As I was in the rewind section of flesh-land. Gun had the firing effect and noise on, but no enemies or movement or trigger appeared.
Ah...don't think it was stuck then...
*Spoilers* You are rewinding, going backwards, so when the "flash" pops up you need to re-shoot the enemy you shot when advancing forward (it could be off in the distance or hidden among "arm" sprites depending on when you shot it!). Basically if there is a crosshair on the screen, that means there is something you can shoot!