This is just my opinion and may depend on people. We rarely use these phrases. ドキュン, バキュン originally come from the sound (onomatopoeia) of a gun and a pistol which shoot your heart so it's an instant thing. I guess キュン is related to the feeling at exact that moment and after that. ●キュンキュン: Your heart CONTINUOUSLY and very QUICKLY beats especially because of love to someone (or sometimes to something) and suffering from that. Two キュンs are repeated so it's like a CONTINUOUS thing. ・例文1. ： 彼女の事を思うと、胸がキュンキュンする。 When I think of her, my heart gets キュンキュン. ・例文2.：　この犬かわいいね。胸がキュンキュンするわ。 What a cute dog! My heat gets キュンキュン. ●c.f. キュン：　I think in this case, it's one-time thing. ・例文：　彼女の事を思うと胸がキュンとなる。 When I think of her, my heart gets キュン. ~ Every time I thing of her, every time my heart gets キュン. ●ドッキュン/ドキュン=ズッキュン/ズキュン: Your heart is SUDDENLY hit especially because of love to someone or something. I guess this feeling comes from your heart itself. Compare it with バキュン. ・例文：　彼女を初めて見たとき、僕はドキュンとした。 When I saw her for the first time, I got like ドキュン. (The nuance is like my heart was SUDDENLY/IMMEDIATELY addicted to her) ●バッキュン/バキュン: This is similar to ドッキュン, but in this case SHE(or something) herself gives out バッキュン. ・例文：　彼女は僕の心をバキュンと打ち抜いた。 She hit my heart like バキュン or my heart was hit by her like バキュン. c.f. For the real sound of a gun (not for feeling stuff), we often use パン: 例文：　「パン」と銃のような音を聞いた。何か重大な事件ではないといいのですが。 　　　　　　I heard "パン", which was like a sound of a gun. I hope this is not a serious happening