The Ultra series of combat vehicles was an early Ajax creation that arose from one of the most ancient and primal urges of mankind; looking at someone else using a really big fucking gun and saying, "Imma get me some of that". They are often held up in engineering classes as demonstrations of the power of envy and metric assloads (to use the scientific notation) of alcohol.
In their efforts to catch up to their new Terran neighbors in an adamant refusal to remain the weak side of relations, assorted Ajax Industries employees took notes from the artillery cannons mounted on their few Thumper Combat Vehicles and attempted to reverse-engineer a new gun with a larger bore. Their attempts were spurred on by the Raider known as Tychus Findlay, who having somehow caught wind of this proceeded to make crass jokes about "penis envy" where said employees could here him. A few arguments and design iterations later (plus one discreet drunken brawl), the firing range they were using to test their first model was rocked by an explosion one quiet night as the prospective gun obliterated the breech and sent the remains of the barrel flying high into the air, only to land in a near-comical upright position with a loud clang.
Some smartass looked at it and said "THE LORD HAS SPOKEN! THE GUN MUST BE SNUB-NOSED!" Thus was born the Ravager (Long Tom Model) Cannon (after a scathing round of reprimands, punishments, and more research), a 180mm field gun which was promptly shoehorned into anything that could fit it.
The Hetzer variant was a comparatively simple modification, needing only an engine swap, a bit less armor, and a modified gun mounting to carry it. While its ammunition load is comparatively small, the longer range of the gun compared to the old AC20 makes it arguably more useful as as an ambusher, and the increased need for resupply ironically improves survivability by making it more likely the vehicle withdraws before the armor is penetrated. If more battlefield endurance is really needed, the trailer hitch allows the Hetzer to tow its own ammunition trailer, as well as whatever else can fit on the hitch. The high gun elevation, up to 78 degrees vertical, also allows it to fill a role of short-range artillery similar to the Light Mortar Carrier. However, this increased elevation and the necessary gun mounting requires a set of "horn" rails that extend above the nominal roof of the Hetzer. While these rails are heavily armored and very difficult targets, crew members must be aware that the higher effective ceiling can give them away in certain circumstances, as well as rendering them ineligible for the vast majority of low-ceiling drive-throughs.
The Merkava variant requires more work, as the original turret is not at all reinforced for the necessary recoil shocks. Eventually, the redesign crew gave up and developed an entirely new turret design (which ended up being somewhat pyramidal in shape) incorporating a series of four pneumatic rams to brace the gun. Fortunately, the space in the old turret meant that the turret ring was large enough to accommodate the new one, and the assorted ammunition bays needed only minor modification. Secondary and anti-infantry fire is provided by an SRM6 with two tons of ammunition, allowing for the use of both anti-armor and anti-personnel warheads in the same combat load. Somewhat unexpectedly, all of these combined changes actually freed up two tons worth of mass - it was eventually decided that more armor was the best use of this. The reduced number of weapons and robust construction of the turret also makes the tank overall easier to repair than its predecessor.
The last of the trio, the oddly-named "Dropbear Ultra" is a testament to that special brand of lunacy best characterized as "So Stupid It Works". It is based on the design of the "Double H", a homegrown series of 60-ton freight-hauling VTOLS used for carrying large loads across the many natural obstructions of Dumassas - especially the assorted ruins and radioactive zones, which present a threat to even prepared travelers. Through a series of miscommunications and lack of oversight too ridiculous to adequately explain, the refit began with removing the internal cargo bay, lift hoists, and part of the extended fuel tank to free up space. All this mass was replaced with a chin turret carrying the cannon and three tons of ammunition, as well as substantially reinforcing and up-armoring the chassis, such that any given section could be expected to take an AC20 shot and remain unbreached (save the rotors of course). The cockpit was expanded to include a dedicated gunner's seat, and a modified recon camera was fitted above the gun. The remaining tonnage was filled with communications equipment and a sensor mast extension; finally, the combat computer is a custom job enabling its crew to consult a selection of range tables and adjust the cannon's orientation on the fly to accommodate "any and every possible atmospheric condition" (a dubious claim at best, but with more backing than most such boasts).
That all this sensory and communications gear makes the Dropbear a well-equipped C vehicle is, rather frustratingly, considered by many to be of secondary importance to the novelty of a flying artillery piece, dropping shells downrange from kilometers away.
This trio of vehicles is rarely called "Ultra" by Dumassans, instead being referred to as the "Drunken Triplets", "Big Drunks" or "D3". This is a reference to the fact that all of the ICE engines used by these vehicles are homegrown Dumassan variants that run on alcohol, which is one of the primary exports of the locals.