“How did he have time to take it apart?”
Practice.
There was a gun in the bag, why have the bag otherwise. I doubt the perp was worrying about bringing home his books to study. I have heard no evidence that the supposedly found Mauzer rifle is actually the kill weapon. If it exists, the bullet that killed CK has to be found and see if it matches the gun. There are other type guns that brake down fast that could have been used.
I don’t think the gun found in the woods was the gun used in the shooting. It was a calling card.
When Charlie was hit, his arms almost immediately came up into the position that usually indicates brain damage or severe concussion.
I saw a close video of him getting shot and specifically looked for the bullet to exit. I didn’t see any sign of it exiting, and my theory is that it hit his spine and deflected upward into his brain.
I’ve seen no mention that they recovered the bullet, or that ballistic matching has been done or attempted with the rifle they found in the woods.