Some common Anti-Tank weapons found in the Italian Arsenal. Any AA weapons I mention here were also pressed into service as anti-tank guns.
1939-1943
Solothurn 20mm - Used in an anti-tank rifle form, usually mounted on a pair of small wheels. Also found in vehicle mounts.
20/65 - 20mm ATR adapted for anti-aircraft work. Almost always found on "Autocannoni" during the war.
37/54 - An AA weapon, rarely found in the field, and usually used as a naval mount. A version of this weapon was used as the primary armament for the M11/39, and was actually a fairly effective piece, though in general the basic 37/54 was not. Maybe the vehicle mount was more stable and hence, had a better trajectory?
47/32 M35 "Elefantino" - Italian copy of the Austrian Bohler. From 1942, supplied with "Effeto Pronto" rounds.
65/17 Field Gun - Older field gun successfully used as an anti-tank piece. Equipped from 1942 with EP rounds. Several of these were mounted on trucks and used as Autocannoni.
75/18 Howitzer - A very good anti-tank gun. Used on the Semovente 75. Occasionally used in field mounts. Equipped from 1942 with EP rounds.
75/34 Howitzer - Modernized version of the above.
77/28 Field Gun - Very commonly found in support of cavalry formations. EP rounds were tested, but I don't know if they were ever actually issued.
88/27 - The German '88'. Obviously an excellent weapon. Not very common. Italian crews preferred their own designs.
90/53 - Excellent weapon. Superior muzzle velocity to that of the 88. Used as an anti-aircraft, anti-tank, and anti-infantry weapon. Found on field mounts, but also used as Autocannoni.
AFTER the Armistice, the Italians received a fairly large number of American 57mm guns (known as the 57/50), and British 17 Pounders, known to the Italians as the 76/55.
The RSI (pro Axis Italian troops) used a mix of Italian and Axis weaponry, and the pro-Allied Italians used their own mix of Allied and Italian designs.
As for the railway gun issue, former user, I must correct myself.
The largest rail gun used by the Italians was the 381/40! (that's right, a 381mm gun!). It was apparently used in long range support roles during the North African and Italian campaigns.
The largest coastal gun to see action was actually the 305/17 Howitzer. Used in both Italy and Sicily.
Hope this is of some use,
Doc