Artillery weapons question

[V4ndall]

So, when designing an artillery model several weapons are available. Initially (at Tech 3 start) it goes up to 150mm, later on it's 240mm and eventually 300mm. The question is however, what specifically unlocks those weapons? Is it tech research, developing models or something else entirely? Any help is much appreciated, because I can't figure that one out.

[Clux]

You get bigger calibers available as your tech level increases (seen in empire resume tab)

[V4ndall]

Thanks, it makes sense.

[zgrssd]

I remmeber the first design having a bug, with allowing only smaler callibres. I think I reported it, no idea if it is fixed now.
I tended to start on T4 nowadays, so I always go a artillery model to itterate on.