The 'most willing' weapon I've seen bump fire was an early 90's Norinco AK. To bump it had to be held firmly on the front grip but very lightly on the handgrip, and very very lightly on or just off your shoulder. This set up an oscillator that effectively let the weapon recoil from your arm (acting as a spring) while letting the trigger move back and froth against your effectively decoupled/ independent trigger finger.
Think of any weapon pointed up, that had a bungee supporting it from above. Then a fixed position stick pressing against the trigger. The recoil (aka bump) moves the weapon back far enough to reset the trigger. The bungee then pulls the weapon back against the stick re-firing the weapon. Same thing but different direction.
I never saw it (or any other for that matter) bump when pulled in tight against the shoulder and firmly gripped at the pistol grip. I expect that is true for your and most any other weapon.
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