Lello,
Good try on the party popper. At least I am not the only one having syncing problems.
Most of my probs are in the trigger. Getting it to fire at a precise point is the goal. I have had it trigger before the strike or after. Mid card strikes have often been a case of an off contact at the trigger causing the flash to strobe.
Arrrh 
. Attatched is such an example in which I also failed to clamp the card hard enough so it moved under pellet impact. Bit of card flying off but no sign of the pellet.
On the accuracy front I have discovered that impact on the foil trigger can fractionally deflect the pellet.
Only really a consideration for the card shot though.
I have 23 cards left to try mods to the trigger. Determined to do this for free, also it takes a different route to the sound trigger stuff that you are doing.
Your drop an object on the trigger would work and I will have a go. While I have the card clamp aligned I really want that shot so will persevere for now. Solution of the trigger problem being key to any exploding shots.
Don