Please provide some facts and figures in the answer.
American artillery was easily the most effective artillery and the most effective weapon of WW2. By "effective" I mean "in terms of killing the enemy".
The Germans scorned American armor and infantry, but they deeply respected American artillery (and air assets) .
Pound for pound, the American proximity fused shell was perhaps the single deadliest weapon of WW2. Not only was it used very effectively to shoot down small, 350 mph V-1 Buzz Bombs sent against London and Jap kamikazes, in N. France it was used with murderous effect against German personnel (the Germans called US prox fused shells "manhunters).
American artillery was superbly crafted. The U.S. had plenty of excellent quality steel. American artillery tubes could be easily replaced when they wore out (not so the Germans or Japs).
Americans deployed many HEAVY and destructive 155mm and 8 inch guns, plus an embarrassing abundance of fast firing and hard hitting 105s. A 155mm shell weighs sixty pounds and packs about 5 kg of TNT; and 8 incher shell weighed 200 pounds and was charged with 15 kg of HE. An 8 inch HE shell had a kill radius of 200 meters. An experienced American 8 inch battery of six tubes could easily send out a ton of shells per minute, accurately, and do so for hundreds of shells before having to stop. I have seen 8 inch howitzers in action during a Marine Corps Reserve exercise. The shell strike is truly awesome - it creates a thick dust cloud a quarter mile wide.
Americans had plenty of motor vehicles and mobilized their artillery better than any other army. The American 4.2 inch rifled mortar could fire six 22 pound shells ( including white phosphorus) per minute accurately out to a range of several miles. The Japanese improvised jungle fortifications were especially vulnerable to white phosphous and they were terrified of American WP. White phosphorus cannot be extinguished ( it burns under water). A dime sized chunk of WP will kill a soldier. First aid for getting hit by a dime size chunk of WP is to use the tip of a knife and try to flick it off. American 4.2 WP shells used in a European town would have created a Hell on Earth.
There was usually (but by no means always) sufficient ammo for American artillery. The Americans manufactured more artillery rockets than any other combatant ( the American 5" rocket was especially high quality). American artillery ammo was manufactured to a high degree of reliability.
Americans did in fact occasionally run out of artillery ammunition during the drive toward Germany, and when they did, invariably the American offensive stopped in its tracks. Americans would become demoralised without artillery support.
American artillerymen were specially selected for intelligence and they were well trained. American artillery fire control was the fastest and most accurate. To direct artillery fire, Americans used an odd system of (accurate) maps (made using photo reconnaisance) and a file cabinet full of tape measures (calibrated for temperature and wind speed and direction). The Germans relied on maps made by cartographic surveyors - that means they had to have surveyors on the ground before they could use indirect fire. The British (wastefully) simply called down every artillery unit on a divisional front onto a map coordinate.
The Japs and Russians could not generally use precise indirect artillery fire. They could not use indirect artillery because they had so few good maps, so few trained forward observers, and so few good radios. Unless they had a working telephone line and pre-registered artillery (easily accomplished on fortified islands like Okinawa) they had to rely on direct fire artillery. The crews of direct firing artillery are very vulnerable to tanks, mortars, and counter-battery fire. American artillery was especially deadly when directed from spotter planes.
American artillery caused 50% of all German military casualties in Northern Europe (Allied air attacks caused 30%). German artillery caused about 30% of Allied casualties in N. Europe (likewise German machineguns).
German field artillery was of very high quality and very well handled, however it was impaired by the needs of German AAA committed to defending the Reich. A full 25% of ALL GERMAN STEEL PRODUCTION went to AAA shells. German AAA occupied 2 million personnel during WW2 (including many adolescents).
The German AAA defense of the Reich was, in fact, effective. Allied bomber losses were high and prohibitively demoralizing to Allied bomber crew. The German air defense of Berlin should fairly be credited as a German victory against Bomber Command and the 8th Air Force (although the British and American histories commonly refuse the Germans that honor). On average, the Germans needed to fire about ten tons of AAA shells (early war) and twenty tons of AAA shells (late war) to shoot down an Allied bomber. Including the wear and tear on the German AAA guns, such exhorbitant expenditure of AAA ammo still worked very much to the advantage of the Germans (including crew, most American aircraft were far more costly than 2,000 88mm AAA shells), and thus it was a strategically effective use of limited resources.
However, the air defense of the Reich severely restricted German artillery resources, especially on the Russian Front. Deficient quantities of German field artillery caused the Germans to incur high casualties when obliged to substitute infantry instead. If the enemy holed up in a town refused to surrender, the Germans had to attack with infantry; the Americans, however, could (and would) just stand off and flatten the town with artillery.
By late 1944 and 1945, the Russians had huge artillery resources and used it to blindly drench suspected German defensive positions with massive fire. The technique was often very effective with several notable exceptions. During the Soviet approach to Berlin, on several occasions the Germans correctly anticipated Russian intentions and simply vacated their lines just before massive (but useless) Russian artillery attacks.
American naval artillery was fantastic too.