The point of asking this was to look at the breakdown of SR when the speed of light is involved.
Eg. in the case of the bullet and gun, either can be considered as stationary (depending on the RF), but, as I understand it, there is no RF in which the photon could be considered stationary, whatever motion one attributed to the emitter
You already answered that to yourself in your answer above, the photon doesn't exist in any real world reference frame you could ever use it is as you called it "light" up until that point. So there is no real way to equate the bullet gun case unless you assume the particle story of light .. it's a nice fairytale but as you know it won't stand up to scrutiny.
I think we have discussed this before we can use the wave as a reference point for things such as the spin during transit but you must use extreme care in making this frame "real". You can only know that is real post photon observation before that point it is a probability reference.
I think you have got the point that there is nothing special about the double slit experiment, your gravity well example as well as countless other examples like Paul started with show the strange nature of light. I find it funny when layman think that the double slit is the only experiment in which there is some strange behaviour with light and if they can just fix that it will all be ok and there classical world will return.
The double slit experiment is but one of a vast array of experiments which show the problems with understanding light and it will never be able to be described in classical terms.
