Jonathan's shotgun was a shotgun that belonged to Hiram Kent that he passed down to his son Jonathan.
Jonathan shotgun

Season One

When Jonathan was under the influence of the nicodemus flower, he was going to Smallville Savings and Loan's bank with this shotgun, the flower's influence escalating Jonathan's anger at having been denied a recent loan application, but Clark stopped him.

Season Two

Under the control of Desirée Atkins, Jonathan shot a bullet out of this gun at Lex Luthor, but Clark incinerated the bullet at super speed with his newly-developed heat vision, saving Lex.

Season Three

In a flashback, Hiram is armed with this gun when he meets Joe the Drifter, who's actually Jor-El. After getting a message from the future, Jonathan protects Lana at his farm with this gun. Adam Knight knocks him out and steals this gun and chases Lana outside with it, shoots it at Lana but Clark super speeds in the way and it bounces off. Clark must've recovered it because Jonathan's seen using it in an episode after it. Jonathan goes to the Kawatche caves with this gun and goes berserk, but Clark stops him before he could significantly hurt Lionel.

Season Four

Stripped of his powers by magic, Clark confronts the three witches with this gun.