Of the 630 guns at Gettysburg, (360 Union and 270 Confederate) There were approximately 120 Union guns lined up along Cemetery Ridge from Culp's Hill to Little Round Top and about 150 Confederate ones along a 2 mile line on Seminary Ridge with the Confederate fire being largely ineffectual due to overshooting. The Union would have started with solid shot, switched to explosive shell/case shot at about 1/2 way across the field and then to canister (single and then double) at about 500 feet from the Union lines. Half of the guns present (both sides) were rifled and half were smoothbore. Usually 2 to 3 rounds could be expected to be fired in a minute and as far as number of guns engaged, it was by far the largest artillery engagement of the War. With he right weather conditions, no doubt that the sounds of the barrage could be heard for a long distance, though I'm not sure about DC, but I have heard accounts of it being held in Philadelphia however.