They put too much much faith in the lengthy cannonade, the largest and longest firing ofcannon the history of the world. The majority of the balls passed over the heads of the union troops on the front line of cemetery Ridge. They landed and exploded into the area behind the front lines were tents and other support facilities were. Why the Confederates didn’t know this is a mystery to me. They had binoculars. So they attacked head on into a line of soldiers that had not been particularly damaged by the cannonade. It was doomed to fail
The artillery bombardment was not the largest or longest in the world. Some of the WWI pre-assault bombardments lasted days and involved thousands of artillery pieces. the largest in the new world, yes it was the largest.
In the spring of 1863, the fuse factory in Richmond blew up.
Confederate Ordnance ordered the arsenal at Selma AL. to ramp up production to supply the ANV with case fuses. The Selma fuses would fly about 200 yards further then the Richmond fuses when cut to the same length. Lee nor Pendelton were not informed of the change in fuse supply.
The Confederate artillerymen would have had a pretty good idea of the range to the Union front line. That range would have been used to set the case fuses for the bombardment.
Once the first volley was fired gun smoke would have obscured the view of Union lines. Almost no wind that day so the gun smoke just stayed close to the ground hiding the view of the target.