There is no evidence to prove that its empty weight exceeds 18 tons; most media point to 17.7 tons. At the same time, the maximum weapon load is indicated to be 8 tons.
Only a few sources mention that the J-16’s maximum speed is 2 Mach or 2.1 Mach, while most sources still consider its maximum speed to be 2.4 Mach.

简明中国飞机手册




Isn’t the weight in game a base weight and not a true empty weight as it takes into account some amount of fuel, ammo, and pilots ect where as IRL its empty weight is literally the plane with nothing in or on it. So wouldn’t the 600kg be the difference in its fuel and ammo?
You are right, but for fuel, you can choose exactly how much to carry in the selection interface. It is not displayed here, or it shows the minimum fuel. We can deduce the weight from the fuel density, but even so, it is impossible to reach 600 KG unless a pilot weighs 150-180 kilograms.
Ammo is pretty heavy that could weigh almost a 100kg alone so now you are looking at only 500kg in weight for fuel to get the in game weight no? and I don’t think the stat card shows how much fuel it’s carrying for that in game base weight, so that seems pretty realistic to me that it could be housing 500kg in fuel on it if it holds even a thousand gallons of fuel at max weight. But even then its the first dev server and most of this plane is probably just place holders the model doesn’t even look complete really.
Shouldn’t basic ammo include more than just the cannon? I don’t think the basic fuel amount would be 400 kg.