantities, the photonic defense curtain equipped on the Rhongomiant's shoulders generates an almost cloth-like photonic field that surrounds the sides of the unit like a cloak, capable of dispersing enemy attacks with relative ease. Due to its nature of being composed of photons, scientists and engineers are also studying further improvements, such as making an optical camouflage cloak, that will fully encase and bend light around the unit, making it appear invisible. Early trials however have proven difficult to implement proper.