About gun research, I suggest 2 tech trees branches :
A- One which unlocks new calibers, bigger and bigger.
B- One which improves one (and only ONE) of the calibers you already "own"
For example, if you unlocked 10inch guns with tech tree A, you can select this caliber in tech tree B and get progressively better and better 10 inchers (including the possibility to make them longer and longer, and more options to make them larger OR smaller). This way, when you unlock a new caliber, you can't immediately make it ultra-powerful, and you have to choose the right moment to start evolving a brand new caliber.