Watch the bullet closly between 12-16 seconds,
There are pressure changes that happen when something goes super sonic.
A wave in front is passed then disrupted as it passes into low pressure behind.
The bullet is supersonic when it leaves the muzzle,, what we see is what happens to the atmosphere around the bullet.
Of course that all doesn't take 4 seconds, it's 4 seconds of film that's taken as the bullet moves just inches from the muzzle.
I noticed too that the muzzle doesn't move in recoil, the barrel must be vice mounted.
A scale or measure to show distance traveled would have added data,(a board with marks) but maybe there was one that's been cropped out of the film.