In the start of World War II in China, the constantly retreating Socialists were being swamped by the seemingly endless waves of War Sage Tanks. The Socialists did not have enough resources to produce enough of their own tanks to counter them and so turned to the Soviet Union for Help. After promising to help destroy the Ultimist hordes after they had unified China, the Soviet Union began shipping over surplus field pieces to help their cause. These so called "130mm Pieces" were very cheap to make and could destroy a War Sage with a single well aimed hit. And the extremely long range of the 130mm meant that it could also function as a potent artillery piece that could be organic to the Infantry, bombarding infantry, vehicles, and structures all alike. And it had some of the greatest range of any artillery piece, being able to reach over 27 kilometers allowed it to outrange the 15cm guns of the Blue Chinese Siege Cannons even if they didn't pack as much of a punch; letting Red Chinese defensive positions decimate attempted breakthroughs and was highly useful in stopping the Blue Chinese, IPTO, and Indian advance under the weight of fire.
When the most of the world had settled back into peace after the death of Mosley, these rifled 130mms were losing their luster compared to the new 125mm smoothbores. Due to its cheapness and reliability it was given a refit with new armour piercing sabot rounds. The old anti tank guns soon proved their worth, smashing wave after wave of War Sages and the vehicles of the IPTO, India, and other south Asian and pacific members of the Alliance as well as what forces the Alliance loaned in from Africa and Europe; to say nothing of the flimsy vehicles of the Syndicate. The 130mms proved more than able to penetrate the armour of anything the Blue Chinese and their support threw at them. A few anti tank guns, towed into a good ambush position by tractors, can decimate a battalion of armour. When serving as artillery, their tremendous range meant that virtually no attempt to attack them would be able to reach them without first sucking up a large number of shells to the face.
In one famous occasion, a small group of anti tank guns stalled a Blue Chinese armoured advance for over an hour before they ran out of ammunition, and reportedly managed to destroy one of the dreaded Emperor tanks. The crews of these defensive guns were known for their patriotism and dedication to the Socialist Federation of China, where they would stay behind to man the guns even as they were swamped by Blue Chinese soldiers. To serve as crew for the 130mm pieces was an honour, and reportedly many wept when the Red Chinese created an autoloader for the 130mm that worked faster than any human crew could without increasing the weight; as it meant that fewer would be able to serve as part of 130mm gun crews, though to compensate Jingwei personally ordered the construction of many, many more 130mms out of spare war factories to give the loaders guns to serve as gunners, spotters, and guards.
As the war dragged on and the Red Chinese dug in further into their defensive lines, the 130mms were relocated less frequently, until by the end of the war most 130mm Guns just stayed in their positions, holding off wave after wave of Blue Chinese armour, day after day, hour after hour. They were among the first to be turned into Cyborgs, so that they could continue to operate the guns with as little issue as possible no matter how little sleep they got. As the years went on, the crews of the 130mms became as cold and mechanical as their own weapons. This also let them push or pull around their own guns quite quickly without needing to wait for tractors or towing vehicles; which was fortunate as more and more of these now mostly carried ammunition rather than guns that hardly needed to go from place to place anymore as all the finesse and maneuver of war was drained away and replaced by simple calculus of numbers.
After the Bombs
After the bombs, warlords, formerly officers of the National Revolutionary Army, began to try and establish their own little fiefdoms in the tattered, radiation scarred wastelands of China. Though many hoped to rebuild, reunify, and revive the Socialist Federation, many more simply slapped on a Socialist coating over newfound powermongering beliefs. Before long, they realised they would need defences to protect their territory from the banditry that roamed the wastelands, as well as other competing warlords and the forces of the Atomic Dynasty, the Heart Washers the Indians, the IPTO, the Dominion, the Empire, and others who sought to pick a fight in the hell that was once China. Having limited resources with which to make new weapons with, they had to suffice for what equipment they could salvage.
In the aftermath of the atomic holocaust, potentially millions of 130mm guns had survived, since they had been out in the field rather than near the cities which had been target to the atomic bombs, still in the same places they had dug in to hold off the waves of Nationalist chinese aggressors. Though many of their crews had either died of radiation poisoning or abandoned their guns for safe shelter, these guns remained, forming a defensive buttress against an enemy that had long been reduced to radioactive ash, slowly rusting away. Others had become Cyborgs who gave little notice to the radiation, being able to operate in some of the most hellish conditions of Earth as if it were just another day at the park. These crew were found by the warlords and revivalists and soon put to work at the guns once more, or if needed; new crews were trained up and attached to the guns.
It was easy enough to form salvage teams to go and retrieve these guns, sending them to dig the guns out of their entrenched positions and haul them back. With vast quantities of ammunition stockpiled in Red Chinese Command bunkers and new munitions coming from the allies of Socialist China as they sought to recover what they could from the ruins and save the corpse of Jingwei's China, they could be supplied easily enough. They often had to be transported by human (or in some cases, animal power, mainly the mutated, cadaverous cows prevalent in China after most of the normal cows died out) since a great number of the vehicles that would have done so normally had either broken down completely in the wake of the atomic exchange, or otherwise had their engines stripped and retrofitted onto recovered Forge tanks.
With this, the 130mms still dig in against the forces of the enemy, holding back wave after wave of tanks. However, where they once did so against the forces of Blue China, and where their crews were once patriotic countrymen, they now destroy anything deemed to be a threat by the survivalist warlords of a post-apocalyptic China, and their crews are desperate, worked to the bone survivors of a nuclear armageddon or machine men who can scarcely be said to care about anything any longer. The firepower these guns provide is invaluable and most Red Chinese commanders would consider themselves practically naked if their armies did not have at least a few dozen at their disposal to crush any attempted enemy under their mighty shells; for many foes still need to be smashed with five inch rounds at long range.