Many of the animations are wrong! They are close... but wrong.
They usually show the slide unmoved until the bullet exits the barrell.
This is close to true but very misleading.
Since Newton's laws of motion are still in effect :-) ...
The mass times the speed of the bullet is equal to the
mass times the speed of the slide (the spring not with standing)
So... the speed of the slide is equal to...
(Bullet Speed X Bullet Mass) / Slide Mass ... at all times!
... again, not accounting for the affect of the spring.
The point here is that if the bullet exited the barrell
before the slide started backward... it would never start!
All the pressure would exit the barrell right behind the bullet.
Unload your pistol and push a wooden dowell down the barrell.
You will be applying pressure exactly were the case applies pressure.
The slide will move... and it does the same when the explosion
applies pressure in all directions.
The bullet moves one way... very fast.
The slide moves the other... very slow.