Meade as well as Hunt, continually expressed concern during the battle about the waste of ammunition, whether by his infantry or artillery. The initial reason with regard to the artillery was a spot shortage, which Hunt described: Early on 2 July, Meade informed me that one army corps had left its whole artillery ammunition train behind it, and that others were also deficient, notwithstanding his orders on that subject. He was very much disturbed there would not be sufficient ammunition to carry us through the battle. I had on my own responsibility formed a special ammunition column attached to the Artillery Reserve carrying 20 rounds per gun, over and above the authorized amount. (Hunt, Battles and Leaders)
The situation was considerably alleviated by the morning of July 3: Lt. Gillett, ordnance officer of this command, was engaged the entire night of 2 July in issuing ammunition to the batteries of the several corps, as well as those of the Artillery Reserve. Seventy wagons were unloaded, which were sent to the rear on the morning of 3 July. (Robert Tyler, Official Report, Artillery Reserve)
On July 3, Hancock actually ignored Hunt and had his batteries fire away most of their long-range ammunition (he tried to get other batteries like those under McGilvery to do likewise, but met with only limited success). When the Confederate infantry advanced, the guns of Cushing and Arnold were silent. They simply loaded their short-range canister and waited. Woodruff in Ziegler's Grove had just a little long-range ammunition left and then his guns fell silent for the time being as well. Fortunately for the Federal cause, there were many more artillery batteries off either flank that had long-range ammunition, which they used very effectively for 10-15 minutes upon the advancing Confederates, while Hancock's guns stood idle.
Federal artillery ammunition was not unlimited, particularly for specific types of ammunition that fit specific cannon. Lt. William Wheeler of the 13th New York battery checked the Artillery Reserve toward the close of the fight on July 3 and found very little if any 3-inch ammunition left for his 3-inch Rifles. There were 22 such Rifle batteries in the Army of the Potomac at Gettysburg. When Wheeler took his battery back to Cemetery Hill late on July 3, Meade was there and asked him if he had a full supply of ammunition. Informed that only a limited supply could be found at the Artillery Reserve trains (upon request by his Eleventh Corps artillery chief, Major Osborne), Meade exploded, saying: "You must have ammunition; the country can’t wait for Major Osborne or any other man. Go immediately to the Artillery Reserve and order General Tyler to send up a wagon load." Wheeler made a show of going and ducked out of sight, not wishing to face Meade's wrath, but he had just come from the Artillery Reserve and knew the situation there. [Letter from William Wheeler, 13 NY Battery]
More ammunition was available at Westminster, Maryland, but that took a considerable amount of time to retrieve at the pace of an ammunition wagon, and the distance would only increase as Meade pursued Lee westward.