First off, kudos on some interesting turrets that aren't just "more damage, bigger range". Things like the electron beam continuing through targets and slowing them is a lot of fun. It would be interesting to have a lower tech/low damage gun that just did the slow. (also having turrets with longer range but dead zone is interesting)

I'm unable to figure out how to make the Heavy Howitzer fire. I found the ammo and filled the requester chest, but the blinking 'no ammo' sign is replaced by a circle with a red cross (presumably no target?). I can see bases on the map within range of the howitzer (according to the red overlay in the map screen).

I'm playing with a Yuoki Modded run, so maybe there is a conflict there. I don't see any issues in the console or in log files.
I'm happy to do some debugging, but I'm really not sure where to start. I just ran a check for updates, and I'm currently running Factorio 0.15.23

I don't think that the howitzer is working. Mine will fire but I haven't noticed a change in the spawner landscape. Additionally, the "kills" and "damage dealt" on the howitzer remain at 0. I tried using a marking capsule on a nearby spawner, and it did make a red X, which eventually faded, but the howitzer never fired at that spot.

Okay to help a little, if your firing the normal howitzer, you place then basic ammo into the chest, then power it, if the X comes up then there isn't any target in range, the Kill and Damage dealt counter will never go up for some reason.

again same thing with the player targeted one, has to be in range, has to have ammo and power, if the marked target is out of range then the marker will eventually fade.

I've looked at the code and the turret only mark enemy bases on Radar check events.
So only if the enemy base is inside the art range and the radar scans that exact position.

Well yes, it does have to be in "LoS", I'm sure I read that somewhere, not entirely sure, as for the player targeted one, it only has to be in range of the Exp. How. for me.