My interpretation of the evidence regarding CE 399 is that at some point this bullet was introduced into the chain of custody.
It may not necessarily have been the case but after examining the evidence I felt it was the best interpretation, the one that made sense of the many anomalies surrounding this aspect of the case.
In order for this to be the case the bullet, CE 399, would have to be fired from the MC in order to match the rifle. However, this couldn't have been done until the rifle reached the FBI lab in Washington on Saturday morning.
By Saturday evening an evidence report compiled by Frazier and signed by Hoover was sent to Chief Curry:
At the end it identifies Q1, the bullet supposedly found on the stretcher, as "a 6.5 millimeter Mannlicher-Carcano rifle bullet. Specimen Q1 weighs 158.6 grains. It consists of a copper alloy jacket with a lead core".
The weight and general description of the bullet is identifies it as CE 399. Which means that if the bullet was indeed a plant it must have been fired from the rifle on the Saturday, before the evidence was returned to Dallas Saturday night.
Until now it has been a matter of pure speculation that the rifle was fired that day in order for it to be possible to produce a bullet which could be planted in the chain of custody.
Whilst trying to track down a decent copy of a photo of the evidence that was taken by Vince Drain to Washington I came across an article by Ken Rahn documenting Drain's recollection of the assassination, which includes this passage:
"...I was taken by helicopter over to the Justice Building and landed on the White House lawn. During this time, I had an armed guard from the Air Force until I got safely into the Justice Building.
I talked to Mr. Hoover briefly and then watched them do a lot of the experiments
such as firing the rifle, looking for prints, ballistic markings, hairs, fibers, blood stains and anything else that later, down the road, might be relevant to evidence which could be used in the prosecution.
By around midnight on Saturday night, they had the plane ready to go..."
It would appear that at some point on Saturday the FBI lab was in possession of bullets fired from the MC found on the 6th floor.
PS: Does anyone have a decent copy of a photograph of the evidence Drain took from Dallas on Friday night?