The problem with all of these concepts are:
1 ) It's a built-in appliance. No mobility. So from the outset you better put it where you want it! Of course, in wartime you always have many months to determine where you want to put a cannon round, right? And if the situation changes? Well just walk away from it, cause you sure ain't taking it with you when you go!
2 ) How do you aim the thing? It's built right into the ground! So from the outset you better not only put it where you want it, also point point it where you want it to shoot!
3 ) How do you hide it? Once it starts firing, it's kind of obvious. And while your pesky enemies might not have been foresighted enough to built a similar gun pointing right at yours, they probably have invested in more flexible munition-delivery systems that can be re-directed your way within a matter of days, if not hours.
The advantage of a cannon vs. missile or bomber is that each round costs you less to deliver. But you really have to have a scenario where the amount of payload you deliver makes sense vs. the cost of the weapon! This is what the Germans found out with their 60cm Karl mortar and their 80cm Dora RR gun. The costs of the systems were way out of line with their utility, given all the other systems for delivering munitions onto the enemy. And those systems were MANY TIMES more flexible than a built-in, buried-in-the-ground cannon.
It sure is a lot of time, expense, manpower and developmental mindshare to put into a system that delivers a cannon shell. I mean .. less than one areal bomb (much less a bomber's whole payload)? Less than 1 missile warhead? But many multiples of the cost? And a minute fraction of the operational flexibility? Sure, sounds like a winning idea to me!
-Mark
(aka: Mk 1)
wonders: Can Fernando do 1/285th?
