Indeed. As Ryan points out, the supply of ammunition was a limiting factor......, even if the position had been adequate to "flatten the hills."
Edward Porter Alexander wrote back to Longstreet concerning the cannonade planned to precede the assault on the center:
"If, as I infer from your note, there is any alternative to this attack, it should be carefully considered before opening our fire, for it will take all the artillery ammunition we have left to test this one thoroughly, and, if the result is unfavorable, we will have none left for another effort."
Then after the cannonade was underway, says E P Alexander: "At the end of 20 minutes no favorable development had occurred. More guns had been added to the Federal line than at the beginning, and its whole length, about two miles was blazing like a volcano. It seemed madness to order a column in the middle of a hot July day to undertake an advance of three-fourths of a mile over open ground against the centre of that line. But something had to be done. I wrote the following note and despatched[sic] it to Pickett at 1.25:
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."
At 1:40 another note was sent:
For God's sake come quick. The 18 guns have gone. Come quick or my ammunition will not let me support you properly.
Two more verbal messages followed.
E P Alexander continues: "I had grown very impatient to see Pickett,
fearing ammunition would run short, when Longstreet joined me. I explained the situation. He spoke sharply, "Go and stop Pickett where he is and replenish your ammunition." I answered, "We cant do that, sir.
The train has but little. It would take an hour to distribute it, and meanwhile, the enemy would improve the time."
Edward Porter Alexander, Military Memoirs..., 1907, page 422. https://archive.org/stream/militarymemoirso00alex#page/422/mode/2up