This may not be true, but the Confederate bombardment as described in the movie and many discussions said they ran low on ammunition and it would take time to re-arm their cassions.
I find this baffeling. If Lee planned and ordered a bombarment prior to the charge, I would expect planners would move the ammunition to location required to support it.
Is it true the Confederate artillery barrage ended earlier than planned??
Much of what happened at Gettysburg (and in any combat situation) was different from what was planned. Here are some useful extracts that might help answer your questions:
1. Lee's Commander of Artillery, William Pendleton, states in his Official Record concerning the July 3, 1863 assault, that he "found.... many batteries getting out of or low in ammunition, and the all-important question of supply received my earnest attention. Frequent shell endangering the First Corps ordnance train in the convenient locality I had assigned it, it had been removed farther back. This necessitated longer time for refilling caissons. What was worse, the train itself was very limited, so that its stock was soon exhausted, rendering requisite demand upon the reserve train, farther off. The whole amount was thus being rapidly reduced. With our means, to keep up supply at the rate required for such a conflict proved practically impossible. There had to be, therefore, some relaxation of the protracted fire, and some lack of support for the deferred and attempted advance."
Pendleton was responsible for having sufficient munitions on hand. My opinion- based on all that I have read- is that he failed.
2. I've never read that there was a set length of time for the bombardment. Certainly Longstreet told his Chief of Artillery, Edward Porter Alexander, to decide when enough damage had been done to launch the assault.
Alexander wrote two works detailing his wartime experience. The earlier book, written for his family and not intended for publication, is one of the best Civil War books available: Gary W. Anderson (editor), Fighting for the Confederacy: The Personal Recollections of General Edward Porter Alexander (Chapel Hill, NC: The University of North Carolina Press, 1989). As I don't have access to it just now, I'll quote some of his remarks from his second book: Military Memoirs of a Confederate: A Critical Narrative (1907 ) pp. 420-425.
According to Alexander, Longstreet's last order to him was: "Colonel: The intention is to advance the infantry if the artillery has the desired effect of driving the enemy's off, or having other effect such as to warrant us in making the attack. When that moment arrives advise Gen. Pickett and of course advance such artillery as you can use in aiding the attack." He responded, "General: When our fire is at its best, I will advise Gen. Pickett to advance."
Alexander records the start of the bombardment: "It was just 1 P. M. by my watch when the signal guns were fired and the cannonade opened. The enemy replied rather slowly at first, though soon with increasing rapidity. Having determined that
Pickett should charge, I felt impatient to launch him as soon as I could see that our fire was accomplishing anything. I guessed that a half-hour would elapse between my sending him the order and his column reaching close quarters. I dared not presume on using more ammunition than one hour's firing would consume, for we were far from supplies and had already fought for two days. So I determined to send
Pickett the order at the very first favorable sign and not later than after 30 minutes firing."
At 1:25P. M. , Alexander sent a message to Pickett: "General: If you are to advance at all, you must come at once or we will not be able to support you as we ought. But the enemy's fire has not slackened materially and there are still 18 guns firing from the cemetery."
He goes on to say "We frequently withdrew from fighting Federal guns in order to save our ammunition for their infantry."
At 1:40 P. M., noting a "decided falling off in the enemy's fire" and seeing Union guns "limbered up and withdrawn" and
"no fresh guns replaced those withdrawn", he sent another note to Pickett: "For God's sake come quick. The 18 guns have gone. Come quick or my ammunition will not let me support you properly."
He states: Soon only a few scattered Federal guns were in action, and still
Pickett's line had not come forward, though scarcely 300 yards behind my guns." Longstreet, after ordering Pickett forward, joined him. "It was doubtless 1.50 or later, but I did not look at my watch again. I had grown very impatient to see
Pickett, fearing ammunition would run short...."
Alexander continues: "The suspense was brief and was ended by the emergence from the wood behind us of Garnett riding in front of his brigade." He accompanied Garnett forward, then "rode down the line of guns, asking what each gun had left. Many had canister only. These and all having but few shell were ordered to stand fast. Those with a moderate amount of suitable ammunition were ordered to limber up and advance.
During the cannonade the reserve ordnance train had been moved from the position first occupied, and caissons sent to it had not returned. Only about one gun in four could be ordered forward from the centre, but from the right Maj. Haskell took five from Garden's and Flanner's batteries, and Maj. Eshleman... sent four somewhat to
Haskell's left."
Alexander joined the guns going forward from the center. When they got to "a swell of ground just west of the Emmitsburg road" they "opened upon troops advancing to attack the right flank of Pickett's division". Eshleman and Haskell "to the left front of the Peach Orchard soon also opened fire. The charging brigades were now close in front of the Federal lines and the musketry was heavy."
He concludes, "we saw them close in upon the enemy in smoke and dust, and we ceased firing and waited the result."
Hope this helps.