Field Gun and 240mm Artillery have very low gun depression (10 degree) which makes them fully unusable on the defense (unless you somehow aim from 100 blocks away). They were originally meant to be used for offense since they can be used in wild, however I do feel like it would be interesting to see a bit more variety for defensive cannons as well. They are quite more chonkier than the regular 57mm cannon which would require more space to use (if you se them in turrets the turret now becomes quite larger and easier to hit). Their current benefit is having more range (200/300 compared to 150 blocks of the 57mm cannon) which is nice but unless its a stationary target that benefit is hardly used. I know that it might not be fully "realistic" but neither is the way warfare is done on the server so it should be adapted to that instead. Giving each cannon a different purpose and playstyle and size would also make it interesting. For example:

57mm Artillery - Close range, small, fast reload, low block damage
Field Gun - Anti personel (canister, shrapnel, smoke, incendiary), medium reload, medium range
240mm Artillery - Long range, slow reload, higher block damage

Range refers to projectile velocity. Higher range cannons should still be able to land hits on closer ranges, but be less efficient at doing so than the lower range cannons