From memory Mike Tesko always argued they replaced a fragmented bullet with a whole one, but why fake the X-rays and at what date was this accomplished?
They were trying to cover up the shooting of June. If as I suspect SC had already committed suicide and was dead on her own bed and and June had no or just a few injuries. The crime scene would look a bit out of kilter. What is more they would not know that the pathologist would help them with the cover-up at say 08.00am since none had been called.
The TOD would also be a factor. If the pathologist recorded a big difference between the TOD of the two women it would be even more problematic.
There may also have been information that June was alive but as we know they started a new case file and the original has never been disclosed.
From the magical PV20 and the Xray and other known facts it is possible to hypothesise how these factors fit together.
I think the initial plan (cover-up) was to say that June had shot SC with AP's Brno during some event (say a struggle over possession) SC is wounded but not dead because the bullet fragmented giving her the opportunity to recover herself sufficiently to kill June. Restage the bodies, PV is sympathetic to the accident and tailors his initial comments to facilitate this scenario.
To bolster this argument they fake an Xray showing a highly fragmented bullet.
This is then sent to the CT some years later because whoever sent it was trying sublimely to help JB