Castenea wrote:Bigger is better, up to a point. That point is determined by your ability to move said guns around. Water borne guns are often larger than guns on land due to issues of moving guns on land. Germany built the largest land based gun ever used, it was used in one campaign, the siege of Sevastopol, due to the trouble of moving that monstrosity.
Bigger is better, if you can move AND FIRE IT.
The larger guns, 6" or bigger, would have several problems. Without motorized transport they were transported in pieces then assembled on site. They also would most likely be restricted as to firing locations as they tended to be finicky about what ground they could fire from. When motor transport comes into the picture, a lot of the problems are reduced but are still there.
An example of animal drawn artillery is the American Civil War which had 20" Rodman guns but the field guns tended to the 4.62" Napoleon smoothbore and 3" Parrott rifles. Field howitzers got up to 6.4"
The largest SPG was the German Karl which was mounted on a 36.5'x10.33" tracked chassis and was strictly a siege gun. The gun you mentioned was the 80cm Gustav, took 25 freight railcars to carry it, took 250 men 3 days to assemble it, took 2,500 men to lay the double rail line for it and had 2 flak battalions to guard it. All for one round every 30-45 minutes.
As for naval guns, with gunpowder either black or brown, sizing up your tube doesn't gain very much in range. Also any vessel big enough to carry a modern gun is too big to navigate most rivers so they are stuck with coastal missions. Lastly with big guns at long range you will have problems with defilade fire.
All in all, big guns are possible but not likely to be worth making for this war if medium field artillery is available.
and, Mangy Fur the Smart Alick Spacecat.