Anti-tank gun
The first anti-tank weapon was used in WWI.  This was the Mauser 1918 T-Gewehr, a heavy, handheld rifle used by German troops to pierce through the thin armor of the time.  Although not an artillery piece, this paved the way for the first anti-tank gun being made.  The 3.7 cm PaK made by Germany was the first artillery piece to be designed specifically for tanks.  This was made after WWI.