The Bubble Gun is an attack used by Taffy in the two Nintendo 64 games (63 1/3 and Sculptor's Cut.)
In the Attack, Taffy draws a Yellow Six Shooter (Presumibly made of gum or more taffy) from his mouth and repeatedly blasts it at the foe. The super makes one shot, if fails or it's blocked, the move stops and allows Taffy to move, but if it hits, makes a lot of shoots and it can cause a Queen Combo.
Also, it's the only super in the two games and in the series where only needs to keep pressing a button instead of Street Fighter-like combinations. Can be realized by pressing punch a few seconds and later releasing it and quickly press kick to perform this super.