The 40-45 “Smelter” Heavy Field Gun's strength lies in its versatility and being able to attack both vehicles and structures, with the fast reload time allows it to out-damage various tanks. The best way to utilize this push gun is to have someone spot with binoculars every so often to look for targets. If they spot an enemy tank, they should call out to the crew and advance towards it.
Once the tank is near, it's important to keep the range of the weapon in mind, as it outranges most tanks by 5 meters. This allows them to hit the enemy without fear of retaliation. Shooting at max range can easily scare off most tanks as the damage of the cannon is pretty high. If unable to destroy the target, the crew could retreat, depending on given situation is too dangerous for the push gun.
All push guns, by their nature, are one of the most vulnerable vehicles in the game in terms of crew protection. They are exposed to the enemy, making them susceptible to all enemy fire. Due to push guns requiring both crew members to move them, the death of a single crew member could result in the push gun being lost. Due to this, it is recommended to only lock the gun when it looks like enemy infantry are going to overwhelm you and can steal it. This tactic is vital if a push gun is decrewed, as it allows a friendly soldier to quickly man it and drive it back to safety.
Push guns have a visibility mechanic, which allows it to operate differently compared to regular vehicles. Similar to infantry, they are affected by vision, meaning their movements cannot be seen unless it is within your line of sight. Due to this, they can hide behind structures, or even bushes (depends on how big the bush is). It's also affected by the time of day, with nighttime making it possible for push guns to become invisible to the enemy if you're far enough from them. Keep in mind that if the enemy decides to illuminate your position, this will expose it until the flare dies out. The push gun's position will be exposed anyway if the crew decides to fire its main armament.
Prior to Update 1.50 ('Inferno'), the 40-45 “Smelter” Heavy Field Gun used to be called the "68-45 'Smelter' Heavy Field Gun" and fired 68mm shells with a significantly higher HV bonus (with less damage than a 75mm round).
It was later changed to fire 40mm shells, along with its HV bonus drastically reduced.
The 40-45 “Smelter” Heavy Field Gun from the front
The 40-45 “Smelter” Heavy Field Gun from the rear
The updated version of the Smelter introduced in Update 1.50 (officially known as 'Inferno'), which now fires 40mm