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Is it that you want the bullet to spawn from the gun on the detonation frame (frame 1) rather than the 1st frame (frame 0)? If so, you could use something like this:
1. On shoot key pressed | set playing sprite animation "shoot" from start.
2 If sprite.animation "shoot" is playing, if sprite.animationFrame = 1 | spawn bullet from sprite at (gun origin).
You will need to add an origin point at the position of the barrel of the gun on the sprite, where you want the bullet to spawn from.
A word of warning about spawning the bullet partway through an animation: unless your animation is very quick and short (or you have a deliberately long wind-up phase for your shot) it is best to spawn the bullet at the first frame because otherwise you may be introducing a noticeable lag into the shooting mechanic, which can make the game feel unresponsive.
If this isn't what you're after please could you link to specific video you're talking about, or include a capx?