530th Field Artillery in Italy

Nigel is indeed correct. A Battery was in the II Corps. The remainder was attached to the 92nd Division. Coincidentally, my father's unit, the 473rd Infantry, was also attached to the 92nd. Weird, huh?

530th Field Artillery Battalion [att] [-1bty] (the one battery, A, was in the II Corps)
Order of Battle • 92nd Infantry Division

 

I'm certainly no expert on weapons of WW2, but it seems there were several types of field artillery pieces. Here is an excerpt that may shed some light for you.
Divisional pieces included the M1 105mm howitzer and the M1 155mm howitzer. Both were excellent weapons, with good range and, particularly in the case of the 155mm, excellent accuracy. Other new weapons were the M1 75mm pack howitzer and the M3 105mm howitzer. Both were lightweight and could be easily broken down into manageable loads suitable for transportation by pack animal (horse, mule, or man as available) or by air, and if relatively short-ranged, were ideal for airborne forces. The M3 also saw service after 1943 in the Cannon Company of the infantry regiment. A SP version of the M1 105mm, the M7 Priest, also equipped the field artillery battalions of the armor division.

Non-divisional artillery pieces included battalions equipped with these same weapons, as well as other, heavier pieces. A companion of the 155mm howitzer was the 4.5" gun (an indigenous 120mm gun was one of the few failures of the inter-war design projects). The tube of this gun was of British design, while the carriage was that of the 155mm howitzer (carriage commonality between companion guns and howitzers was one of the hallmarks of U.S. artillery designs). Unfortunately, the 4.5" -- although well liked by American artillerymen - was not a very efficient weapon for its size. The shell (also of British design) was of low-grade steel, thick-walled and with a small bursting charge compared to the shell weight. The 4.5" projectile weighed 54.90 pounds, but had only a 4.49 pound bursting charge, while the 105mm howitzer projectile weighed 33 pounds, but had a 4.8 pound bursting charge. Its range was insufficient to compensate for the relative ineffectiveness of this round and as a result it was withdrawn from service soon after the end of the war.

A much more effective weapon was the M1 155mm gun, known as a "Long Tom" (an appellation with a long and glorious tradition in the U.S. artillery.) It combined long range, accuracy, and hitting power with a well designed, mobile carriage.

A different 155mm gun was the M12 SP. Developed in 1942, it was an interesting amalgam of the old and the new, utilizing the tube of the pre-war French designed GPF (Grand Puissance, Failloux), itself developed in World War II, and the chassis of the obsolescent M3 Grant tank. It was an experiment by the Ordnance Department that had been turned down by the AGF in October 1943 on the grounds that there was no requirement for it. However, in early 1944 urgent requests from U.S. Army forces in England for a heavy SP gun resulted in 74 being rebuilt. They eventually equipped seven field artillery battalions in the ETO and proved invaluable. An improved model, the M40, based upon the M1 gun and M4 tank, was produced in 1944 and deployed in limited numbers to the ETO in March 1945.

Heavier supporting artillery pieces were the M1 8" howitzer, an excellent and accurate weapon; the M1 8" gun, which was developed as an answer to the superb German 17cm gun, had greater range and a more lethal shell than the German weapon, but suffered from poor accuracy and excessive barrel wear; and the 240mm howitzer, a good, if very heavy, weapon.

Nearly all US artillery battalions were organized with three firing batteries and a total of twelve tubes. The exception was the eighteen-tube armored field artillery battalion and the six-tube 8" gun and 240mm howitzer battalions. A major advantage for the American artillery was that it was fully motorized and highly mobile. All 105mm and 155mm howitzer battalions in the ETO were truck-drawn, although a Table of Equipment (TE) for a tractor-drawn 155mm battalion existed. The 155mm gun battalions were almost all tractor-drawn, although a few evidently were also truck-drawn. The 4.5" gun, 8" gun, 8" howitzer, and 240mm howitzer battalions were all tractor-drawn, although, again, a TE for truck drawn battalions existed. The standard prime mover was a two-and-one-half ton truck for the 105mm and a 4-ton Diamond T truck for the 155mm howitzers. Tractors included the M5 thirteen-ton prime movers, which were utilized for the 105mm M2 howitzer, the 4.5" gun, and 155mm M1 howitzer, and the M4 eighteen-ton hi-speed, full-track, heavy prime mover, which was utilized for the 3" AA gun, the 90mm AA gun, the 155mm Long Tom gun, 8" howitzer, 8" gun, and 240mm howitzer. Redundant M3 medium tank chassis, without armament, and M31 and M32 armored recovery vehicles were also utilized as prime movers for the heavier artillery pieces.

Non-divisional artillery battalions were normally subordinated to field artillery groups. The groups were formed in 1943 from the headquarters battery of the broken up field artillery regiments. The field artillery group consisted of an H&H Battery, with a command element and a fire-direction center element, and a Service Battery. A group was usually assigned from two to six battalions, although one or more of the battalions might be attached for direct support of an individual division. Usually, the groups were assigned howitzer and gun battalions of companion caliber, that is, 155mm howitzers were grouped with 4.5" guns, 8" howitzers with 155mm guns, and 8" guns with 240mm howitzers. The normal ratio was one gun battalion for every two howitzer battalions, although this was not always firmly adhered to. Separate 105mm howitzer battalions were normally grouped together, but were almost always assigned to direct support of divisions. The 155mm SP gun battalions were assigned to groups as the tactical situation warranted, or were frequently attached, by battery or battalion, to armored or infantry divisions.
Military History Online - US Army in World War II