The U.S. fielded the most capable artillery in the world, during WWII. In all variations, the weapons were excellent. U.S. ‘Arty’ was mobile, unlike the Germans and Russians, at all divisional levels. The fire control system, where artillery forward observers operated close to the infantry, used powerful radios and telephone via land-lines. The redundant signals system meant that, even when all other contact with front-line units malfunctioned, artillery communications usually worked. Superior also was the U.S. artillery fire-direction system with swift engagements of targets via coordination firing from many ‘arty’ units from many widely separated firing positions. This included the deadly ‘Time on Target’ were shells arrived on target at nearly the same from different and widely dispersed units. Use of the top secret proximity fuse, employed for anti-aircraft fire beforehand and now massed produced, was deployed first in December 1944. This proximity fuse triggered a explosion at a preset distance and proved deadly against infantry and light armor.
Source: THE UNITED STATES ARMY IN WORLD WAR II: Author Rich Anderson -Militaryhistory Online
The artillery demands at Simserhof, the reduction of a dense fixed target, did not play to these U.S. 'Arty' best in class capabilities. The fortress was immobile, a static target. The U.S. had no siege artillery. Only the German army, despite the huge cost and niche use, still had siege cannon, which were originally designed to destroy Maginot forts in 1940. Lacking ‘one punch’ knock out capabilities, the original tactic at Simserhof was to drive into the open the defenders by the concussions of the heavy artillery. Then, once out , destroy them with the lighter artillery. This failed. Simserhof was impervious to general bombardment. The approach changed to utilize the sheer mass and volume of U.S. artillery and to smash, bludgeon and destroy the fort’s and bunkers cannons. Only after the Fortress' heavy guns, the dual mounted 75s and 133mm retractable cannons, were destroyed by artillery and tank destroyers units, did the ground assault proceed. U.S. artillery provided lavish support for the attack despite a time of severe artillery shell shortages. Artillery units were organized into a "Parks Group" and consisted of the 156th Artillery Regiment, the 242nd attached and the 157th, 182nd and 272nd reinforcing. The cannons employed started with the 75's mounted on Sherman Tanks, 105's, 155 "Long Toms", 8 inch howitzers, 4.5 inch heavy cannon and 240mmhowitzers 'on loan' from XV Corp Artillery had little effect. The defense was subjected to 78 P-47 fighter-bomber sorties using 500lb bombs, which "bounced off" Simserhoff. The infantry would, as always, do the bleeding to capture and reduce the fort.
Dora, the largest gun ever built, was designed to destroy Maginot Forts. Not ready for action in 1940, Dora was employed by the Germans in Crimea on Fort Molakov with great success. It took 25 trainloads of equipment, 2000 men and up to six weeks to assemble. Dora: Worlds Largest Cannon