I didn't forget it, I left out Tentacool and Tentacruel and The Tower of Terror by purpose because while they were banned after 9/11 they were brought back once everyone's feelings had cooled down a bit.
Episode 19 - Tentacool & Tentacruel
"After the events of September 11, this episode was taken out of rotation. In the months following the attack, Kids' WB! refused to air it due to the destruction of several buildings (particularly Tentacruel smashing a skyscraper) and the use of machine guns. When Cartoon Network started to air Pokémon, the episode was put back into rotation. It is also on the VHS "Seaside Pikachu" and the Indigo DVD boxset." - Bulbapedia
Episode 23 - The Tower of Terror
This episode was temporarily banned after September 11. The official reason has never been confirmed, but many believe that the episode's title may have had something to do with it. The episode is no longer banned and continues to air.
It is also speculated that it was also banned because of Meowth's Hitlerish moustache:
Actually there were only revolvers involved as far as I know. I guess you saw the second picture and drew the conclusion that it was a shot gun.
However there are other Pokémon episodes featuring guns that aren't banned:
Episode 34 -The Kangaskhan Kid
Episode 54 - The Case of the K-9 Caper!
Also, Ash imagines his friends getting shot by a sniper in episode 55 - Pokémon Paparazzi.